>'<'-*'i>^^^'''i^'^ " THOU HAT TE ache: ANOTHER BEST THOU : 'LIBRARY I ANNEX "■ ••c;^*<;^'<:i'S;^»t^-'<|'«s^'>*^'l^'i-''^*i>$-'*>^ New York State College of Agriculture At Cornell University Ithaca, N. Y. Library Cornell University Library LB 1519.R40b Object lessons and how to give them, seco 3 1924 013 393 909 Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924013393909 THE OBJECT LESSON BOOK By GEORGE RICKS B.Sc. (Lond.) NATURAL HISTORY OBJECT LESSONS A Manual for Teachers and Pupil Teachers OBJECT LESSONS AND HOW TO GIVE THEM First Series for Prinnary Schools OBJECT LESSONS AND. HOW TO. GIVE, THEM Second Series for Intermediate and Grammar Schools OBJECT LESSONS AND HOW TO GIVE THEM Seconb Series For Intermediate and Grammar Schools By GEORGE RICKS B.Sc. (Lond.i INSPECTOR OF SCHOOLS TO THE SCHOOL BOARD FOR LONDON AUTHOR OF "NATURAL HISTORY OBJECT LESSONS " ETC. Q>ri?ell iJpiuersity, BOSTON, U.S.A. D. C. HEATH & CO., PUBLISHERS ^^95 CONTENTS. MB80K PAOa TNTRODTTGnON TO *' OBJECT LESSONS, AND HOW TO GIVE TTTEM " ix INTRODDCTION TO OBJECT LESSONS FOR JUNIOR AND SENIOR SCHOOLS xiv FIRST STAGE. I. SOLID AND LIQUID . . 3 II. " PROPERTIES " OF EOnrES 6 III.— WEIGHT .. .7 IV. POROUS. ABSORBENT. .9 V. POROUS BODIES. FILTERS 11 VI. NON-ABSORBENTS . . . . . . . . . LJ VII. SOLUBLE 14 VIII. — SOLVENTS WATER, ALCOHOL, &C 15 IX. ADHESIVE .. 17 X. HARD AND SOFT 18 XI. BRITTLE, TOUGH, FLEXIULE •••.... 20 XII. — ELASTIC 21 XIII. — PLASTIC 23 XIV. FUSIBLE ... . ... 25 XV. ON SOME FURTHER PK'.i-EKTIES OF THE COMMON METALS . , 26 XVI. — LEAD ... 27 XVII SULPHUR ..••80 SECOND STAGE. I. ^WATER. ITS PROPERTIES ........ 35 n. WATER. — A SOLVENT ..37 VI CONTENTS. LESSOH PAOB ni. WATBR AH TAPOriU DEW 4" IV. WATER AS FOG, MIST, CLOTID, RAIN, AND STEAM ... 41 V. WATER AS SNOW AND ICB 46 VI. — MBRCCRT, OR QOICKSILVEH 46 VII. AIR — A SUBSTANCE, INVISIBLE, OOOUMBS SPACE, HAS WEIGHT . 48 VIII. — AIR PRESSES IN ALL DIRECTIONS 60 IX. AIR IB ELASTIC 62 X. — GAS . . . . I . , .... 54 XI. — COAL-GAS ...........66 tn. TAR 58 JCni. — CARBONIC ACID .....60 XIV. — PARAPPIN OIL 62 XV. — CANDLES 64 XVI. —SOAP-BUBBLES AND WHAT THEY TEACH ..... 65 XVn. — BALLOONS ...........68 THIRD STAGE. I, — MOLECULES ......... .73 n. — STATES OF MATTER 76 in. ADHESION ....77 IV. — CAPILLARY ATTRACTION 79 V. — PROPERTIES OP SOLIDS EXPLAINED 81 VI. — FORCE OF GRAVITY— WEIGHT. 83 Vn. — THE SURFACE OP A LiaUID AT REST IS ALWAYS LEVEL . . 86 VIII. — PRESSURE IN LIQUIDS. 1 87 IX. — PRESSURE IN LIQUIDS. U. 90 X. — BUOYANCY OP LIQUIDS 92 XI. — WEIGHT OF THE ATMOSPHERE 94 Xn. — PRESSURE OF THE ATMOSPHERE . 97 XIII. — THE BAROMETER ......... 9S XIV. — THE SYRINGE ......... 101 XV. THE COMMON PUMP 103 XVI. — FORCE-PUMPS 106 XVII. — THE SIPHON 108 XVIII. — THE AIR-rUMP .....,.,., HI CONTENTS. Vll P0T7RTH STAGE. LESSOir PAGE I. — EFFECT OP HEAT ON BODIES (1). EXPANSION AND CONTRACTION . 117 II. — EPPEOT OF HEAT ON BODIES (2). — LIQUEFACTION AND VAPOBIZA- TION . . . 119 III. — THE THERMOMETEB. . . . . . . . . .120 IV. — FREEZING OP WATER . . . . . . . . .124 V. BOILINO OF WATER — CONVECTION . . . . . .126 VI. — DISTILLATION . ....... 128 Vn. EFFECT OP PRESSURE ON THE BOILINQ POINT OF LIQUIDS . . 130 VIII. — STEAM AND THE STEAM-ENGINE . 132 IX. — CONDUCTION OP HEAT. — CLOTHING 136 X. — RADIATION OP HEAT. — RADIATORS 138 XI. HOW HEAT AFFECTS THE ABSORPTION OP WATERY VAPOUR BY THE ATMOSPHERE .... .... 410 Xn. — HEAT THE CAUSE OF MOTION IN THE AIR 143 XUI. — DEW AND HOAR-FROST 145 XIV. — RAIN, BNOW, HAIL, SLEET . 147 XT. — SPECIFIC HEAT 149 XVI. — LATENT HEAT .......... 161 XVn. COOLING BODIES AND FHEEZING MIXTURES. .... 153 XVm. — SPECIFIC GRAVITY OF SOLIDS ....... 156 SUPPLEMENTARY LESSONS ON PROPORTION. I. 168 n 161 m 163 IV. 166 FIFTH STAGE. 171 II. — CHEMICAl ATTRACTION I. POEOB • • . 173 nl. — ELEMENTS AND COMPOUNDS 1'° jy. — THE AIR A MIXTURE OF GASES 178 T. WATER A COMPOUND OF TWO OASES 180 VlU CONTENTS. LES301T PAGB VI. — COMBUSTIOIT a . • 182 VII. — THE CHEMISTRY OP A CANDLE 184 VIII. — ELECTRIC AND MAGNETIC FOEOES 186 IX. — CENTKE OF GEAVITT 188 X. — LEVERS AND THEIR USES 1 192 XI. LEVERS AND THEIB USES. II 195 XII. — LEVERS AND THEIR USES. — lU 197 XIII. — THE PULLET 200 XIV. — THE WHEEL AND AXLE '. 203 XV. — THE INCLINED PLANE ...>.... 205 XVI. — THE WEDGE 207 XVII. THE SCHEW 208 XVUI. — FRICTION ....a...... 210 INTRODUCTION TO OBJECT LESSONS AND HOW TO GIVE THEM, Our knowleclgeoftlie material world is obtained through the senses. The organs of sense are the eye, the ear, the nose, the tongue and palate, and the nerves of touch located in the skin. The special nerves of these organs are acted on by things external to the body; the effect is conveyed to the brain ; and mental impressions or ideas are the result. Thus a red colour acting on the retina, the sound from a whistle acting on the auditory nerves, or the smell of an onion on the olfactory nerves produces a definite mental impression. The five sen- sory organs, then, are so many doors and windows by which knowledge enters the mind. There is, however, another source of knowledge of materia] bodies. In this case the mental impressions are derived from within the body, and are due to muscular exertion. It is bj muscular feeling that we estimate the amount of force required to overcome resistance. Thus we get ideas of elasticity and weight from the amount of active energy put forth by the muscles to overcome inertia in the one case and gravitation in the other. If a weight is placed in the hand we are con- scious of a certain amount of force expended to keep it from falling ; if the weight is increased we are conscious of an increased expenditure of muscular energy. The mental impressions, formed by and through the senses, including muscular feeling, are called sensations. By the organs of sense we are said to perceive, or to make mental notes of external bodies, and these mental notes we X OBJECT LESSONS. call perceptions. Perception is tlie first step in knowledge : attentive perception leads to observation ; observation is the forerunner of comparison ; while comparison is the basis of classification ; and these together constitute the foundation of all knowled ge. The primary purpose of lessons on common objects and natural phenomena is to cultivate the senses, to train to habits of attention, intelligent observation, and accurate comparison, and so to lead up to the higher processes of the mind — reason and judgment. Of course the acquisition of information is an important aim ; but the object lesson is designed to assist and guide the child to discover properties of things, and thus acquire knowledge for himself, rather than to pour information into his mind like wheat into a sack. Mental impressions are formed at a very early period of childhood. A bright light or a shining object attracts attention before the child has acquired the power of taking hold with its hands; and a certain amount of dis- crimination, enabling it, for instance, to distinguish the face of its mother from that of a stranger, quickly follows. The power of recognising resemblances and differences rapidly increases, new ideas are as rapidly acquired ; and when the child enters school, he enters it with his perceptive faculties, to a certain extent, cultivated, and with his mind a treasury of simple ideas. The natural course for the teacher would seem to be to gather up into something like order, and to perfect, that which has been so far imperfectly accomplished; and then, starting from this as a basis, to evolve a systematic course of training, proceeding step by step in a natural order, each step being a logical sequence of the preceding. Further, the teacher who would best succeed must take childhood's method of imbibing knowledge and adapt it to her own use. Restless activity, insatiable cariosity, and love of imitation HOW TO GIVE THEM. XJ ctaraoterize cbildliood. What the child sees he wants U> know about, to handle, and examine, and, if possible, to take to pieces, or otherwise experiment upon ; what he sees done he wants to do, and if opportunity be not found for the indulgence of his natural activity, he will find the oppor- tunity for himself. An object lesson, then^ besides fulfilling some definite purpose in training the perceptive faculties, should provide something for the children to do to satisfy their innate activity, something to examine and discover to arouse their curiosity, and something to copy to gratify their desire for imitation. Herein lies the secret of securing attention, of begetting a state of vigorous mental activity, and of associating pleasure with instruction. Theselection of lessons, and their adaptation to the capacities of the scholars, or to their different stages of advancement, is another point of fundamental importance. A child of four years of age is a different being, intellectually, from a child of seven ; and a lesson suited to the capacity of the one must be totally unsuited to the mental condition of the other. The mental faculties of a child are strengthened and invigo- rated by proper exercise, but are weakened and depressed by being exercised on subjects beyond his powers of compre- hension. To graduate the lessons to the mental condition and previous training of the scholars necessitates a complete system. It is not sufficient to select a lesson at random, no matter how skilfully it may be handled. Each lesson, whilst fulfilling its own special purpose, must form a link in the chain, a unit in the whole. Nor is the method of giving the individual lessons of less importance than their selection and adaptation. Occasional information given about things of every-day life does not serve the distinctive aim of object lessons. To be a passive recipient of information gives no pleasure to a child. To hold an object before it, and enumerate its general properties — what it is composed of, or where or how it is made — and XU OBJECT LESSONS. then to get the information returned by questioning, is at best but a mere exercise of the memory ; it does nothing in the way of exercising and developing the more important mental powers. " To tell a child this, and to show the other, is not to teach it how to observe, but to make it a mere recipient of another's observations — a proceeding which weakens rather than strengthens its powers, of self -instruction, which deprives it of the pleasure resulting from successful activity, which pre- sents this all-attractive knowledge under the aspect of formal tuition, and which thus generates that indifference and even disgust with which these object lessons are sometimes re- garded. On the other hand, to pursue the true course is simply to guide the intellect to its appropriate food, and to habituate the mind from the beginning to that practice of self-help which it must ultimately follow. Children should be led to make their own investigations and to draw their own inferences. They should be told as little as possible, and induced to discover as much as possible." * Having formed new ideas of things by the method of obser- vation and experiment under the guiding hand of the teacher, our next step is to endeavour to fix these ideas in the minds of the children by means of language. But in every case words nmeii follow ideas; in fact, terms should not be given till the necessity for them is felt. Thus, suppose " a liquid " to be the subject of the lesson. The children are led by experiment on several liquid bodies to note that they all have certain common properties — such as flowing in a stream, finding the lowest level, spreading out and filling up hollows, easily flowing in drops, having no definite shape, but taking the shape of the vessel iato which they are poured — and the necessity is felt for one term which at once embodies all these properties. The extension of the children's vocabulary in this way is one of the minor advantages of object lessons; • Herbert Spencer. HOW TO GIVE THEM. XUl anrJ furtter, to secure freedom and accuracy in speech, the children should be encouraged to answer all questions, as far as possible, in complete sentences, or at any rate in complete phrases. The teacher may commence object lessons by taking some familiar object — such, for instance, as the black-board — and lead the children to observe its colour, shape, substance, surface, and so on ; but then we have so many properties in combination that the scholars are not likely to get very clear notions of any. It is desirable, therefore, if not actually necessary, that lessons on objects should be preceded by a special training in colour, form, size, weight, hardness, and others of the more conspicuous properties of bodies. Lessons on objects may then be introduced gradually, and, to a large extent, they may be made to constitute simple practice in the application of previously acquired knowledge. In dealing with the proj^erties or qualities of objects, those only should be dwelt upon which render the objects valuable for the several uses in which they are employed. Thus, all children are alive to the fact that we cannot see through sponge, cork, india-rubber, or leather; but to stop to describe these objects as opaque is a waste of time. On the other hand, although the children know equally well that we can see through glass, its property of transparency must be made a cardinal point in a lesson on glass, because it is thia property which makes glass specially usefuL INTEODTTCnOlT OBJECT LESSONS FOR JUNIOR AND SENIOR SCHOOLS. " The practice of every art implies a certain knowledge of natural causes and eifects, and the improvement of our arts and industries depends upon our knowing the properties of natural objects which we can get hold of and put together." " No line can be drawn between common knowledge of things and scientific knoicledge, nor between common reasoning and scientific reasoning. In strictness all accurate knowledge is science, and all exact reasoning is scientific reasoning." " The method of observation and experiment by which such great results are obtained in science, is identically the same as that which is employed by every one, every day of his life, but refined and rendered more precise." " The way to science lies through common knowledge, we must extend that knowledge by careful observation and EXPERIMENT, and learn how to state the results of our inves- tigations accurately, in general rules or laws of nature, and we must learn how to reason accurately from these rules, and thus arrive at rational explanations of natural pheno- mena, which may suffice for our guidance in life." * On these principles laid down by Professor Huxley the following simple lessons on natural objects, and natural phenomena have been constructed. • Fiofes9or Hnxley. INTRODUCTIOls XV Their primary purpose is to develop the faculties of the mind, to quicken the intelligence, to train to habits of accu- rate observation, exact comparison, and sound reasoning, and to excite an interest in all those objects with which we daily come in contact, and those natural phenomena which constantly appear before our eyes. The lessons are suggestive rather than exhaustive ; some are little more than outlines ; a few are worked out more fully as models for young teachers. The experiments are numerous and interesting, yet simple and inexpensive. The lessons of the First Stage* deal with the more common properties of solids, and show how these properties make them specially useful in the arts and industries. They are lessons almost entirely of observation, experiment, and comparison. Those of the Second Stage deal similarly with the properties of liquids and gases illustrated by water and air. The Third and higher stages demand a closer observa- tion ; the reasoning faculties are gradually brought more fully into exercise; and the simplest facts and laws of nature are explained in the simplest possible way. * The standards for whicli the stages are suited must depend in a great measure on how far the children have heen trained in the infant school, but, generally, Stape 1. -will be suitable for Standard II., Stage II. for Standard III., and so on. Lessons suitable for Standard I. •mil he found in " Object Lessons, and How to give them, Jint Series, for primary schools." FIEST STAGE. FIRST STAGE. LESSOT^ I. SOLID AND LIQUID. AsricrES for rinfm.rion : any specimens of •well-tnown solid bodies, vTater, apiece o; sponge, and one or two gl^ii vei-scli. L To show that tie minute paxtides of which solids are hnilt ap are held more or less firmly together. Ejrpen'menf 1. Take two lumps of loaf sutrar ; and, rubbing them tog-ether, fhow that the lumps are mide up of praiiis. Exp. 2. CtusIi or pound tlie grains to show that these uiay be divided into smaller bi*s as fine as flour or dust, and too small to be easily seen as separate ; i}?-ticies. -E.-o. 3. Eub lumps of chiUt together, to show how ex- tremely fine are the tiny partiflts of which chalk is made. £xp. 4. Hand a small cubical lump of lo.ii sugar, or a piece of brick, to a scholar, and ask biin to separate it into tiny ti:s. He fails. Why ? The sug-.ir, or brick, is too hard. The ffi-isi .i, or j>ar:.\.'ts, are held together too firmly. IL To show that water and oil are also composed of minute particles ; hut that these are held together less firmly than are the grains of sugar and salt £xp. 5. Take some water in a tumbler, and sprinkle a little on the tioor, or blow a little through a svringe, to show the tiny droi-S. 4 OBJECT LESSONS. Eijp. 6. Into a test-tube three-parts full of water pour a single drop of olive oil. SLake well; the single drop is divided into thousands of tiny drops, giving the water a milky appearance; but the drops are too small to be seen separately. " Now which is the more easy to divide, the oil and the water into tiny drops, or the sugar and salt into grains ? The water and oil. " And why?" Became the drops of oil and water are not held so firmly together as are the grains in the sugar and in the salt, " Here are pieces of stone and wood. You can handle them, pass them round, throw them up, and they are not altered. Now, can we take a piece of this water out of the glass and pass it round the class? No ; it would break into drops at once, and fall on the floor." " I want you to remember that all those things in which the tiny particles are held firmly together ice call solids ; and that all those things which are made up of tiny drops not held firmly together we call liquids." III. To show that solids have shapes of their own; but that liquids, having no shapes of their own, take the shapes of the vessels in which they are placed. The teacher may proceed somewhat as follows : — " Here is a small block of wood.* What is its shape ? " A cube. " I put it in this tumbler ; is its shape altered ? " iVb. " What shape has it still ? " A cube. " I take out the cube, and fiU. the tumbler with water. What is the shape of the water in the tumbler ? " T/ie same shape as the tumbler, * Einder-gaiten cube, for inetanos. FIEST STASB. 5 ""Sow I poTir some of the water into this •c." They are unlike in taste. " Xame one property which the following pairs of bodies have in common.'' 1. Soda and ice. Both are transparent. 2. Cork and spniire. Both are light. 3. Lead and gold. Both are heart/. ■i. Wool and sponge. Both are soft to the touc'h. 5. Rif)e orange and sugar. Both are sweet to the tasta 6. Lemon and rhubarb. Both are sour to the taste. LESSON HI. WEIGHT. Articles for illnstratioii : pieces of metals, woods, coA, and sponge. I. Weight is pressure downwards. Ejrp. 10. Direct some of the scholars to hold in their hands any heavy substiiuces withiu convenient reach. What cjtu they s-iv about them ? They are heavy. 8 OBJECT LESSONS. Next take lighter bodies, such as light wood, bark, cork, &c. What can be said of these bodies ? They are light. Exp. 11. Place these bodies in a vessel of water ; some float, some fall to the bottom. Distinguish again the light and heavy bodies. When placed in water in what direction do bodies press ? Dotcnwards. Ejc-p. 12. Place a heavy weight on some soft yielding substance such as puttv, clay, snow, or sponge. In what direction does the weight press ? Dotcntcards. Erp. 13. Direct a boy to place his right arm and hand in a horizontal position. Place weights — ^books for instance — on the hand until it is visible to the class that the hand is pressed doicmcards. TTe call this pressure downwards Weight. Weight is a property common to all bodies. II. Actual weight depends partly on size. " Wood floats on the water, iron sinks to the bottom. Which is the heavier body ? " Iron. " Here is a small piece of iron, and here a large block of wood. Which is the heavier ? " The tcood. " What do we mean, then, when we say that iron is heavier than wood P" We mean that when tee fake pieces of iron and icood of the same size, the iron is header than the wood. When we compare the weights of bodies it is always understood that we are speaking of pieces of the same sise. III. The meaning of heavy and light " Here is a small block of wood. Has it any weight ? Does it press down ? Now take this small block of lead of the same size. Which presses down the harder ? Which FIRST STAGH 9 has the greater -weip-lit ? Compare the weight of the lead and the wood." The had is hea^t, t/ie wood is light. " What do we mean when we say that wood is light? " We mean if /las litti.k weight. ■• And what do we mean when we sny the lead is heavy ?" We tneini that lead has Mica weight. All bodies, then, have weight ; and when we sit that bodies such as wool, feathers, cork, and sponge are light, we mean that they possess but little weight, compared with such bodies as stones and metals. IV. The uses we make of bodies as dependent on their weight. The teacher will illustrate how heavy substances are used for weights. Ac.: and light substances, as cork, for instance. for making life-buoys, cork jackets. i.*ic *,* As these lessor.? have a natural sequence, ihe introduction of any one lessoa should, ai a rule, be a recapitui.itioa of the sali cut points in the preced- ing lesson. LESSON IV. POROUS. ABSORBENT. Articles forillnjtration : spoiiire. bread, piece of cane, sugar, salt, chalk. water, a couple o. plat^ and a couple of tumblers. I Meaning of porous Fcroiis bodl s absorb wafer. Select substances in- which the pores can be readily seen — such as spouge. crumbs of bread, and a piece of cane — to show the meaning of porous. Let the children handle and examine the specimens, and discover the holes for themselves. Exp. 14. Pour a little coloured water into a plate, and 10 OBJECT LESSORS. in it place the sponge and the bread. The children will see the water graduallv risinff. " Where does the water go? " If fills up the tiny holes. " I place a piece of flint and a piece of lead in the water. Do these substances suck up the water ? '" JVb. "Why not ? ' They have no little holes* Exp. 15. Place the piece of cane, together with a slate pencil, in a bottle half fiUed with spirits of turpentine. In a few minutes the turpentine will hare ascended through the pores to the top of the cane, and on the application of a lighted taper will burn with a smoky flame. The turpentine does not ascend through the slate pencil. Why not ? Exp. 16. Xext let the children examine pieces of chalk, loaf sugar, table-salt, &c. Can they see any pores ? Xo. but they are there. Show this by placing the chalk, &c., in the coloured warter. The water is absorbed, we can see it rising ; hence the pores must be there. They are too small io be seen. Erp. 17. Take a piece of loose twine, and, after immersing it in water place one end in a glass of water, and the other end in an Fig. 1. """"^ empty glass placed at a lower ele- vation (Fig. 1). After a time it will be found that all the water has been transferred to the lower glass. How has this been brought about ? The firine is porous ; the icafer ascended thraugh the pores to the top of the glass, just as it went up the cane; and then trickled doicn through the pores into the hirer glass. * Note. — All substances are more or less atsirbent. It -srill be sufficient, however, at this stage that the children s'r ould distinguish betwfea substances manifestly absorbent, and those which absorb so very little as to be practi- callv non-ahsorbeni._ FIRST STAGE. 11 II The uses to wMdi we put some porous bodies 1. From this last experiment the children can he led to see -why we make candles with wicks in the centre ; also the use of wicks in oil-lamps. 0. Most articles of olothiiij are porous. Show the use of the ■' house-tlannel." 3. Blotting-paper is a' sorb>'nf. Write upon it. The ink spreads about — runs into the pores How is blotting-paper useful r " Writing-paper" is not porous, the pores have been filled up with *.'s('. The teacher will doubtless have other substances at hand still farther to illustrate the tact that porous bodies absorb water. LESSO>; V. POROUS BODIES FILTERS. Aktictfs for illuf,v-;ii'n : a small flowei-pot, sponi,tr, cnarcoal, sand, tiom'. bloiiii;g-papt:i, and a fu;;i".l funnel. I. A sponge filter. E^p. IS. Take a small common flower-pot — clean ol course. Put a piece of sponge at tie bottom. Pour in a little dirty water. When the pores in the sporge become filled wiili the water, the latter passes slowly through, but is not cleansed. "Why?" The por-f: are too /'>rge to mrrenj fue tint/ parfMes qfmiid/roi/ij:s^:nii t', rough. Put sawdust or s;ind with the water. These substances do not pass through. "Why?" TJ.i p'-'iYf (iir too small to aJioif t!<<: pariU'tS ofsiJirdii^f or gtind to p,.>^ through. 12 OBJECT LESSO>'S. II. The charcoal filter. Exp. 19. Show that charcoal is porous by standing a piece in water on a plate, as shown in the previous lesson. Exp. 20. Phue layers of powdered charcoal and sand on the sponge in the flower- pot. Pour iu water ; this, as it slowly trickles out at the bottom, will be found at first to be coloured \\ith very fine particles of charcoal, but presently the drops will be clear and colourless. Prepare a mixture of flour and water — half a tea-spoonful of flour well stirred into a tumbler of water. The mixture when poured into the filter will have a milky appearance, but the water will trickle out clear and bright. III. Blotting-paper filter. Exp. 21. Cut circles of blotting-paper say 83 or -1 inches in diameter. Take two thicknesses and fo'd twice, as in the cut (Fig. 2). Then open out to form a cone. The filter will hare the paper two thick- „. „ nesses on one side, and Fig. 2. . ' six on the other side. Place the cone in a small funnel and pour in the flour- mixture. Clear water passes through, but the flour is left behind in the filter. The pores in the blotting-paper are too small to allow the flour-dust to pass through. IV. The earth-fllter. Have you ever seen a spring ? The water comes out of the ground clear and bright. Where did the sprins get its water ? From the clouds. The rain fell on the soil and became muddy and dirtv : but it trickled slowly throuffh the soil and the sand, and gravel, and rocks, and, as it comes out. FIRST STAGE. 13 we see it quite c\ ean again. Ho w is this ? The earth through which it has passed has acted as a huge filter. v. Uses of filters. To drink dirty water makes people ill. We can cleanse the water by tiltering it. Spring water is best to drink, because it is clean, it has been filtered. When water is supplied to us through pipes, and we have to keep it in cisterns, it is best to filter the water before we drink it, because the pipes may not be clean, and dust and dirt may be present in the cistern. LESSON VI. NON-ABSORBENTS. Articxes for illustration : any non-absorbent substances, as metal?, glass, leather, clay, putty, &a I. Some non-ahsorhent bodies. Show by experiment that many substances, such as glass, india-rubber, leather, metals, horn, ivory, vie, do not absorb water. IL Non-absorbent substances do not allow water to pass through. Refer to glass, earthenware, and china vessels, which we use for holding water and other liquids. IIL Uses to which some other non-absorbent substances ar put. India-rubber for making waterproof clothing. Leather for boots and shoes. Refer to the necessity for 14 OBJECT tESSONS. keeping the feet dry. Bottles and drinking vessels -were formerly made of leather. Paint and tar are pat on wood to prevent absorption of water, and consequent decay of the wood. Dry wood absorbs water and swells. Window frames not painted would not fit closely. They would be too large in damp weather, or too small in dry weather. Putty is used in glazing to prevent water passiag through between the wood and the glass. LESSON VIL SOLUBLE. ABTicrrs fnr illustration : soluble and insoluble snbstonces ; engar, alum, salt, soda — camphor, chalk, marble, wood. L The meaning of soluble. Exp. 22. Put a teaspoonful of salt into a medium-sized test-tube three-parts filled with water. Stir, or shake ; in a short time the salt has disLippeared, and the water is just as clear as it was at first. Where is the saltP Clearly it is in the water. We can taste it, but not see it. It is invisible. Repeat Exp. 6, p. 4. The oil is split up into such tiny drops that we cannot see them separately. In the same way the salt splits into such tiny particles that, although we can taste them, we can neither see nor feel them. Ejep. 23. We can recover the salt from the water. Boil a little brine in the evaporating dish until the water has aU been converted to steam. The salt is left behind.* • Another and a pretty experimfnt to show that water may contain solids in solution, although we cannot see them. To the solution of salt add a few drops of nitra'e of silver. A dense white curdy-looking sohd is seen flosiiing aLout. Add a litUe ammonia solution anJ ihe solid is again dissolved. FIRST STAGB. 16 n. Some substances are soluble in water, and others insoluble. Srp. 24. Show the solubility of suirar, alum, soda, salt, iltc. Use glasa vessels for clearer illustration. Then the insolubility of other bodies, such as stone, chalk, coal, and wood, may be demonstrated. Ask the children to name some of the things they know which dissolve in water, and others which do not so dissolve. Arrange the names in two columns ou the blackboard. IIL Manufacture of salt and sugar. The teacher may illustrate the use made of the solubility of substances by showing how salt is prepared from the water of brine springs and from sea water by evaporation ; and :ilso how sugar is prepared from the juice of the sugar-cane in a somewhat similar manner.* LESSON VIII. SOLVENTS— WATER, ALCOHOL, Ic. Abmcus for iUusi-.auon : small iiuautitv ot alum, saltpetre, lime, .iuuj'lior, spirits ol wiiie, benzine, and naphtha. I. Water can dissolve only a certain quantity of a solid. i:rp. 25. Show this by putting more salt or sugar in a test-tube of water than the water can dissolve. [Hasten the solution by boiling in the dame of the spirit- lamp.] n. Some substances dissolve best in hot water, others in cold water. E.!-p. 26. Make a hot saturated solution of aluiu, and set • The manufacture of salt and sug;ir may form subjec-s tor separ.nic. These will foim the 2/((/ dirision. Erp. 40. Those which the iron nail will not scratch, viz. liint, glass, steel, Otc. These will form the 3rrf division, including the hardesi substances. Lastly, show the class that we can tell the harder of two bodies by rubbing them together. The harder will cut, or scratch the softer. Exp. 41. Try iron with copper, brass with copper, iron with glass, and so on. Tell the children that of all known bodies the diamond is the hardest. It will cut or scratch every other known substance. Instance the glazier cutting glass for windows. Describe the diamond as looking like beautiful clear glass. A\^hT so tiny a bit in the glazier's tool ? If a glass-cutter can bo borrowed for the occasion, and its use shown, so much the better. II. How some metals are male harder. 1. If steel be made red hot, and then cooled quickly b\ plunging into cold water, it becomes much harder. '2. Pure gold is almost as soft as lead, and if used for coins would soon wear away ; a little copper mixed with it makes it much harder, and it does not wear away so quickly. Silver it. hardened in the same way. Copper is also hardened 20 OBJECT LESSONS. by mixing with it a little tin and zinc — two other metals. The mixture is called bronze, and pennies, halfpennies, and farthings are made of ic. LESSON XI. BRITTLE, TOUGH. FLEXIBLE. Articles for illustratioii : pin, needle, various wires, old " kid " glove, chalk, and glass. I. Meaniag of brittle, tough, and flexible. Erp. 42. Take a pin ; ask a child to break it. It bends, but does not break. Try a needle ; it bends and then breaks. In the same way try a piece of lead wire, or copper wire ; and a piece of chalk, or slate pencil. The lead wire and the copper wire bend, but do not break. The chalk and the slate pencil break easily. Exp. 43. Test by striking each article with a hammer. The same result : the j)'n and the lead and copper wires bend, but do not break ; the needle, chalk, and slate pencil break into pieces. Tell the class that when we can break articles into sharp pieces with the fingers, or hy throwing them on the floor, or by striking with a hammer, we say they are brittle. When they hend, but do not break, we say they a,Te flexibk. Exp. 44. Next compare lead wire with copper wire bv bending backwards and forwards. The lead wire breaks easily, the copper does not. We say the copper wire is tough. Exp. 45. Next try to tear a piece of thin leather — an old "kid" glove for instance — and then a piece of brown FIRST STAGE. 21 paper. The paper is easily torn, the leather is not easily torn. Both the paper and the leather are flexible ; but the leather is tough also, it is not easily torn. II. Bodies which are Drittle, &o. By similar experiments the children may be led to see that such substances as 1. flint china jlass are hard and brittle. cast iron 2. chalk \ salt > are soft and brittle. bread ) 3. eopper wrought iron brass hard wood 4. cork \ sponge > are soft and tough, india-rubber ) ■ are hard and tough. I.KSSON XII. ELASTIC. an Artict.es for ilhtstration : balls of wool, india-rubber, clay or pntty, orange, a piece of spouge, strip of glass, cork, and piece ot " elastic." I, Meaning of elastic Exp 46. Take balls of wool, india-rubber, and clny or putiy. an orange, and a piece ol' spouge. Let individual 22 OBJECT LESSONS. scholars be called in front of the class to try the effect of squeezing each. The orange, the wool and india-rubber balls, and the sponge take their own shapes again when the pressure is removed. They are said to be elastic. Clay, and putty, and butter are not elastic. WhyP -EiTjo. 47. "Pull india-rubber, woollen cloth, or flannel. What is the result ? " These substances stretch or become longer. " Let go with one hand. What follows ?" They go back again to the length they had before being stretched. Try a band of india-rubber by actual measurement. Why do we say that india-rubber, woollen cloth, flannel and such like articles are elastic ? Ea^. 48. Take a flat ruler, a cane, or a piece of whale- bone. Bend them and then let go with one hand. What is the result ? They spring back again. These bodies also are elnstic. Why do we say so ? Call the attention of the scholars to the three kinds of elasticity here illustrated, and give other examples of each kind. II. Some hoiies are more Siastic than others. Exp. 49. Test an ordinary wooden penholder, a qnill pen, a strip of glass, and a slate pencil. The quill pen can be bent almost double before it breaks, the glass bends a little then snaps, and so of the wood ; the slate pencil bends scarcely at all before it breaks. In the same way compare other substances, such as— cork with sponge, leather with flannel, a book cover with a sheet of paper, and so on. III. Uses we make of elastic substances. 1. Cor\ for stopping bottles. Show how the cork is com- pressed on passing through the narrower part of the neck of FIRST STAGE. 23 tte bottle, and how it opens out and fills the larger part of tho neck. 2. India-rubber for bands, "elastic," &c. [Cold has a curious effect on india-rubber : it makes the rubber non- elastie. Advantage is taken of this in the manufacture of "elastic.'' India-rubber threads are stretched, wound on rollers, and kept in the cold for a few days. They are ihen woven with the woollen, cotton, or silk threads into bands. The bands are passed over a hot roller and the rubber becomes elastic again.] 3. S/iiii(ic for iras/iiiig purposes. We are able to squeeze out the dirty water. The sponge expands again and is roaly to take up more wafer. LESSON XIII. PLASTIC. Articles lor illustration ; well-kueaded clav, and one or two moulds. I. Meaning of plastic. E.rjK 50. Take a lump of clay [previously well kneaded], and having sprinkled over it a little tine sand press it into a mug, or cup, or, "mould" of any form. Press well in in order that the clay may take the exact form of the inside of the vessel in which placed. "^ ' •H> i|* ^' fit/ Break the mould, or if of v' ' J: Jf' %__^>f^ shape to allow it, turn out "TuIg. CLAr. tihe •• cast." [Plaster of '^' Paris may be used in tlie place of clay.] We have made the clay into a certain shape, the shape of 2i OBJECT LESSONS. the vessel into whicli it was pressed. We call the vessel a mould, and because the clay can be formed or moulded, we say it is plastic. Plastic fneans capable of being moulded, or formed into shape. Clay can be moulded into shape by the hand. Show this. II. Things made in moulds from clay. Exp. 51. Bricks. Show how bricks are made. A common slate pencil box with the bottom removed will serve as a mould. Drain-pipes, tiles, &c., made in moulds from clay. Explain that these things are baked to make them hard. Contrast the bricks before and after baking, with regard to properties. Before baking, soft, plastic, and twn-porous ; after baking, hard, brittle, and porous. III. Things made from clay by the hand— earthenware and china. Make a rough tea-cup to show the process. Stick on a handle. Describe the manufacture of earthenware. A fine kind of clay, and burnt flints ground to powder, are well mixed. A tough paste is thus made, and from this the articles are formed. The articles are now baked in an oven. Xext they are dipped in a mixture and baked again. The second baking produces the glaze, and renders the articles non- porous. Colours are next put on, and the articles again baked. Ornaments, vases, figures, &c., are made of potters' clay mixed with fine sand, and then baked. These are called ferra cotta, which means baked earth. When warmed, gutta-percha is plastic ; ornaments, solea !'or boots, &c., are made from it. FIRST STAGS. 26 LESSON XIV. FUSIBLE. Articles for ilhistration : lead, tin, cast articles, salt, and sugar I. Meaiiiag: of fusion. Krp. 52. Melt lead in an iron spoon. Pour it ont. " It flows in drops. In what state is it ? " In a 'liquid state. " lu what state was it before melting ? " In tlie solid state. •' Then what change have we brought about by heating P '' We have changed the kadfrom the so/id to the liquid state. Things which can be changed from the solid to the liquid state by heating are said to he fusible. Ice is fusible. It does not change to water on a very cold day. except we bring it into a warm room, or hold it in the warm hiind. It does not require much heat to melt or fu^e it. II. Common substances wMch axe fasible.* 1. The mefais. Some require very great heat. Melt a little tin ; the iron spoon does not melt. ^Vhy ? Show articles made of cast or fused iron — nails, hinges, grates, i£c. Show anv articles cast from bronze or bell-metal. 2. Salt is fusible at a great heat. Makes a glaze for so;jje kinds of draiU-pipea. 3. S.'.ijai: Melt a little in the evaporating dish, and compare the fused sugar with the original. 4. A mixture of tf:ii:s and sjda is fusible; and when fuied it makes a beautiful triuisparent glass. * Of course, in sn-iotuess the term i- used eoinpaiatively. Some substaDces are nier.- fusibV than o;liei-s ; and it is only to the former, those which are evident-lv fns:We, that the tejm i- here applied. With s.ilfieient. heat all solids are fusihie. But tliose which are combustiMe '• tuke lire " before reaching the point of fusion, unless totally excluded from the access of oxvsren. 26 OBJECT LESSONS. LESSON XV. ON SOME FURTHER PROPERTIES OF THE COMMON METALS. Articles for illustration: any specimens of malleable and ductile metals, various wires, a thin rod of glass. I. malleable. Exp. 53. Take a piece of lead, place it on a block, and hammer it. What is the result ? It is flattened or spread out. A bit of copper wire may be treated in the same way. Refer to the blacksmith heating iron to a white heat, and then hammering it into various shapes. Most of the metals can be hammered out without break- ing; but gold and silver can be hammered out into thin leaves finer than the finest tissue paper. Show gold and silver "loaf" and tin "foil," and plates of any other metals. Substances which spread out without breaking when hammered might be said to be hammerahle. And that is just whit "malleable" means. It is formed from the Latin word [malleus^ for a hammer. But there is a kind of ham- mer made of wood, used by carpenters, called a mallet (little hamm:r), and gold and silver are beaten out with wooden hammers or mallets ; hence we say malleabk, and not ham- merable, but the two words have the same meanins. The teacher should now show the manv uses to which these metals are put because of this property of iKalleability. II. Ductile. Exp. 54. Take a thin glass rod. Hold it in a lamp or gas-flame until it is soltened, then gently draw it out into a thin thread. Tell the children then the word ductile is used for ««» be drawn out. Solid glass is not ductile. Glass FIRST STAGE. 27 softened by heat is very ductile. Most of tlie metals are ductile, some much more so thau others. GoKi and silver ici/r oau be made as fine as the finest thread. Show steel, copper, lead, and zinc wires. m. Tenacious. Erji. 55. Direct some of .he children to test the strength of some of the specimen wires hy trying to break them. Ghiss ■wire snaps very easily. It can scarcely hold together. Lead and zinc wire break more easily than copper, and so on. ^Te say that lead holds together more firmly, or is more fenaeion^. than glass; and cojiper is more tenadoiis than lead. The tiny particles in some metals hold together more firmly than in others. TVhy cannot we make so fine a wire or so thin a leaf of lead as we oau of gold or copper ? The tinv particles of lead do not hold together so firmly as those of gold or silver. Lead is not so tenacious. Direct the attention of the children to some of the more common uses of wire.* XcTE. — Lessons on any of the more common solids may now be introduced. Li a great measure they should serve as the medium for a recapntulation of the ideas developed in the previous lessons. The following lessons on lead and sulphur are given as examples. LESSON XVI. LEAD. Articies for ill ]?tration : lead in a? many forms as may convenientlv be procured, a.* pipe, sheet, foil, and wire ; ;dfo g-.ileiia. L Its properties. (a) "I want Hirrv to come to the ti^^le, and take this • The couimon uso.s of meia.s may form the subject for *,'thex lessous. 28 OBJECT LESSONS. piece of pipe in his hand. Look at it. I think vou can tell me of what it is made ? " It is made of lead. " What can you say ahout its weight ? " It is eery heavy. (b) " Take this nail, and try to scratch it 2s ow take this knife and try to cut it. What else does this teach you about lead ? " It is a soft metal. (c) " John shall take this piece and hammer it on the block of wood. Wbat happens ? " It spreads out, or flattens. " What do we say of lead because we can hammer it out ? " It is malleable. " Here is a piece of lead ' foiL' What does that teach us?" That lead is malleable. (d) '• Here is a piece of lead wire. What can we learn from this ? " That lead is ductile. (e) "Bend the wire. It bends easily. What do we learn from this ? " That lead is very pliable. " It does not go back again to its former shape. What does this teach us ? " That lead is not elastic. (/) "Bend it backwards and forwards two or three times. What happens ? " It breaks. " What does this teach us ? " That iead wire is not very strong. It is not tenacious. (g) " I melt this piece of lead in the iron spoon. What does this teach ? " T/mt lead is easily melted. " What difference do you see between the freshly melted lead and the piece of lead before it was melted ? " The fresh piece is much brighter. It shines more. (h) " Lead pipes and sheet-lead plates on roofs of houses last for very many years. What does this teach us?" That it does not icear away quickly. " Yes, and when a thing does not wear away quickly we say it is durable." " We have learnt a good many things about lead. Tell me what they are once again." Lead is heaiy,soft, malleable, ductile, pliable, fusible, and durable. FIRST STAGS. 29 II. Its uses. TTe have now to fine! out how all these properties of lc;ui make the inot;il useful for v;irious purposes. '■ Here is our piece of lead pipe. Xow why is lead specially useful for making pipes ? " [The teacher should here trace the course of one of the gas-pipes in the room, and show how it has to be bent and turned. Cast-iron pipes would do for straight tubes, but they could not be bent. Wrought iron would be much dearer, and would not bend so easily as lead. Silver or copper would do for gas-pipes, but they are too dear. Silver would be better than lead for water-pipes, but would cost too much. Copper would not do, because it would rust, and the rust of copper is a poison.] Lead is very useful for making pipes, because — 1. It is eiisily bent. '2. It is soft enough to be cut with a knife or saw. 3. It does not rust. i. It does not allow gas or water to escape. 5. It is cheap and durable. For similar reasons it is useful as sheet-lead for covering the roofs of houses or floors, for lining wooden cisterns for holding water, and so on. m. Alloys of lead. [Mixtures of metals are called alloys."] Lead and zinc melted or fused together make a verv tiudik alloy. Advantaire is taken of this to make wire, which, besides being cheaper, is softer and more easily bent aud twisted sbout than copper or iron wire. Useful in the garden for rvinff up trees and shrubs. Why better than twine ? "When a little of another and very hard metal c:died nntiiic 30 OBJECT LESSONS. is fiised with lead, the alloy is harder than pure lead. Thia alloy is used for making shot. [Test the hardness of common shot by hammering.] When another metal, very much like arsenic, called antimony, is fused with lead, it produces an alloy — ti/pe- metal — used for making type for printing. [If possible, show specimens.] Solder is a mixture of lead and tin. [If possible, show its use.] Pewter is an alloy of tin and lead. IV. Whence obtained. Tell the children that we get lead from mines — not pure lead, but lead mixed up with other substances. [Show any ores of lead. The more common one, Tiz. galena, from which lead is smelted, is very plentiful.] The lead is melted out, and run into moulds. LCSSON XVII. SULPHUR. Articles for illustration : roll sulphur, " flowers " of sulphur, an olive- oil fls.sk, and the spirit-lamp. I. Its properties. The children will discover the more obvious properties of sulphur under the guidance of the teacher. It is of a pale yellow colour, hard, brittle, injlammable, insoluble in water, and heavier than water. Exp. 56. To show that sulphur is fusible. Put powdered roll sulphur, or flowers of sulphur, in an ordinary olive-oil flask ; heat gently over the spirit-lamp. The sulphur easily FIRST STAGE. 31 cliaiiges into the liquid state, when it has the colour of amber. As the temperature rises it becomes darker in colour, and takes about the consistence of treacle. Pour into cold Mater, and the once hard and brittle yellow sulphur ia now soft and tenacious, and much like india-rubber. Let the children compare the properties of this changed substiuice with those of roll sulphur. £.rp. 57. Although sulphur is insoluble in water, it is soluble in alcohol, or spirits of wine, and some other liquids. The teacher will dissolve a little in alcohol, or in bisulphide of carbon. It readily takes iire, and from this property follow its chief uses. II. Its uses. 1. Lucifer matches. The children will be interested in learning how their grandfathers and grandmothers managed to get a light before " matches " were invented. Describe the " flint and steel" and the tinder-box. Show how sparks were ob- tained by striking a piece cf steel. Use the back of a knife on the sharp edge of a broken flint. ^Next, show how the light was obtained by using strips of paper, or splinters of wood, the ends of which had been dipped in melted sulphur. Xext followed the improved sulphur matches, and in mmy places they are still used. The splinters of wood were dipped in melted sulphur as before ; but in addition just the ends were dipped in a mixture which, when it became dry, ignited on being rubbed on a rou^jh surface. This did awav with the flint and steel, and the tinder-bux. Xow the best matches are made without sulphur. "Why ? A^'e are triad to be rid of the sulphur because of its suf- fcKMting smell. 2. C+unpowder. This is an intimate mixture of atout 15 parts, by weight, 32 OBJECI- LESSORS. of nitre, 2 parts sulphur, and 3 of charcoal. These are well ground, then well mixed, and made into a paste with water. The paste is pressed into hard cakes, these are broken into grains, and then dried. Exp. 0$. The teacher may make a little of the paste, not too soft. It burns with a hissing noise, and throws off showers of sparks. 3. Sulphur is also used in bleaching straw and wool, &c. Exp. 59. Show this property of bleaching by holding a flower in the fumes of burning sulphur lor a few seconds, most of the colour disappears. 4. Occasionally used as a medicine. The children may have heard of " brimstone and treacle." SECO.TP STAGE. SECOND STAGE. LESSON I. WATER.— ITS PROPERTIES. Articies for illustration: a "pop-gun" or substitute, a piece of glass lulling, a piece of lead or wooden pipe, and a bottle aud cork. I. Water is a liquid. [See First Stige, Lesson 1., page 3.J TTe may say water is a liquid because — - 1. // luay be made to Jfoir in i/ro/^s. Show this by lettinw .vater drop Irom a sponge, or from a bottle. 2. It Ci'iiiiot be graspi'd by the hand. Why ? Its p.irticla do Dot hold together tirmly enouo-h. 3. It cannot be made t-o form a hfap. Try it on a slate or plate. It spreads out. and seeks to find the lowest place. 4. It has no shape of its own. It takes the shape of the \ essel in which it is placed. II. Water is clear, colourless, transparent, tasteless, and odourless. All these simple properties may be re.idily elicited from the children by directing them to use their senses of sight, taste, and smell. in. Water cannot be squeezed into a smaller space. E.rp. 60. To show this the teacher will require a tube and piston of some kind. A child's " pop-gun " will answer 36 OBJECT LESSONS. I'Hi verv well ; or take a straight quill, and a slice of raw potato, with, a little stick for a piston. Plug one end of the quill by pushing through the potato. 2s^early fill the quill with water, and then plug the other end by pressing the potato on the quill till the latter cuts through. The plug may be forced out or broken, but the column of water cannot be made shorter. A piece of strong glass tubing,* with pellets of "tow"T instead of potato, and a piston rod made of hard wood shaped as in the cut i Fig. -l) will form a better instrument, and will be useful in future lessons. Exp. 61. Fill a bottle with water and try to force the cork in. Some of the water is squeezed out by the side of the cork as the corkis pressed in. Fis. 4. I IV. Water presses sideways as well as downwards. Children will appreciate the fact of the pressure down wards by trying to lift a bucket full of water. Ejcp. 6'2. To show the pressure sideways take a piece of tube — any kind will answer ; it may be cardboard, wood, or lead, provided we ciin readily pierce the walls. Plug one end of the tub? firmly ; make tiny holes in positions as shown in Fig. o, and plug with wooden spikes. FiU with water, and remove the spikes. Direct the children to note carefully what follows. Three tiny streams spout out. Are they alike ? How do they differ ? The top stream does not run out with so much force. The bottom stream seems to be in a greater hurry harder and so rushes out farther. • Conntn- boys mate their pop-guns by foreitii; out the pith from a strsight stick of eldi^r-woi d about an inch iu diameier. t Uniavel a piece of o.d twine. Fig. 5. it is pushed SECOND STAGE. 37 Note also that as the water in the tube lessens in volume the toree of the streams lessens. " What do we learn from this experiment P " Two things. (1) That water presses aidcinii/s us well as doirmcardi,. (2) T/iaf the deeper the wafer the greater the pressure. XorE. — Pressure in aU directions is dealt with in a future lesson. LESSON II. WATER— A SOLVENT. Arttctfs for illustration ; subftaiu-es soluble iu water, acetite of lead, liquid ;uumonia, tumbler, jug, aud water. I, Action of water on salt. sug:ar, alum, soda, &o. Exp. 613. Show the solvent power of water by dij^solving a lutle salt, sugar, alum, aud soda in separate glasses or test- tubes. * How has the water changed the solid? Can we see the salt or alum in the solution ? The water has broken up the salt, lic, into such tiny particles that we cannot see them. It has made the solids inri^ible. How do we know that the salt or sugar is everywhere in the water ': Taste the smallest drop. It is salt or sweet as we dissolve the one solid or the other. We should remember then that even the brightest and clearest liquid luav contain some solid in solution although we cannot see it. Sometimes, indeed, we cannot discover the solid either by sight, taste, or smelL JE.rp. 64. ■■ Here is a liquid which contains a solid." ["Dissolve lead in white viuegar or acetic acid. J 1 ou can- not see it ; but it is there as I shall show you. Here is another liquid [liquid ammonia], but this contains 38 OBJECT LESSORS. no solid. I pour a little of this into the first, and what do you see ? A white solid shows itself, which slowly sinks to the bottom ; the solid is painters' " white lead." Exp. 65. " Here is another clear liquid [lime-water], of which vou may drink. It is quite harmless ; but it contains a solid in solution, which you cannot diseoyer by sight, taste, smell. A boy shall blow through it by means of this glass tube. It gets milky-looking. By-and-by a white dust will fall to the bottom. This is chalk." We can recover solids by evaporation. Show this by means of the evaporating dish. Bodies which can be broken tip or dissolved by liquids are said to be soluble, and the liquid which dissolves the solid is called the so/rent. TTater is therefore a solceid for salt, soda, &c. II. Action of soluble bodies on liquids which dissolve them. Exp. 66. Fill a tumbler full of water to the brim. Care- fidly pour it into an empty jug and add a couple of ounces of salt. A tumbler of water weighs about half a pound, so that the weight of the salt solution will be about ten ounces. Xow pour the solution into the tumbler. It is again exactly full and no more. Haifa teaspoonful of water would have caused an overflow, but two ounces of salt have made no change in the size [volume] of the water. TThat can we learn from this experiment ? 27iaf when we dissolve salt in icater, the salt does not increase tlu bulk of the water. This is true also of other solids which dissolve in water. They do not increase the volume of the water. TTe may conclude from this that water is porous, although the pores are too small to be seen; and that the tiny particles uf the solid fill up the pores. If we add another ounce of salt to our solution we shall find SECOND STAGE. 39 thnt tVie whole is not dissolved. A boy can eat only a certain quantity of bread at a time, and water can dissolve only a certain quantity of salt nt a time, and no more. When the pores are full no more salt can be taken up. Time perinittiii;::. the teacher may here show the different solvent power of water on different solids, and how heat affects this solvent power. III. Uses of water dependent on its solvent power. Pure water is obtained by boiling, and then changing the steam back to water [distillation]. Rain water and snow water are nearly pure; but river and spring water always hold solids in solution. Where do these waters get their solids from ? When rain falls what becomes of it ? 1. Part runs away in streams to the river and thence to the se 1. 2. Part " dries up " — erapomfcs into the air. 3. Part soaks into the ground. That which flows into rivers dissolves certain substances as it moves along, rubbing against the soil and sand and stones. Tliat which sinks into the ground dissolves a good deal more of rocks. i*ie., than river water. This water rushes out in spriuijs. The best drinking water conies from springs. Spring water contains solids. Instance the "'fur" on the kettle. Let the children taste dislilled water. They will find it not agreeable ; in fact, it will remind them of rain water, and thev will learn that the best drinking water holds suh- stances in solution. This occasion may be taken to show what are the impurities that make water bad for drinking, and what is the difference between mineral water and dirty water. Refer to the fact that plants are dependent for much of their food on the sol\ent power of water. 40 OBJECT LESSONS. LESSON III. WATER AS VAPOUR. DEW. Articles for illustration : freshly cut leaves, flower in water, lump oi ice. I. Vapour, evaporation. Suppose we hang out a wet cloth to dry. The cloth dries ; but where does the water go to ? You sprinkle a little watei on the floor; it soon dnes tip. The roads may be watered; but they are soon dry again. What do we mean by dries up ? Where does the water go ? Yoa say it is gone away ? I will tell you how it went away, and where it is gone. You will remember how the water split up the salt into such tiny particles that we could not see them, and how even the tiniest drop of water got its share of the salt. Well, verv much in the same way the air splits up the water into very fine particles, too small to be seen, and then the water mixes with the air, as the salt mixed with the water. "I want you to remember that the water which is in the air, but which we cannot see, is called vapour ; and that the change of water to vapour is called evaporation." II. Other sources ot vapour. We see that the ground, and houses, and trees, and plants are all wet after a shower ; and we see them dry very soon after, and we know that much of the water has become vapour ; but there is water also going into the air in the form of vapour from leaves and from the bodies of animals. Exp. 67. Place a few freshly cut leaves under a dry tumbler. The inside soon becomes covered with moisture. Why ? Iq the same way, if the naked arm be inserted iu a jar, the SECOND STAGE. 41 jar aftor a time will become covered with moisture, showing that, like the leaves, the skin gives off water. Water also is always coming from the lungs. Breathe on cold slate or glass. What is the result ? The children will now be prepared to answer such questions as the following : — " Why do flowers soon droop and wither after they are cut ? " " AVhy do we put flowers in water when we want to keep them fresh and bright ? " " Why does the water in the vessel decrease in quantity ? " " Why do we desire more to drink on a hot than on a cold day?" ■ " What do we mean when we saj that ' ink dries ? ' " III. How to collect vaponr from the atmosphere. Exp. Ci8. The teacher n-.ay show how to get vapour from the air, by bringing a glass of iced water into a warm room. The moisture soon covers the outside of the gliss.* In a similar way vapour froHi the air is settled at night on the cold grass and leaves. This is called deicf We often see moisture on the windows of a warm room, or on those of a closed carriage. Where does it cume from ? AVhat causes it to settle on the window ? LESSON IV. WATER AS FOG. MIST, CLOUD, RAIN, AND STEAM. Articles for illusimtion : apparatus for boiling water. I. What is fog? "What did we learn about vapour in our last lesson P" It is in thf air but tee cannot see it. • Thi> moisture is bettv is given in the Fourth Suige, Lo^son XIII., ptge 145. 42 OBJECT LESSON^S. " What do we say of things which cannot be seen ? " They are invmble. " Can you tell me why vapour is invisible ? " Yes, the little particles when floating about in the air are too small to be seen. " In this lesson I am going to show you hoiv this vapour changes back to water, and in what forms we see it in the air. " But first of all I must tell you that when vapour changes back again to water we say the vapour condenses. To condense means to press into a smaller space, and so to thicken. Every one has heard of * condensed milk.' The substance of the milk — the nourishing part — is pressed into a smaller space, and therefore thickened. Then it is said to be condensed. Just so when vapour falls back into a liquid state its particles are pressed together into smaller space. It is ' condensed.' " You know that vapour condenses, because you have seen the water on the cold tumbler and on the window panes ; but what causes the change ? " Breathe on this hot slate." No moisture. "Breathe on the cold slate." "We see the water. " B i-eathe on this cold glass." We see the water. " Where did the water come from that is on the slate ? " From the lungs. " Have you ever seen what looks something like smoke coming out of the mouth on a cold day ? " Zes. " That is the moisture of the breath condensed by the cold air." " It is the cold, then, which condenses vapour." And I must tell you now that cold air makes the tiny particles of vapour join together — in companies, as we may say — to make tiny drops of water large enough for us to be able to see a, mass of them together, and not large enough to be seen singly. The cold air changes the vapour into what we may call water-dust. This water-dust is fog. SECOND STAGE. 43 On a frig£ry morning: you may see a little white substance settled on the loose fibres of wool on your coat. Under a miignifying- glass we see that this is formed of rows of water- bejiJs, so tiny that it would take tifty of them to make a drop the size of a piu's head. Then on spiders' webs you may see water-beads a little larger. II. What are clouds? The teacher may lead the cb.ildren to answer this question for themselves by some such simple narrative as the fol- lowing : — " The morning was misty, but there was every prospect of a tine day, so we — that is, my brother and I — made up our minds to climb to the top of one of the high mountains we had seen a few miles off as we entered the village on the previous evening. Immediately after breakfast we trudged oif, and soon arrived at the foot of the hill. So far we had walked in a thick nii-t ; but, after climbing for about an hour, we walked, almost suddenly, out of the mist into the briirht sunshine, and a glorious view burst upon us. Above were cloud-capped mountain-peaks : around, on every side, the lesser hills were bathed in a Hood of sunlight ; below, a iiroat white sea of fog hid every house and tree. The moun- tain was steep, and we were glad now and then to take a rest, and to watch the fog as it slowly melted away. In another hour it had entirely disappeared, and we saw below us lesser hills and valleys, lakes, and stie-ims, with villages dotted here ;uid there, stretching for miles away. " When we had reached within half a mile or so of the top of the peak we were climbing we entered another fog, very much like the one we had left in the valley in the morning, oulv this was colder and wetter. It was not pleasant, tor we could see but a yard or two before us. However, we stTuiTLrled on to the very top and rested awhile, hoping the fo^: would disperse. Eut we waited in vain, and were obliged 44 OBJECT LESSONS. to descend without enjoying the splendid view we had promised ourselves from the top. We soon got hack into the hright sunshine of the valley, but on looking behind us, there, on the top of the peak, resting Hke a nightcap, was the cloud through which we had passed." III. Mist and rain. " ^Vhere does the rain come from ? " It comes from the clouds. " How does the rain fall ? " It folk in drop'<. " Are the drops always of the same size ? " Xo ; they are sometimes small, at other times large. " Do clouds always send down water ? " JVo. " How often can we see clouds ? " Almost alicatjs. " But it only rains now and then. How is this ? I will tell vou. When the clouds do not rain, the particles of water are all in small companies ; but when the clouds get colder, then the particles gather into larger companies, so as to form drops. These drops fall, and we say it rains. When the drops are small we say it is a misty rain, or a ' Scotch mist.' " IV. Steam. Fxp. 69. Show, by boiling water in a kettle or " Florence flask " * that the steam, as it issues from the spout or neck, is invisible. It is commonly said that we see the steam as it issues from the steam-engine or the kettle, but this is not strictly true. As the steam spreads out in the air it gets a little cooled, and a steam-fog is formed. The steam-fog is what we see. Steam-fog is just like common fog, only it is hot. Steam-fogs soon change to vapour, and then are, of course, invisible. Common fogs, too, evaporate ; but more slowly than the steam-fog. • Fl.isks m;ide of thin glass in which olive-oil is imported. Being thin, tliey are less liahle to cracu SECOND STAGE. 45 [Tlie fuller explanation of steam and its uses, and of the formation of rain, is given in the Fourth Stage, Lessons VIII. and XI'Y., pages 132 and 1-17.] LESSON V. WATER AS SUOy^J AND ICE. Articles for illustration : w:uer and a lump of ice. Snow if possiWe. I. Snow, ice. and water compared. Under the guidance of the teacher the children may first compare snow, ice, and water as to their chief properties. For example — 1. TTater cannot he grasped by the hand ; snow may be pressed into hard balls. 2. "Water and ice are alike clear and transparent. 3. Ice is lighter than water, and therefore rests near the top of the water. II. Meajiing of " frozen," " TThen ice is taken into a warm room, what change takes place in it ? " If tiii'/fs. " And what is it when melted ? " Water. " What do we call the change of ice to water ?" Melting. " If water is piut in a very, very cold place, what change takes place ? " The trater cfiaiiacs to ice. " And you know, I think, what we say when water is changed to ice ? " We sa;/ it is/rozeii. "Yes, and when we say that any substance which we usually see as a liquid is changed by cold to a solid we ?;iT it is frozen. Thus milk or quicksilver may be frozen. But when melted lead becomes solid again we do not say it is frozen. It is soNdified. But frozen and solidified mean the same thing — a chnige r'l om the liquid to the solid'' state 46 OBJECT LESS0X3. III. What are snow, hail, and ieel " Ice, you know, is solid water, but what are hail and snow ? " " Rain falls in drops — large and small. Sometimes rain- drops have to pass through very cold air in coming down. What happens ? " The drops freeze. The water becomes solid, and these little solid balls of ice we call hail. " But suppose the vapour as it condenses into fog and mist to become frozen and to fall, what then ? " We hare a snowstorm. " There is much more to learn about snow, and hail, and ice ; but all I want you to remember now is that ice is solid water, that hail is solid rain, and that snow is solid fog and mist." IV. Uses of snow and ice. The teacher may lastly call the attention of the children to the uses of snow and ice. Although snow is so cold it is like a blanket in this, that it does not let warmth pass through it easily. Sheep are often buried in the snow, and are found to be much warmer than they would have been in the frosty air. Snow keeps the earth warm, and partly protects the plants from the frost. Ice covers the water. It shuts off the cold air and so keeps the water beneath waimer for the bsnttit of the animals and plants which live in it. LESSON VI MERCURY, OR QUICKSILVER 4.RTICI,ES for illustration : mercury and tin-foil, and, if possible, scales for weighing, vermilion, and cumabar — an ore of mercury. L Its properties. The more evident properties — such as its great weight, its SECOND STAGE. 47 state as a liquid, its easy dirisihilify into small drops, and ifi beautiful sihery lustre — miiy be elicited from the children. Hrp. 70. If the teacher has scales at hand he will com- pare the weight of mercury -with the weight of water. A small cup, or small bottle of water weighs, say, half an ounce ; the same volume of mercury will be found to weigh nearly fourteen half-ounces ; that is, mercurj' is nearly four- teen times as heavy as water. This metal is a liquid at ordinary temperatures, but in the very cold regions of the world it freezes in svinter and becomes a solid metal like a bar of tin or lead. In this state it is malleable like most other metals. This the teacher can only tell the children, but he may s/iow them that, like other liqu Ms, it can be made to "boil" and change to invisible vapour. £!i~p. 71. If the experiment be conducted in a test-tube, the vapour will condense ai^ain in tiny silvery drops on the cool glass near the open end of the tube. II. Whence obtained. Alwavs obtained from mines. Sometimes found as pure liquid mercury in little hollows in rocks : more often as an ore. This ore consists of sulphur and mercury. The ore is roasted, the sulphur burns away, and the mercury bet omes Tapour. This vapour condenses in cool earthenware pipes as liquid mercury. II r. Uses. 1. For "silvering" looking-glasses. A piece of tin-foil, of the same size as the gln=s to be " silvered," is spread on a perfectly flat and smooth stone. Mereurv is pioured on the tin-foil and made to cover it. The ^lass plate is then caused to slide gently over, not quite 48 OBJECT [.ESSO^•S. toucHng the tin-foil. The glass thus sweeps off a large proportion of the mercury and all the air, leaving but a thin film of the liquid metal. I^ext the glass is heavily weighted. In a short time the mercury and the tin-foil form a solid amalgam, which adheres to the plate. Exp. 72. The teacher can illustrate the formation of amalgams by working up a little tin-foil, such as is used for wrapping round tob.icco, with mercury until the mixture has the consistency of putty. 2. For the extraction of silver and gold from their crushed ores. The mercury forms an amalgam with these metals. To obtain the precious metals the mercury is driven off by heat. It is, however, collected to be used again for a similar purpose. 3. Mercury is also used for the preparation of a very bright red-coloured powder called rermilion. Vermilion is used as a paint, and for culouring sealing- icax. Other uses will appear in future lessuus. LESSON VII. AIR— A SUBSTANCE, INVISIBLE, OCCUPIES SPACE, HAS WEIGHT. Articles for illustration : a tumbler and basin of water, test-tubes, and a little aniiuunia and hydrochloiic acid. I. Air— a substance, invisible, occupies space. Hitherto we have dealt with things which we can see ; now we propose to find out somethiag about some bodies which we cannot see. SECOXD STAGE. 49 "T have here a tumbler. Is it full or empty ?" " Empty YOU say. I tliiuk not: I shall show you that it is full of soniethiui:." Hj-j). 7o. See ! I turn it bottom upwards, and press it down into this basin of water. " Does the water till the glass ? A boy shall come to the table and press it further down. Can he make the water quite till the i;lass." Xo. " Then tliere must be something in the glass. I lean it on one side, and out something comes in a great bubble. What was it." Air. ••Then what was there in the glass ?" Aii: " yS bat do you say about the air because you cannot see It ? " J}~e say it in inrc^il'le. It is difficult for little children to appreciate the fact that bodies do exist although invisible, and the teacher .-bould therefore multiply instances. He niav refer to salt and sugar in solution, and to vapour and steam. -F.r/i. 74. A pretty experiment may be shown as a further illustration. Hold an inverted test-tube for a few moments over the mouth of the bottle containing ammonid-u-iifcr, and another over a bottle containing hydrochloric acid. The tubes become filled with the gases that rise from the bottles, but nothing can be seen. Place the mouth of the first over the mouth of the second, and then invert. A white cloud appears in the tubes, which gradually falls as a white flak\ solid to the bottom of the lower test-tube. Or. fill a bladder with air. Prick it. The children may fed ;md hear the air rushing out, although they cannot see it. II. Air has weight That air has weight is readily shown by weighing a flask from which the air has been taken, and then weighing it sg.iin when the air has been iillowed to enter ; but as few teachers will be able to command the necessai-y apj^aiatus, it L 50 OBJECT LESSONS. must suffice here to show by a diagram on the blackboard how the weight of air is ascertained (,Fig. 6). Fig. 6. A box measuring a foot in each direction will hold about an ounce of air. LESSON VIII. AIR PRESSES IN ALL DIRECTIONS. Articles for illustration : tumbler, water in large basin, piece of thin card, a bov's sucker. I. Prtssure downwards. Exp. 75. Fill a tumbler with water, invert it and, raise nearly out of the water (Fig. 7). The water does not fall out of the tumbler. Why not ? There must be some pressure on the free surface of the water. And this can only be the air, for nothing else rests on the water. Exp. 76. Repeat the same experiment, using a wide tube, securely corked, or covered with a piece of bladder at one end. Bemove the cork or prick the bladder ; the water SECOXD STAGE. ■M falls. "U'li^y !' If the tube is not too wide, the tkiimb placed over cue end will ;ins\ver equally well. The air presses doiniinirds ou the free surface of the 7. Fi> ?. water with sufficient force to hold the water up in tl.o tumbler or tube. II. Pressure upwards. -E.rj>. 77. Fill a tumbler, wineglass, or wide test-tube to the brim with^ water. Press over irs open end a stitf piece of piper or a card. Hold the card in its plac£ and invert the glass : the water will not run out ^Fig. > Why r It is beoi'use the air presses /^^'/fifrrfs on the paper, and keeps the water in. III. Pressure sideways and in all directions. E.rp. 7^. Cut a circle of about four inches in diameter from a piece of moderately thick leather. Soak till it i^ soft aini flexible. Tie a knot at the end of a piece of string, and pass the other end throutrh a sniall hole out in the centre of t:ie leather. Dip the leather in water, and then press it down on TO a piece of slate or smooth stone. We can lift the slate or stone by means of the s'tA-rr. and the sucker adheres equally well, no matter in what position the sL.te or stone is placed. Xow. why do we press down the sucker, and how is it that 52 OBJECT LESSOXS. the leather holds en so firmly ? Leather is not adhesive, neither will water stick the leather to a stone. AVe press the leather to squeeze out all the air. We then raise the leather a little by pulling the string, and the pressure of the air on the leather on one side, and the slate or stune on the other, hold the two firmly to- gether. It is just as though I held the leather with my left hand and the stone with the right, and pressed the two together. You will learn more in a future lesson about the pressure of air. What I want you to re- member now is that air presses in all directions, doicmcards, upicards, and ddeicays. The teacher may refer to the manner in which limpets fasten themselves on the rocks when the tide recedes ; and to the suckers on the feet ui flies enabUng them to walk on the ceiling body downwards. Fig. 9. LESSON IX. AIR IS ELASTIC. Articles for ilhiftration : sponge, pop-gun, or cylinder and air-tigLt piston, bladder, or any air-tight bag. The teacher should introduce the subject of this lesson by referring to the elasticity of solids. Sponge may be taken as an example, because the elasticity of air is somewhat similar to the elasticity of sponge. Like sponge, air can be pressed into a smaller space. It is t'onqyressible. And, like >poii ge, when the pressure is removed it opens out again. That is, air is elastic. jS.i-jk 79. The first property is easily shown by meano of the pop-gan. SECOXU STAGE. 53 How does the pop-gun work ? When the cork is placed in the end of the gun, the barrel is full of air. If the cork were not in, the air would all be pushed out by the rod. But the cork keeps the air in. As the rod is pres^^ed in the air is pvossod closer together, and occupies a smaller space. When the rod is pressed half way. the air occupies half the space it occupied before. Xow when air is squeezed in this way, it tries to open a way out for itself, and as the rod is [n'ossod in still farther, the air forces out the cork all at once, and so makes the popping sound. It is not the rod alone which forces out the cork, for it is not long enough to touch the cork. It is the air between the rod and the cork trying to expand to its former size which makes the latter tly out. In other words, it is due to the elasticity of the air. The teacher may explain the action of the pop guu made out of a goose quill, as described in Lossou I., p. oti. The ditierent action of water and air in the pop-gun, should also be shown. V E.rp. SO. A more perfect apparatus for showing the elasticity of the air is an air-tight brass tube with a closely fitting piston, as shown in Fig. 10. When the piston is pressed down considerable resistance is felt, and if it is pressed down quickly and then released, it springs back again. The teacher may further illustr.ite the elasticity of air by means of any suitable articles within reach, such as a bladder, or an air-tiirht india- j.^^, jq. rubber cushion filled with air. The bladder or b.i£r may be pressed at will; but the air within always forces it into its original shape when the pressure is with- drawn. 54 OBTECT LESSOXS. LESSON X. GAS. Articles for inustratioii : sulplnmc acid, a little Etilphide of iron, botlles, and water. I. What is a gast The teacter should call on the children to reproduce the ideas already acquired about solids and liquids. A solid is a substance which retains its form and size, unless acted on with more or less force. A liquid is a substance which keeps its own size, but takes up the shape of the vessel in which placed, and spreads itself out so as always to have a level surface. We have now to consider some other substances which are neither solids nor liquids. TTe call them gases. Most of you know one gas — that which we burn to give us light. Air is one of these gases. What have we already learnt about air ? It has iceight. but is very light. It takes up room for itself, viz. occupies space. It is comfre^sibli and elastic. All gases have iceight, [but as we shall learn by-and-by, some are heavier than others,] and they all occupy space. Like air, too, they are all very compressible and very elastic. There is one more fact to learn about gases, includin? air. They are always trying to spread out more and more, so that very little of a gas wiU fill a large space. We can half-fill a bottle with a liquid, but we cannot half-fill a bottle with a gas. The gas will spread out and fiU the bottle. A rough idea of the constant tendency of gases to expand may be shown by allowing a little coal gas to escape. It soon fills the room, as may be detected by its unpleasant smelL SECOND STAGE. 55 ^.rp. 81. A better method i^ to fill a bottle* with some strong-smelling gas. such as sulphuretted hydrogen,! and allow it to escape into the room. It will soon be discovered in every part of the room. We c^n now answer the question, '• What is a gns ?" (ti?.-; ("s a subsfancr whicJi (unless confined) retains neither form nor sisc, ami which //as no iiiirfdf'f. II. How liquids and gases are alike. Show how water m.iy be made to fioir in a stream. E.rp. S'2. Then make a little carbonic acid gas by pour- ing very dilute sulphuric, or hydrochloric acid, on a few pieces of chalk, and show how this can be made to tfoir into another tumbler. We cannot see the gas flow out of one vessel into the other, but we can see its eifect when poured * As in fatur? lessons wc shall often have to fill bottles -with g:ises. it will be well to show the method hei-e. 1 1^ X^Hien the gr-.is is not soliiUe in n ater, take any vessel, snob :!S a wooden bucket, and tix a ^helf across it two or ihree inches from the lop. Cut a hole in the shelf. !N\.aly fill the vcsstl with wa' or. This forms a "pneumaiic trough. " Fill the bottle with -water !/ Fiff. 11. FiiT. 12. in the trough, and place it iiiotith downward on the shelf orer the hole. The iMs fixini the gerer.iting: bottle i :is*es along a tnbe, and bnbh'es up into the boitle threiiLib the hole in the shelf. iSee Fiirs. 11 and li.'^ (i" ^Vhen 'he ff?)s is soluble in w.ater it must be collected by displaeemi-nt ot air. (See Fiir. 67 ' If the jnis is lighter ihan air the jar must be inverted and the g-.is poured unwards- t SnlphnrettiHi In drogen is formed in abundance when diluted sulphnr'e aoid is poured on sulphiiie ot irxsn — both inexpensive sul-stauccs. Forpre- jvarition see Fig. tiT' Heat is net required. •56 OBJECT LESSONS. on a lighted taper. The flame is extingnislied. Both liquids and gases mar be made to flow; hence they are called ^u/flfe. The word fluid means flowing. The teacher may conclude this lesson by showing how extremely useful in nature is the constant expaimon of gases. Stagnant pools, heaps of refuse, decaying vegetable and animal matter, give of gases which to breathe in quantity would cause illness, and perhaps death. But they soon expand, and are lost to the senses in the vast atmosphere around and above us. LESSON XI. COAL-GAS. Articles for illn5tratin]i : long clay pipe, small coal, clay, soda-watei bottle, wire gauze. I. Its properties. Attach a piece of india-rubber tubing to a gas-burner and collect the gas in a bottle, as described in the foot-note, page 00. Like air, coal-gas is invisible; but, unlike air, it has ar unpleasant smell, and burns with a bright flame. II. Its manufscture. Sxp. 83 Take a clay tobacco-pipe with large bowl and long stem. Fill the bowl nearly to the brim with crushed coal, and stop firmly with well-kneaded clav. Put the charged bowl into a clear fire, and direct the children to take note of the result. First steam pours out of the stem. This is from the moisture in the coal. When this has ceased, a stream of coal-gas follows, which may be ignited. It burns like a candle. SECOND STAGE. 57 At the present stage it ■will be sufficient to let the children iinderstimd that coal-gas is manutactured on a large scale in a manner similar to that we have employed in making a tiny quantity. Great iron vessels, called retortx, are used instead of the bowl of the pipe, and long iron tubes instead of the stem. III. Coal-gas in mines. Fire-damp. Refer to what sometimes happens when coal-cas escapes into a room. It mixes with the air, some one brings in a light, and there is a sudden expl'sion: the windows are blown out, or the walls thrown down, and perhaps people are injured. £rp. 84. Show s/ight exp/osion of a mixture of coal-gas and air in a soda-water bottle. Fill the bottle with water, admit air to till about two-thirds of bottle, and then till up with coal-gas [see nc^e, page 65]. Apply a lighted taper to the mouth of the bottle. " What happens in mines ? " " Firstly, the owners of a co il-miiie sjet all the fresh air they Can into the mine, or the men could not breathe, and would die. "Secondly, gas often escapes in large quantities from the coal in the mine without any heat. " Thirdly, it is dai-k, and the men must have lights to see to work."' " Here then we have everything wanted for a dreadful explosion. Fortunately when a light is placed in a lamp made of wire gauze it will not set fire to the mixture. But then sometimes the men are careless, and explosions happen, and many of the poor miners are killed."' Ej-p. So. If a piece of wire gauze be held in a jet of ga#, an inch or two above the ''burner,"" the gas may be burut.l above the ijauze without isrnituiij the jras below. ■58 OBJECT LESSO>'S. IV. Usefal lessons to be learnt. 1. Never sleep in a room into whicli coal-gas is escaping. It may poison you. 2. Whenever there is an escape of gas open the doors and windows. Gas expands, and soon mixes with the air outside. 3. Xever take a light to see where the leakage is ; the mixture of gas and air might explode and kill you. LESSON XII. TAR. Articies f(fl- illustTation : tar, spirits of wine or ether, naphtha, and if convenient, carbolic acid, and any aniline colours. I. Whence obtained. Tar is one of the products formed during the manufacture of gas from coal. It comes over from the retort with the gas, and is col- lected in water through which the gas is made to pass. It may be made from wood in the same way. That is, the wood may be enclosed in an iron retort and heated just as we heat coal. In the one case charcoal is left behind, in the other colie. The teacher should show these sub- stances. Planks used in ship-building are covered with wood-tar made from logs of pine-wood. AYood-tar is obtained as follows : — a hollow or kiln is SECOND STAGE. 6fl made of sugnr-loaf shape (see Fig. 13) in tlie side of a hill, having a small opeuing at the bottom which leads to a tank. The kiln is iilled with logs of pine-wood and covered with turf, a hole being loft in the top, where the fire is kindled. The wood smoulders, and becomes charred from the top downwards, while the tar oozes out at the bottom. II. Properties. ^lany of the properties of tax* may be elicited from tht children by le;iding questions. Wood-tar is of a bkicJiish-hruirn colour. Coal-tar is bhck, in each ease a n'scous fluid — viz. a fluid which is thick and sluggish — having the consistency of liquid glue, or treacle. Uupkasant sin-ell, and bitter burning taste. Sums freely, giving off volumes of /leacy smoke. A little header than irater, and therefore sinks to the bottom when poured into water. Does not mix with water ; and soap and water will not remove it from the fingers. DissoJres partly in alcohol, and partly in ether and spirits of turpentine, and mixes freely with oi/s -Ani fats. To cleanse the hands of tar, rub with turpentine, or with a little oil or fat, and then wash with soap and water. The most important property of tar is its power of pre- venting deciiy. Salt and sugar are used to preserve meat : but tar is a more powerful preservative than either. Tar is not usoil, however, for this purpose because of its unpleasant taste. III. Its uses. 1. For preserving wood. Timber is sometimes steeped in tar. Somerimos the tar is spread over woodwork like piiint. 0. JS'iiphtha, useful for dissolving india-rubber : carboln afid. used for disinfecting purposes and in the making of * Tar is a mixture of many bodies. As these T;iry with the source whence the tar was derived, luid the amount of heat used in dis:ilLition. its piopertics raiv somewhat. 60 OBJECT LESSONS. carbolic soap ; aniline colours, used for dyeing calico, and cloth, and other important substances are got from tar. How far the teacher will pursue this subject must depend on the capacity and intelligence of the scholars. LESSON XIII. CARBONIC ACID. Articles for illustration : bottles, tumblers, chalk or marble, and hydrochloric acid. I. Its properties. To show its properties collect one or two bottles of the gas. (See Fig. 12.) Ea^. 86. [Carbonic acid gas is readily obtained by pouring dilute hydrochloric acid on lumps of marble or chalk.] The gas is invisible and tcithout smell. It is also a hmcy gas ; it can be poured from one vessel to another. (See experiment Lesson X.) Its most striking property can be shown by plunging a lighted taper into a jar or bottle of the gas. The flame is at once extinguished. Or a stream from a jet may be made to play on a burning match. The flame is extinguisbed. K^ot only does carbonic acid gas not burn itself, but it prevents other bodies from burning. [A portable fire extinguisher has been constructed which makes carbonic acid gas, and pours it out through a tube.] We breathe out carbonic acid gas from the lungs. Exp. 87. Send a stream of carbonic acid gas into lime-water. The children will see that the gas makes the water look milky ; they may be told that the water held lime in solution, and that the gas united with it and formed chalk, which is insoluble. Hence the white colour. SECOND STAGE. 61 ^v'o.w call on one of the scholars to breathe out through a f-olution ot lime-water and note a result similar to the above. What coni'lu#ion can the children diaw from the experiuu-nt !' T/'c breafhe out carbonic acid ^. A saucer of lime-water left to stand for a few hours will become milky-looking on the surface. Why!' l/icre is a/iciii/s a s/na// amount of cat hoiiw acid gas present in the air. II. Carbonic acid produced in bnrnins^. We have seen in previous lessons that we breathe out vapour of water from the lungs, and that water is produced bv dame. In this lesson we have le.irut that we broa.he out carbonic acid gas. and now you have to learn that Hame also pro- duces carbonic acid gas. Erp. S9. Take a bottle with wide mouth — a large " pickle- bottle "" will answer very well — invert it over a lighted candle ^Fic- 1-i). When the candle is extinguished pl.:ce the bottle over a wine glass of lime water (Fig. 15). In a few minutes we see the milky looking surface, showing that c^u-bonic acid gas was present in the brittle. This :;as was produced by the burning of the caudle. III. Cai'bonic acid gas is poisonous. The teacher will refer (1) to the tact that chalk or limestone when iieated in kiins gives otf carbonic acid gas ^draw the 62 OBJECT LESSONS. outline of a lime-kiln on the blackboard], and that people who have gone into kilns, recently emptied, for the sake of the warmth, have gone to sleep, and never wakened again ; (2) to the choke-damp (carbonic acid gas) formed by the explosion of fire-damp (coal-gas), which probably kills more miners than the explosion itself. The amount of carbonic acid gas in the air is not suffi- cient in quantity to do us any harm ; but if we shut ourselves up in a close room, and breathe the same air over and over again, the amount of the gas is iacreased, and we become heavy and sleepy, and perhaps get a headache. IV. A usefullesson. It is not healthy to live in a close room. Allow the foul air to escape, and the pure air to come in, by opening windows or doors. If opened ever so little the air is kept purer and more healthy than when closed. LKSSON XIV, PARAFFIN OIL. Ahticles for illustration, benzoline, paraffin oil, paraffin candles, and, if possible, benzoHne and paraffin lamps. I. Properties. The properties more easily discerned should be educed in the usual way. Paraffin oil is a colourless liquid, lighter than water, having an unpleasant smell. The oil is not explosive as is often supposed, and as a liquid it does not burn. Exp. 90. Pour a little into a cup, and plunge into it a lighted taper. There is no explosion, and the oil does not SECOND STAGE. 63 burn ; on the contrary, tlie taper will be extinguished, just as if it had been plunged into water. Whence, then, do we obtain the beautiful light given by pai-affin lamps? It is from the invisible rapotir or gas into which the paraffin liquid changes when heated. A mixture of the vapour from paraffin oil and air is explosive, just like a mixture of coal-gas and air ; and it is the paraffin gas which burns just as coal-gas burns. II. Whence obtained. The children will remember how coal-gas was obtained from coal. A crude, viz impure, oil is obtained from coal just in the same way, only the coal is not heated so much. The poor kinds of coal are used for the purpose of extract- ing this oil. In America the oil is obtained from oil-wells. It is called petroletini, that is, rock-oil, and millions of gallons are brought to this country every year. The crude oil is separated into benzoHne, paraffin oil, and solid white paraffin. III. Uses. Beiisoh'iie is used for sponge lamps. A lamp should be shown, and the teachers should point out the danger in carelessly using benzoline. It gives ofi' vapour in hot weather, and may thus form an explosive mixture with the air. It burns readily also on the application of a flame. Hence it should never be handled by candle-light. Benzoline is useful in removing grease-spots from clothing. It dis- solves the grease. This should be illustrated. Paraffin oil is used for burning in lamps. A lamp should be shown. Paraffin, when puritied, is a pure white solid. Ii is used foi making candles. 64 OBJECT LESS0X3. LESSON XV. CANDLES. A RTicLES for illnstration : as many kinds of candles as can be obtained , the stem of a tobacco-pipe. I Kinds of candles. 1. The rushlight. This is now seldom used, but it was the light by which our grandfathers and grandmothers had to read and sew. It was made of the pith of rushes dipped in fat. 2. The common dip. Dips are made like the rushlights ; onlv instead of the rush-pith the wicks are made of loosely twisted cotton threads. The wicks are dipped in the melted tallow two, three, four or more times, but allowed to cool between each dipping. The cooliag allows the tallow to set, and in the next dipping more tallow adheres. 3. Mould candles. What is a mould ? Look at the candle. What is the shape of the mould ? Where must the wick be placed. Why called mould-candles ? The vdck is plaited. Why ? It saves snuffing. It causes the wick as it burns to curve slightly outwards, and the wick is com- pletely consumed. [See Fifth Stage, Lesson TIL, page 1S4.J Paraffin, wax, oil obtained from the head of a whale, and many other fats and oils are used in the manufacture of candles. II. How a candle bnrns. The teacher should have a candle burning in front of the cl.iss, and call children to note first the solid fat, then the cup at the top and what is in it. What liquefies the hard fat? J^est they should note the liquid fat going up the wick. Place a twist i;f cotton in water to sho^v how the water a;Ct'ii.ds. SECOND STAGE. 65 At the top of the wick the liquid is changed to a gas by the heat. Show this by inserting a small tube iuto the middle of the tlame ; the gas pours out at the other end and maybe ignited. Compare with I he parathn oil going up the wick, and with turpeutiue up a piece of cane. The teacher should draw the attention of the children to the fact that whether we use candles, or oil, or coal-gas to light our hiiubcs, we always burn i/an of one kind or another. LESSON XVI. SOAP-BUBBLES AND WHAT THEY TEACH Artioi ES for illustration : tobacco-pipe, ^o,lp and hot vvaur, a Httle oil, water, and quicksilver. I. How made. •• To-day our lesson is to be on soap-bubbles and what they teach us. I must tirst of all show you hi w to make soap-bubbles, and then I dare s.iy you will try and make them for yourselves." Erp. 91. "Tou all know what this is?" A tobacco- pipe. "And this I dare sav you can toll bv its colour and smell?" Scup. " Yes, and here I have water. I will heat a little in this test-tube over the spirit lamp.'' " Now I sliall dissolve some of the soap in the warm water. Xext I warm the pipe and put a drop or two of the so.ip mixture in the bottom of the bowl." ■• Xow I blow gently. There it is. What do you see ': '' A ball. '■ Yes. and we call this ball a so-p-b I hie. Look. I shake it off. There it goes. In what direction is it geing r " P 66 OBJECT LESSONS. " Ah, where is it now ? " It has burst. " I will make another. There it goes, up again." " \Vatch it. In what direction is it going now ?" It is coming dotcn. " We will find out something more about these pretty balls. I put a Kttle water in the pipe and blow. I make bubbles, but they break at once in the bowl of the pipe. Why ? I will tell you. The particles of water alone cannot stick together firmly enough to make the ball. The soap sticks them together, so that the thin covering of the so:ip- bubble is just a very thin sheet of soap and toater." " But what is there inside the ball ? Think for a moment. How did I make the soap bubble ? " Jiy blowing through the stem of the pipe. " And what did I blow through the stem ? " Air. " Now breathe gently on your hands ? How does the air which comes from the lungs feel ? " It feels tcann. " Then what kind of air did I breathe into the bubble ? " Warm air. "Now you can tell of what the bubble is made P Wh;;t is the covering ? " A sheet of soap and wafer. " And what is there inside ? " Warm air. II. What they teach. " And now we have to ask the soap-bubble why it first went up, and then came down again." "And first why does the bubble ascend?" " Here is a test-tube, what is there in it ? " Nothing. "Oh ves, there is something in it although you see nothing. Whatfi'llsit?" ^(>. " Now I pour in a little oil. Where does the oil go?" To the bottom of the tube. "■ And what has become of the air which was at the bottom where the oil now is ? I will tell you, the oil has pus/ied it up." SECOND STAGE. 67 "Now I pour in a little water. Where do vou see the >T;)ter;- " At t/w bottom. " And where is the oil ? " Jiisf above the water. " And how has the oil bee i raised higher up ia the tube P " T/ie water has pushed it up. " Lastly, I pour in a little quicksilver. "Where does that go ? " To the bottom. " And what has the quicksilver done to the oil and the water? " It has pushed them higher up in thf tube. " Xow I will shake the test-tube. What do vou see?" The qiti'lsiher is at the bottom, and the oil and inter mi.red up. "Wait a minute or two. Xow what do you see ? " Thi oil is ri^iihi to )"/k' top of the fcnter, " From these experimeuTs you see that the heavier bodies ahvavs JB/Yss up the lighter ones and take their places. And this is what I want you particularly to remember. Xcif/icr light bodies nor heart/ bodies (V^ccud of theiiiselres. If they go up tliev are always pus'icd up." " On a cold day stand under an open window. What dc vou feel ? " The eold air eoming down. "Yes. it comes down and takes the place of the warmer air oi the room. But where does the warm air go ? // ;> p-f-'u'.i up to the top of the room, and squeezed out tehereiv) there are opetii»gs." " When the air of the room is warm open the door about an inch. Hold a lighted candle near the top ; the dame is blown outwards. Hold it near the bottom; the flame is blown inwards. The cold air is coming in at the bottom and the warm air is going out at the top, and the cold air forces up the warmer air just as water forces up oil, or quicksilver forces up water. Stncf we kjioir that the warm air is lighter thou t/ie colder air." " We will now return to our soap-bubble. What kind of air had it mside P " JTarm air. fiS OBJECT LESSONS. " And which is the lighter, warm or cold air ?" Warm air. '' Then why did the bnbble ascend ? " The cold air pushed up the warmer, and therefore lighter air in the bubble. " Now we have to ask, Why did the soap bubble come down again ? When you put hot water out in the cold does it remain hot ? " N'o, it soon gets cold. " And the warm air soon gets cool and heavier, and the soap-and-water-covering of the bubble helps to make it a little heavier still, and down it comes. " I think you must have learnt in this lesson why bodies lighter than water ascend in water, and why bodies lighter than air ascend in air. You will learn in the next lesson about other bodies which are lighter than air, and to what use we put them because they are light enough to ascend in the air. That is, they are light enough to allow the air to push them up." LESSON XVII. BALLOONS. Akticles for iUnstratdon : a small collodion balloon.* I. A model ballooD. JSrp. 92. To inflate a balloon and despatch it to the ceiling is an interesting experiment; but, unfortunately, it is somewhat difEcultof execution. The difference between the weight of coal-gas and common air is not sufficient to carry a balloon of less diameter than 18 inches, but a small coUodion balloon wiU ascend if filled with hydrogen gas.t • These can be purchased at a shilling. Great care must he taken in iinfolding, as they are extremely delicate, t To prepare hydrogen gas place a little of gi-anulsted zinc at the bottom oi SECOND STAGE. 69 ^I. How balloons ascend. The teacher can make clear to the childreu the exceeding buoyancy of hydrogen gas in an ocean of air by comparing it with the buo3'ancy of cork in water. Cork is four times lighter than water, but hydrogen is fourteen and a half times lighter than air. Xow coik from its lightness compared with water can be made to hold up or carry up weights Fi^^ 16. Fis. 17. in the water [show this by experiment], and just in the same way halloons are able to earn/ up irrights in the air. Br comparing the weights of equal volumes of air and livdroii'en. it is easy to see that hydrogen has a great lifting power. "NVe have to remember that so long as the balloon, n boltle ^FiiT. 10). c o.ver it with w.iter. Iiisn-t cork with tubes as in figure. Pour in thioueh the lovitr Inhe a little sulphuric acid. Hydmi:™ r,.mes oft iu qnantiiv. Alow .sufficient time for the bottle tn be filled with hydropen to the exclusion of air. and then tie the collodion balloon over tlie taperiug end of th." shov:cr uliss tuhe wiih a thread of silk. When inflated tie tiie month qnicklv 1 w firmly. If the gas is required free from moisture and arid it nitist be passed throiigh water, and a tube ivintaiuing either lumps of unslalced linie, or of cilcic chloride. 70 OBJECT LESSORS. the hydrogen it holds, and the weight it carries, weigh less than the volume of air displaced the balloon will ascend, because the lighter body is always pressed up and its place occupied by the hearier body — ^in this case the air. Let the capacity of the balloon be 100 cubic feet. 100 cubic feet of air weighs, say . . 7^ lbs. 100 ,, ,, hydrogen ,, ■ ■ i lb. If the weight of the balloon itself is 4 lbs., the carrying power wiU still amount to 3 lbs., or thereabouts. Of course the larger the balloon the greater the carrying power. Coal-wrfs is now always used because of its cheapness. Its density varies from one-third to two-thirds that of air ; hence its carrying power is much less than that of hydrogen, and the balloon has to be made so much larger. The teacher may enlarge on the management and uses of balloons : how, when they arrive in rarer air, they are made to ascend stiU higher, and how the aerial voyager manages to get down again. *,* A 'balloon IS in. in diameter, made of gold-beater's skin, costs about three shiUings ; but this can be used many times. THIRD STAGE. THIRD STAGE. Uitliorto our lessons have dwelt almost entirely on tacts evident to the senses, or made evident by simple experiment. "We have now to otfer simple explana- tions of these facts, leaving the more difficult points for consideration in the higher stages. We start with the hypothesis that all matter is made up of molecules. LESSON I. MOLECULES. Articles lor illustiMtiou : nieroury, small piece of cli.imois leather, test- tube, drop of olive oil, &c., according to expeviments selected. The aim of the teacher in this lesson will be to show hon- minute paitiolesot matter may he divided and subdivided until we can just distinguish them by the naked eye, then only by the aid of a common lens, then with the help of a microscope ; and, secondly, to show that we can further subdivide, so that with the aid of the most powerful microscope we fail to detect them, and thus lead up to the still smaller particle — the i>hile< tile. (l.j Hxp. 93. Squeeze a drop of mercury through chamois leather, and let che drops fall on a piece of black cloth. Thousands of the tiniest drops are made from one drop. They are easily seen shining like silver on the black ground. Take oue drop, the size of a pin's head. Spread 74 OBJECT LESSONS it over the cloth with the blade of a knife, as you would spread butter on bread. Thousands of tiny drops from this one drop may be seen with a common lens. Exp. 94. Heat a few drops of mercury in a test-tube. In a few minutes the mercury boils and begins to change to invisible vapour ; but as it comes in contact with the cold glass in the upper part of the tube the vapour condenses into hundreds of thousands of tiny globes ; some join together and become large enough to be seen by the naked eye, while others come into view only with the aid of the magnifying- glass. Here, then, we have mercury divided up, first of all into particles (in the shape of vapour), too small to be seen at all, and then condensing into what seems to the eye the tiniest of silver balls. (2.) Ejtp. 95. Take a large test-tube, nearly fill it with water, and add one drop of olive oil. Shake violently, and we have split the single drop into thousands upon thousands, many of which can be seen under a lens as silvery globes gradually rising towards the surface of the water. (3.) Water changes to vapour, ihe particles of which are absolutely invisible under the microscope. It condenses into drops (in fog) very small, but not too small to be seen under the magnifying-glass. (4.) Exp. 96. When water is added to a solution of gum-mastic in " spirits of wine," the gum-mastic becomes visible as very fine whitish particles. If we add one drop of the solution to half a pint of water, stirring well when we add the drop, the water assumes a milky tinge. This milky tinge is given by the particles of gum-mastic, but they are too small to be seen even under the most powerful microscope. Now the best microscopes will show solid bodies so small that the hole made in a sheet of paper with the point of a needle will hold many thousands. The particles of gum- THIBD STAGE. 75 mastic must be smaller still, for they cannot be seen at aU. The teacher may refer also to solutions of solids in liquids ; the solids in solution are invisible. From the above or similar experiments the children will be led to see into what extremely minute particles we can subdivide matter, and they must be told that for reasons of which they will learn more in future lessons, we suppose that all bodies are made up of minute ball- shaped particles — particles so minute that it would take millions of them to make the tiniest drop of water we can see. These small particles are called molecules, a word which means litflf LESSON II. STATES OF MATTER. Articles for illubtr.ition : lead, uiercury, water. If we agree to suppose that all bodies are built up of molecules, we can easily explain many facts not otherwise capable of simple expianatiun. I. Cohesion. Take a lump of lead. It is not easy to break. TThy ? The molecules are held firmly together. Are the mole- cules tied together in any way ? '^o, but there is some power or force which holds them together just as if they were tied. If not, what would happen ? The lead would break into fine dust. What is the force that holds the molecules together ? We know not. We only know it is there, and we name it the force of cohesion, because the word eo/ieswii means holding together. Let us break the lump of lead, and then press the broken ends together. Do they unite and hold together again ': 76 OBJECT LESSORS. Xo How is tbis ? If you could see the broken ends nndei the microscope you would leam the reason at once. The broken surraces are roiiirh, and the molecules are not brought near enough to each other to hold together. Exp. 97. Boys sometimes amuse themselves by cnttrng a piece off ixom two bullets ; then, scraping the cut faces as smooth as possible, thev press them together. The pieces hold together so well that it takes a pretty hard pull to get them apart. How is this ? TVe have brought some of the molecules near enough together to hold on to or attract each other. Sometimes when sheets of glass have been pressed together it has been found impossible to separate them without break- Exp. 98. In the same way if we divide a piece of india-rubber, making a smooth cut with a sharp knife, we can press the cut faces together and make the pieces adhere. n. Solid, liqmd, gas. We can now better understand the difference between solids, ligvids, and gases. As we saw in the lump of lead, the molecules in solids are held firmly tog tt her. It is this, in fact, which makes them solids. Here is a drop of mercury. I just pat my finger on it and it is broken into many smaller drops. The molecules of mercury are not held so firmly together as the molecules in lead. It is the same with water : you can break it into pieces with the slightest touch. But is there no attraction or drawing together in the molecules of liquids ? I squeeze a drop of mercury between two pieces of glass. It is flattened. I remove the pressure, and the drop becomes a glnbe again. I pour a Httle oil on water ; you see the oU floating in drops THIKD STAGE. 77 I throw water on the floor. The moleci^les combine to form ball-shaped drops. If the molecules of mercury, oil, and water did not attract each other they would be w glass marbles, a Ixig of small shot, a lamp diimney, and a boy's sucker, " I. Pressure downwards and sideways. '■ A^Tiat is 'weight ? " Pressure doiniirards. Pre^ure (oiatrds the centre of the earth. " I plac« a lump of lead on tJie table ; in what direction does it press ? " Dotniirards. " Is there any pressure sideways or upwards ? " Xoiic. " "Why ? " Because the attraction of cohesion among fhf parficJf .-> 4' stronger tJian their gradty, or attraction towards thi rarfA. 88 OBJECT LESSONS. Exp. 104. " Here is the tube which we used in a former lesson (see page 36). I set it upright and place this iron rod in it. Does the rod press on the sides ? " 2\'o. " I fill the tube with water. Xowif I make a hole in the side of the tube, what wiU happen ? " The vaier trill run out. " "What does that show about the water ? " That water presses on the sides of The vesseh ichich hold it. " Liquids then differ from solids in this, that they exert a pressure sideways as well as downwards. Solids press only downwards. I will endeavour to explain how this comes about. " Tell me what we learnt about the molecules of water in the last lesson." The^ are very minute. They are round and smooth. They more about amongst each other with perfect ease. Exp. 105. A little experiment will now help us to see how water presses sideways. Take three marbles, place them side by side in the shape of a triangle on the smooth table, or on a piece of glass. Now put a marble on the top ; what happens ? Those below are pressed out sideways. And this is just what happens with the fine " water-balls " only the water-balls are so much smoother than the marbles that they move much easier. Exp. 106. The teacher may further illustrate by making a hole in a bag of small shot. The shot close near the hole will be pressed out by those above, and these in their turn will be pressed out by those above them, and so on. Just so it is with the delicate irater-.'ir.<:scd up the sp.iut till the liquid in the pot and in the spout have the same level. (d) E.rp. 1(S. Cover one end of a ghiss cylinder — a lamp chimney will answer the purpose — ^vith the leather of a boy's ■■ sucker," and let the string pass upwards throui^h tlie chimney. Hold the leather firmly against tlie base of the chimney, and lower carefully into a jar of water. Drop the string, and the leather is held in place by the upward pressure of the water (Fig. It should of course be noted that this up- ward pressure in liquids operates only up to the natural level of the surtace and not bovond. At the surface there is no upward pressure. This upward pressure in water explains why solids weigh U ss in water than out of it. Water, then, presses downwards and siJe\A ays aud upwards. If J'lY^tl Fit;, i-2. all I, '\CiiOHS. 90 OBJECT LESSONS. LESSON IX. PRESSURE IN LIQUIDS. II. Articles for illnstration : thimney-glass and boy's Backer, hollow india-rubber ball, and bent glass tubes as in Fig. 23, glass vessel for holding water. I. Pressure increases as the depth increases. It is quite certain ttat the pressure dmrmcards in a column of water must increase with the depth, for a tall column must be heavier than a shorter one. £xp. 109. The teacher may next show that the pres- sure sideways increases with the depth by the tube as described on page 36. The chimney-glass and boy's sucker (see the preceding Lesson) wiU serve to show that pressure upwards increases with the depth. Press the chimney-glass a short distance into the water. Pour water into the chimney-glass and note how tall a column of water is required to overcome the upward pres- sure and set free the sucker. Remove the glass from the water, and again holding on the sucker, press down further into the wt«er. Pour water into the chimney until the sucker is released, and compare this column of water with the previous one. It wiU be longer, and therefore heavier. Why must it be longer and heavier ? Because it has to overcome a greater upward pressure. And it will be found that the deeper we press in the chimney- glass the greater will be the upward pressure on the leather sucker. Sailors sometimes amuse themselves by partially filling a bottle with water and then after corking it tightly, letting it down by means of a long string into deep water. The great pressure of the water forces in the cork, and the bottle THIRD STAGE. 91 comes up full of water. Xo matter in what direction the nock of the bottle may point when let down, the result is the same. This shows that the water presses with equal tacility in all directions, and that, deep down, the pressure is much gre.itor than near the surface. II. At the same depth the pressure is equal in all directions. JExp. 110. Take two glass tubes bent as in Fig. "Jo Put meivury in the lower part of each tube, so as iust to fill the short arms. Lower both tubes into a ijhiss vessel of Vi^l Pig:, -23. water. Tlie water will force the mercury up the longer arm of each tube, and if tlie mouths of the short arms are at the same level, the mercury in the long arms will stand at the same height, showing that the downward pressure and the side pressure are equal. III. Liquids transmit pressure. The teacher will nrst explain the meaning of the word ^■i7)l-- 'y. 115. Xow instead of the test-tube let fcfe^s^^ us use a tube open at both ends, onlv we cover one end tightly with a piece of wet bladder (Fig. ~5). Now fill and raise as before. So far this is the same experiment as the hist ; but now prick a hole in the Wadder. The water runs down, because the air has got at it and forced it down.t If we couhl use a tube about oo feet long we should find that the pressure of the air would keep the ^^ tube full of water; but make it about o4 feet ''^ long and the w iter would siuk one foot in the tube. Hence the air can support a column of water of anv length up to 33 feet, but no more. What do we learn from this ? That the weight of the air is suificient to balance a column of water oo teet high, but not a column -34 teet high. It is not easy to experiment with tubes so long, although such tubes have been made for the puipose of testing this weight or downward pressure of the air It is more convenient to use mercury. This metal is about 13i times as heavv ss vater, and so we must divide 33 feet • This o;ui be done unde>- Jne receiver of an air-jump. t -\ oi rk. or tlie palm cl the h;ind- may be r.?. d iu^tead of the blf. idcr. 96 OBJECT LESsOXS. by I'Sj to find the length of a column of mercury equal in weight to a column of water 33 feet high, both being held in tubes of the same diameter. It will be found to be between 29 and 30 inches. Exp. 116. Take a tube about 30 inches long — with small bore, because less mercury will be re- quired — closed at one end. Fill the tube by pour- ing in mercury. Place the finger firmly over the open end. Invert and place in a small cup of mercury (Fig. 2(3). The column of mercury will be supported by the weight, or pressure down- wards of the atmosphere. Notice, we say the column of water is about 33 feet long, and the column of mercury is ahout 30 inches. Tou will learn the reason for this in a future lesson ; but I may now just tell vou that the weight of the atmosphere is not always the same. It changes a little day by day, so that sometimes it will support a column of mercury 31 inches high, and at other times perhaps only ','9 inches, or even less. We can now easily find the actual weight of a column of air of any size, reaching from the sur- face of the earth to its utmost limit above. Take a tube square in section, and each side of the square measuring an inch (inside measure) (Fig. W7) ; we may say that the tube has a base of one square inch. Xow the mercury which j ust tills a tube of this size and 30 inches long weighs about 15 lbs., and as this can be supported by a column of air of the same size, we conclude that the column of air weighs 15 lbs. It is more usual to say, T/ie atmosphere Fig. 26. Fig. -27. THIRD STAGE. 97 prc.'isr^ on ereri/f fling irith a force equal to about lb lU. on tht iqt.are iiw/i. LESSON XII. PRESSURE OF THE ATMOSPHERE. Afttolfs for ilhist ration : boy's sucker, tumbler, water, and a thin card to cover moutli of tumbler. The teaelier should refer Ixtclc to Le<;=on TUT., page 51, and repeat one or two of the experinieuts to show that air presses in all directions. Secondly, he should refer to Lesson IX., paije 91, where it is shown by experiment that, at t/'iesiii/ie depth, tlie pressure of water is equal in all directions. The atmosphere presses with ec[nal force in all directions, when measured at the same distance ahove the level of the sea. The same law holds good in all fluids, including, oi course, the air : but we cannot prove it in the atmosphere bv simple experiment because we eannot easily git to different heights The law must be true at the surface of the earth. We have seen that the pressure of the air on every square inch is ahou : lo lbs., hence the pressure on a square foot will be more than a ton. The pressure, then, on the bottom of an '■ empty "' bucket me.tsuring a square foot will be over a ton. How, the;], cm we lift the bucket ? J ust because the pressure upward is equal to the pressure downward, and we feil to feel the pressure of the air at alL Pressure side- ways, too, must balance or we could not stand upright. The pressure of the atmosphere on every child in the class is several tons ; but it is not felt because there is an equal pressure outwards from the air within the body. [If an air-pump can be obtained, the great pressure of the H 98 OBJECT LESSONS. atmospliere caa be demonstrated in a variety of ways. See Lesson XVIII., page HI. J If the air whioh surrounds the earth had the same density* everywhere, it would follow that, as we ascend, a given column would become just as much lighter as it is shorter ; but the air is by no means of the same density everywhere. It could not be, because it is very compressible, and the air above must squeeze the air below into less space by its weight. Therefore the air near the sea level is much heavier than the air at a great height. We m.iy compare roughly a column of air to a column of bales of wool. The bales near the bot- tom will be squeezed, and made thinner than those above (Fig. 28). It we go to the top of a high mountain we leave the denser part of the air below us ; and a column of air should weigh much less at the top than it does at the base of a mountain, and so it does. The weight is lessened much more rapidly than the length of the column above us. At the top of Snowdon the column with square inch section weighs about 13 lbs. instead of 15 lbs. At the top of Mont BLinc the column weighs only 71 lbs This is shown by the height of a column of mercury which the air supports. For every thousand feet we ascend the column of mercury shortens by about one inch. But if the air were equally dense everywhere, we should have to ascend more than a mile before the mercury fell as much as this. On the other hand, if we descend into a mine the column of mercury lengthens one inch for every thousand fed. Figr. 28. * Tliickness, closeness, the same number of moiecules occupying a similar space. THIRD STAGE. 99 LESSON XIII. THE BAROMETER. Aktici 1 s for illustration : ;i narrow tttbe tliirty-four inches lontr. a iluillow oup, ;uni enough mereurv to till the tube and t\vo-thir.l> of the cup. I. The weight of the atmosphere very much varies with the amount of water-vapour it contains. ■■^^ot only iloes the \veight of the atmosphere rarv aoeordiug to the height above the sea level at which it is measured, but it varies, as I have alreadj- told yoti. from day to day. and even from hour to hour, at the same place, 'fo what is this variation due ? It is due to two or three causes, but mainly to the variation in the quantity of vapour in the air. ••The atmosphere is a mixture of certain ga>es and rapoi/r, and you know that vapour is lighter than air. How do you know that vapour is lighter than air? "' I!ci-inisc it oscc/iJ.< in flie ('//•. •■ Then which will be the heavier, dry air or a mixture of air and vapour ? "" T>i-i/ aii: •• And how will the weight of the mixture vary ? " If will Vfiry i7,-- the (juantifi/ of ritjioiir in the atnmsjfiirrc roriis. •• When will the mixture be heaviest'.'" Tr7/e» it mntaiiis t/ic !ctK moisture me^^r.s we .^haJI jyivi'olily Jiare jinc irentJier. That is. the barometer tell^ .<■',;/( f/t/vj of the kind of weather we are likelv to have. When the mercury falls " it nsnally betokens A7i«y >reafh,: when it •■ vises " it ttsnally points to fair ,r, .jflier. ^^"hen the merourj- falls very i]^uickly it often betokens a severe storm. LESSON XIV. THE SYRINGE ARTlctES fi)r illnstration : long guifs tube?, one sti-.-.iglit and one bent, wiUi rather small bore ; a syriiue of any kind. I. Its parts.* Consists of tube with small hole at one end, a piston-rod and a sucker. The teacher should take his specimen syringe to pieces and show its p,arts. • The :e^-.cV.e-r may o:r.-truct hi? own svrinir?. Take a ir';\«< tii'be eig-ht or nine inches Ions:, -w-iti about a >..i".:-inc . bore. Fix neatly into one end .i short cvlinder ot wood with s sn ail hole through the oentre. lake a stiaight rod for pi>ton. and hind worsted round one ena to mafce the suLkej. 102 OBJECT LESSORS. II. Its action. Exp. 118. See that the piston-rod is pressed, down. Place the point of the syringe heneath the surface of the water. Draw up the piston-rod ; the water follows the sucker. Press the rod back and the water is pushed out in a stream through the small hole. Wliy docs the water enter the cylinder when the piston-rod is drairn baeJ; ? And why does it require a small hole, or holes, through- which to force the irater. It is commonly said that the sucker sucks up, or draws up, the water ; but this cannot be, as I will show Tou. E.ip. 119. Take this glass tube, place one end in this bottle of water and the other end in your mouth. Kow ''suck up" the water through the tube. Wliat makes the water come through the tube into your mouth? I will tell you. Tou "suck out" the air, and the water follows. But the water cannot *!?■ 32. juove of itself, something must force it up the tube. It is the air pressing on the water in the bottle. I will now prevent the air from pressing on the water in the bottle. I pass the tube through a hole I have made in this cork and cork the bottle. Both tube and cork must fit tightly. Now try and suck up the water. You cannot. Why r Because although you re- move the air from the tube, the ^ater cannot follow, there being no force to push it up. Fig. 33. Fisr. 34. Erp. 120. We may show the same thing another w..'! H.ie i~ a bent tube. 1 partly fill it with water (see F-g. 35). THIRD STAGE. 103 iT. o.i. So long ;is T leave the short end open I can drink th water ; but when I place my thumli tlrnily over this end I ean no longer got the \v;itev to ascend the longer leg. rrccisoly the snmo thing happens in tlie work- ing of the syringe as in the oottle when not corked. The sucker removes the ulr, or the greater part of it, fron-. the cylinder, and the prossuie of the air on the suitace of the water in the vessel forces the water into the svriiige. £.rp. 1'21. To answer the second question the teacher will till a test-tube with warer. and, placing a disi- of paper over its mouth, invert. The upward pressure of the air keeps the water in its place. If I remove the paper I don't remove the pi cssurc. and yet the water falls out. It falls oat because the gravity of its molecules is greater tlian their cohesion; and when there is nothing to preserve the level surface of the water, the air breaks in and 1'oroos itself up among the particles, and actiuilly turns the water out to take its place. Small holes are used in the syringe btvause they allow but a small water-surface for the air to act upon, and so the water does not run out freely unless pressed by the sucker. in. Its uses. W.iteriniT plants, cleansing their leaves. For cleansing our ears. Occ. 1.ESSON XV. THE COMMON PUNIP. AKTiet.FS for illustration : a L:las* nuclei of a common ?t;uion-pnmp :* or the teacher may jkoteh on the Wnekl'vwl ^Fii:. 39). I. Its parts. Like the svvino;e. the comnicin pump coiissts of a cylinder * This can Iw punli::ftii l"ov about t >to ^hilliIll;s. 104 OBJECT LESSONS. and a piston with sucker ; but, unlike the syringe, it has little doors working on hinges, which we call valcen. The teacher should, at the outset, explain the working of Fig. 36. calves, sketching the shapes of two or three of the most common on the blackboard (Fig. 36, a, b, c). The valve (Fig. SGb) is the one most often used in the common pump. It is called the bellows valve, because used in the common bellows. When you pull the handles of the bellows apart, as shown in Fig. 37, you make more room in the bel- lows, and the air forces the valve open and rushes in to fill up this space. When you push the handles of the bellows together, as in Fig. 38, you compress t4ie air, and it closes the valve and rushesout in astream through the nozzle. The teacher will nest show the class the position of the valves in the model, or in the sketch. He should also draw attention to the facjt that the lower part of the cylinder, or the suction-pipe, is smaller than the upper part, commonly called the barrel. II. Its action. In Fig. 39a the pump handle is down, and consequently Fig. 37. Fig. 38. THIRD STAGE. the sucker is as high as it emi be raised. Both valves arc closed. Eaise the handle. The air between the sucker and the lower valve will be compressed, consequently it will force open the vulve in the piston and escape above. When the Pig. 39. sucker is at its lowest point this valve will be again closed bv the weight of the air above it. Press the handle down. The valve in the piston is kept closed by the downward pressure of the air, and the space between the piston and the lower valve will be deprived of the groiter part of its air. The air in the suction-pipe, bv ITS elasticity, will open the valve, and part of it will escape into the " enipity '" space in the barrel. It follows that the pressure of the thinner air on the water withiu 106 OBJECT LESSONS. tte suction-pipe will be less tban the pressure outside, and then the water \rill be forced by the outside pressure up the suction-pipe until the pressure outside and inside are equal. Repeat the action with the handle, and the water will rise higher in the suction-pipe, till after a few strokes it forces open the lower valve and enters the barrel. When we raise the handle again the piston presses upon the water and forces it up through the valve in the piston (see Fig. 39 b). The next down stroke actually lifts the water, and when sufficient is raised above the sucker it flows out through the spout. The teacher will now show the class that the principle of the pump is just the same as that of the syringe, or of " sucking " up water through a small tube. It is the weight of the outside air which presses up the water in the suction-pipe. What is the Irngth of a column of water which the pressure of the atmosphere will sustain ? From 33 to 34 feet. Then we cannot raise water with an ordinary pump from a greater depth than 33 or 34 feet. As a matter of fact we cannot raise it so high, because we fail to keep out aU the air. The average is about 27 or 28 feet III. Its uses. The teacher will give the uses, e.g. raising water from wells, draining mines, &c. LESSON XVI. FORCE-PUMPS. Articles for illustration : — Models of force-pumps are rather expensive, and the teacher will probably have to substitute diagrams on the black- board. I. Porce-pnirip. Fig. 40 represents a very simple form of the force-pump. THIRD STAGE. 107 CC ^ ^ "SnwW The action, while the piston is asL-endiug, is like that of the common pump. In de- scending the action is different. There is no valve in the piston. This is placed somewhere in the disoliLir^e pipe which leaves the barrel near the bottom. In the diagram b represents tlie suction-pipe, c the barrel, r the piston ; D is the discharge pipe from the barrel, v and \ ' are valves opening upwards, cc is the "condensing '" chamber, and A is the discharge pipe into the air. ^^ hen the piston is raised the water is forced up by the atmospheric pressuvo outside Fifi. 40 into the suction-pipe, and thence, after a few strokes, into the barrel. "When the piston is pressed down the valve v closes and the water is forced through the pipe d and into the chamber ir. This chamber at first is full of air ; but after a few strokes of the piston this air is compressed b^• the water forced in. The compressed air in its turn presses on the water and forces it out through the tube A — which is smaller than d — in a continuous stream. II. The lifting pnnip. ^ The workiuiT of this pump can be seen Fig- ii- at once from the diagram (Fig. 41). Water may be pressed up, or lifted, to almost any height 108 OBJECT LESSORS. in the small tube, provided sufficient force is exerted on the piston, and the machine is strong enough to bear the pressure. III. The fire-engine. The fire-engine is a kind of force-pump- Fig. 42 represents the hand fire-engine. It is a doiihle force-pump of the kind Fig. i-1 shown in Fig. 40, and its action is precisely similar. The water is forced alternately from either cylinder into the central, or condensing chamber. LESSON XVII. THE SIPHON. Articles for ilhistration : bent glass tubes. [See cuts.] I. Experiments with water in bent tubes. Exp. 122. Take a narrow glass tube isay t^ inch bore) THIRD STAGE. 109 about eight inches in length, and bend it as shii'svn in Fig. 43. Fill the tube by innuersing in water. Eaise it geutlv out of the water; the waier does not run out of either end so long as the tube is kept upright. /'^^^ Or the points of the fore-tingers may be placed ' | over the ends wliilst liftimr out of the water. a K.rp. IJo, ISoxt take a wide tube bent in the ^anie way. with logs of equal length. Place the t' i Place a open ends upwards and p7/ the tube with water. thin card over each end and invert (Fig. 44). as the logs are kept upright the water does not run out. But if the tube be inclined either wav the water runs So long B C Fii-. 44. Fia:. 45. out of the /oirt'i- end. "\Then we incline the tube we makt one IciT longer than the other (Fig 4;';. II. Action of the siphon. (a.) How 'is ihc iraicr n taincd in ho:h i,-,js of the tube when tin- ,', > are of the i-a:in length / [h.) When ice male one kg logger than thf otlnr. why tioii the KCter run out ot the loirer end ' A few questions on the pressure of the air in every direc- tion will lead the children to see that the pressures upwards 110 OBJECT LESSONS. on B and c (Fig. 44) are equal. There is also a downward pressure on b and c, riz. the weight of water in each leg. And this will be equal when the legs are of equal length. [That is, of course, provided the tube is of the same bore throughout.] Suppose these pressures of the air upwards to be equal to a weight of 4 lbs. on each leg, and the pressure of the water downwards in each leg to be half a pound. Then this just amounts to the same thing as a pressure upwards on the water in each leg of 3g lbs., and the one pressure just balances the other. &p. 124. Next suppose one leg to be twice the length of the other (Fig. 46). The pressures upward as before will be equal, say equal to a weight of 4 lbs. on the water in each leg. But the pressure downward in the longer leg will be double that of the other. If the water in the shorter leg weighs ^ lb., then the water in the longer leg weighs 1 lb. This amounts to the same thing as a pressure up- wards en B of 4 lbs. less 5 lb., and on of 4 lbs. less 1 lb., or -i^ lbs. on b and 3 lbs. on c. And the pressure on b being greater than that on 0, the water is forced out at the lower end. Fill any V-shaped tube having one leg longer than the other. Place the fingers over the ends. Dip the short end into a vessel of water, and let the long end hang outside : the water will be taken from the basin in a continuous stream so long as the mouth of the short tube is below ihe surface of the water; or, if the water be received in another vessel, until the surface of the water in both vessels occupies the same level. Any bent tube having one leg longer than the other is called a niphon. THIRD M'AtiE. Ill III. Use of the siphon. U^ed by brewers and wiue and spirit mercliants for emptying easks too heavy to be lifted. 0:in be used for carrying water any distance from a higher to a lower level, passing over any elevations not higher than Fi? 47. the length of a column of water which the atmosphere will sustain. What height is this 'r The teacher may also s^li:go^t other u-os of the siphon, such as taking the clear parts of a licpiid from the thicker pans. leaviuLT the muddy parts behind; or taking a liquid from beneath the fat which may be tio.iiiug on the top, leaving the fat behind. LESSON XVIII. THE AIR-PUMP. Ariiclfs fov ilhistnitiou : ;i diagram, and. if posfible, an air-pump. I. Descriptioii. An air-punip is a machine for extracting the air from closed vessels. Fiir. -it? represents a section of one of the more simpile forms. 112 OBJECT LESSONS. R is the glass " receiver " from whicli the air has to be exhausted ; b is a brass cylinder, called the pump barrel. P is the piston, which is worked by the handle h attached to the rod r. c is a brass plate on which the receiver is made Fig. 48. to fit very accurately ; s and t are valves opening upwards like small doors, s is in the piston itself, and t is near the bottom of the barrel, a is a screw which closes the tube d when necessary. II. WorkLag. Exp. 12.5. Suppose the piston to be descending. The valve t is closed and the comprossion of the air in the barrel will open the valve s and the enclosed air will escape. ?Tow raise the piston. The pressure of the external air closes the valve s, and all the air above the piston will be forced out through the hole in the lid of the barrel through which the rod works. A vacuum would thus be made in the barrel, but the air in the receiver expands, opens the valve i, and fills the barrel. A. double stroke of the piston removes a portion of the air remaining in the receiver, because each time a vacuum is made by the piston the air in the receiver expands to fill it. This will go on until the tension of the air in the receiver is too feeble to raise the valve t. The receiver will never THIRD STAGE. 113 Vieeome quite ernptv of ;iir, although what it contains will bo exceedingly rarefied. The air being rarefied, the pressure on the internal surface of tlie receiver is but little, while at the same time the pressure on the outside is Id lbs. per square inch. Hence the receiver will he fixed firmly by atmospheric pressure on the brass plate The teacher may here explain that the action of the lungs and the tubes leading thereto from the mouth in "sucking"" the air from a tube is just the action of an air-pump. 1 he lungs are expanded, and the air in the tube expands and rushes in. The tongue acts the part of the valve, and stop.-- the mouth of the tube, whilst the air is expelled from the lungs, and the latter expand again. III. Its uses. The teacher will tell the children that the air-pump has many uses which will become apparent in future lessons. At present he may exhibit some experiments further illus- trating the pressure of the atmosphere. The following are suggested : — Ej-p. r.?(_i- Using a glass jar open at both ends as a Fiff. 49. receiver (Fig- 49 , the open end a may be covered with I 114 OBJECT LES^SOXS. sheet india-rubber, or soft bladder. As the air in the receiver is removed the effect of the atmospheric pressure on the bladder or india-rubber is very striking. If the palm of the hand is substituted for the bladder the pressure may he felt. Exp. 127. Two hollow half- spheres (Fig. 50) are made exactly to fit one another. One of the half-spheres, b, is screwed on to the plate of the air-pump, the other, a, is then placed firmly* on the top. A receiver is thus formed, which is exhausted by the pump. Turn the tap, T, to shut out the external air and unscrew from the pump. Screw on the handle H, and call upon a couple of scholars to exercise their muscles in trying to separate the half-spheres. ¥is. 50. * It 13 exceedingly difficult to get trass and glass ground ao perfectly true as to be air-tight when fitted together. A bit of lard spread over the surfaces will, however, remove all difficulty on that score. rOUETH STAGE. FOURTH STAGE. LESSON I. EFFECT OF HEAT ON BODIES (i). EXPANSION AND CONTRACTION. Articles for illustration : liladder, flajk. bottle, glass tul>e. chalk, ami nitric or sulplinric rtciii. tlieriuometer. I. Gases. E.rp. 12S. Half fill a bladder with air or gas, and place in front of the fire. It begins to swell almost at once, and is soon quite full. Why ': Hcnf crpaihh tjascs. On being removed from the fire the bladder slowly returns to its original size. Why ? Co/i/ coiifracfs gases. [The teacher should here explain that when we speak of fo/t-tube and an iron siioon. I. Liquefaction. Heat ij-j>aitd-< solids, but it does more. Tt liquefies nearly all. The teacher may take any or all of the following solids and liqiufy or melt or fuf-e them : — ice. sulphur, sealing-wax, tin, zinc, lead. The metals may be fused in an iron spoon. With sufficient beat nearly all bodies can be melted. Some bodies, such as iron, glass, sealing-wax, illc., soften before melting. The teacher may show how we take advantage of this ciixumstance in the case of iron to fashion various articles by hammering ; and in the case of sealing- wax by making an impression in the wax by means of a seal. II. Vaporization. Ei-p. 132. Heat applied to ice changes it to water. Heat applied to water changes it to a gas or vapour, which we call steam. The process is called vapoiizafion. ilany solids may be changed to vapour. In addition to water the teacher mav vaporize any or all of the following : — sulphur, mer- 120 OBJECT LESSONS. cury, alcohol, iodine, camphor, and zinc. The vapour of iodine has a very characteristic colour when seen in a flask. The vapour of zinc hums with a bright green flame. The teacher will now lead the children to see that the state in which any matter exists, whether in solid, liquid, or vapour, depends entirely on heat. Further illustrations may also be given. Thus : — Palm oil is really a liquid oil in Africa (whence it comes). In our country it has the consistency of butter. Butter is almost a liquid oil in the hottest summer weather iu this country. In winter it is quite hard. Olive oil, again, is a clear liquid in summer ; in winter it is solid. Mercury is a liquid in EngLtnd. In the coldest regions, in winter it becomes solid. Like water, it is said to be frozen. Ether is ordinarily a liquid, but under a summer's sun it boils, and becomes a gas or vapour. We may say generally that — Heat added to a solid gives a liquid. Heat added to a liquid gives a vapour Heat subtracted from a gas gives a liquid. Heat subtracted from a liquid gives a solid. LESSON III. THE THERMOMETER. Articles for illustration : mercury, tube with very narrow bore, and a Fahrenheit's thermometer. I. Our senses are not exact measures of heat. Exp. 1^3. Take three basins ; in one basin put water at about 50° and in another water at about 90°. Direct a scholar to place one hand in the water in one basin and the other in the water in the other basin for a few seconds. FOURTH STAGE. 121 N'ow pour botb waters into the third basin, and let the scholar put both hands into the mixture. To one hand the water will seem warm, to the other it will feel cold. This shows that our sense of feeling is not a correct measure of heat. Again, the atmosphere at say 50° will feel cold to a per- son coming from a warm room ; but to a person emerging from an ice-bousie it will seem warm. Or again, marble, wood, and flannel may all be of the same temp lature ; but to the band the wood will feel colder than the flannel, and the marble will seem colder than the wood. II. Howwe measure heat. The amount of expansion which bodies undergo under varying temperatures is the best measure of heat. But we must select a substance which undergoes considerable ex- pansion, and which is not easily changed into another state by either heat or cold. Solids would not be suitable. ^Vhy ? They undergo too little expansion to be easily seen. Gases are not convenient. Why? They are aSected too much by atmospheric pressure, which is constantly changing. ATe select liquids. "Will water answer well? Why not? It changes to solid ice at a temperature not very low, and to steam at a temperature not very high. Spirits of wine answers well for very low temperatures, but changes to vapour at a lower temperature than water. Mercury is not easily frozen, and does not chaxige to vapour till a high temperature is reached, and is therefore the substance best suited for measuring expansion, and thereby the amount ol beat. III. The mercurial tnermometer. Take a eapUlary tube open at one end but with a bulb blown at the other. Exp. 134. To fill the tube. Heat the bulb. The air 122 OBJECT LESSONS. expands and part of it is expelled. Plunge the open end at once into mercury. As the air cools the pressure of the atmo- sphere ■without forces some of the mercury* up the tube into the bulb. Xext heat the mercury in the bulb until it boils, when its vapour will drive out all the air and fill the tube. Plimge the open end again into mercury. The bulb and tube will now be filled with mercury. Why 't Seal the open end, by melting the glass, before the mer- cury has time to cool. To graduate the thermometer, viz. to mark the steps or degrees of heat. Put the thermometer in a vessel containing melting ice. The column of mercury falls because heat is withdrawn. When the column becomes stationary mark with a file on the tube the position of the top of the column of mercury. This point we call the /neltiiig point of ice, or, which is the same thing, the freezing point of water (Fig. •32). Xext, suspend the instrument in steam rising from boiling water. The column rises because the heat expands the mercury. TThen it again becomes stationary mark the position of the top of the column. This point is called the boiling pioint oi water (Fig. 53). We have now fixed two poiats on the tube corresponding to the boiling and freezing points of water respectively. Our next business is to divide the space between these two points into steps, or grades. In one thermometer the freezing point is marked and the boiling point lUO, and the space is divided there- fore into 100 steps. This thermometer is called the Centi- grade, viz. having a hundred steps. * T\'e cannot //o«r mercury info a capillary tuiie. BOIL ihq' Fig. 53. FOUKXH MAL.E. r; He m;iT also jho^v how to find the tempera- tui-e on one s. ale That corresponds to a given t.emperature on the other. 1S|> F.ihr.=r UH>" Cent. The thermometer most commonly used for household pur- poses in our country is called after its first maker, Fahren- heit. On this thermometer tlie tVeezinir point is marked o"^ and tie boiling cjoint JIJ. Thus the sp.ioe between is divided into ISO parts or degrct ^. Other degrees are marked (if ueces- s;iry^ above "Jl^ and below )2 The cipher (or zero) on Fahrenheit's thermometer was erro- neously supposed to represent the lowest tem- perature attainable. It is about the teiipaa- ture resulting from the melting of a mixture of suo" . or orr.^hed ice and silr. The teacher may sliow how the marks are made on the tube. It is coated with wax, and then scratches are made in the wax with the point of a needle. The tube is then placed in a solution ibydrofluoric acid) which eats into the cl.-ss. but does not ailect the wax. ¥ Fijr. 54. CEXTIGIUDE THEKMt'METEK. And I'' Cent. = i'-^ Fahr. We must remember, however, that the number which represents a Certain temperature on Fahrenr.eiT s scale does not. as on a Ceni;g:ad.c scale, represent ihe nrmiber oi dcirrees above freezinc- It is o.2° too many. Hence in changing from Fahr. to Cent, suhfi-ac: o2 jrorn the gir,H number and muJ-'^p'y hy ^ Ar.d to cLangr ir.iu Cent, to F.ibr. Hiuir,j' ihc girii iiumh')- hit f and add o'2. T'-e dejre.s are usually marked on the " frame '" in which the tube a:.d bulb : re lixcd Fi:r -341. 12 i OBJECT LESSONS. LESSON IV. FREEZING OF WATER. Articles for illnstration : if possible a thermometer tube, and a freezing mixture ; a tlieruxonieter. The teacher will elicit from the children the following general laws given in preceding lessons. 1. Bodies expand as they receive heit. That is, the molecules get farther apart, and the bodies become less dense. 2. Bodies contract as heat is subtracted. That is, the mole- cules get closer together, and the bodies become more dense. 3. That a denser body has more weight than an equal bulk of a less dense body. From this the children may be led to see — 1. That as water in lakes, and ponds, and streams cools first near the surface, the particles become more dense and sink to the bottom, forcing up the less dense, because warmer, particles from below. In this way the whole body of wtiter becomes cooled. 2. That without some change, or deviation, or special exception, from the general law that bodies contract on the removal of heat, the process of cooling under a cold atmo- sphere would go on until the entire mass got just below 32°, when the whole would be changed to ice. 3. That in frosty weather this would change all the water of our ponds, and streams, and shallow lakes to solid ice, killing the fish, and many other of its inhabitants, and m iking it probable that the heat of summer woiild scarcely be sufficient to re-convert the whole of the ice to water. The teacher may now tell the children of the wonderful exception to the general law in the case of water. Water contracts and becomes heavier, bulk for bulk, on cooling, till it reaches about 4U° Fah., that is about 8° above freezing FOl'ETH STAGE. 125 point, and then us it still further cools it expands till it gets below 3"J°, when it changes to ice.* A pint of water at 40° becomes 1-jip pints of ice at S'2°. The contraction and expansion of water in cooling from say 50'^ or 60° to o'i''^ may be shown by using water instead of mercury in a thermometer tube, and placing the bulb and tube in a freezing mixture of ice and salt. Thus it is that ice always forms at the surface of water, and there remains as a kind of coating to keep off the cold winds from the water below. "SVith a little help the children will now be able to explain — 1. ir//.v a bottle filled with water and tightly corked will burst if placed in a ft-eezing mixture. 2. W/ip water-pipes frequently burst during fristy weather.t 3. Wht/ rocks and stones often split in winter. 4. Hoir frosts pulverize the soil. 5. TVJii/ shallow water freezes over very quickly in frosty ^veather. 6. TTht/ lakes of very deep water seldom freeze over, even in severe winters. LESSON V. BOILING OF WATER— CONVECTION. Akticxes for illustration : Florence flask, narrow tube, aniline solution, large test-tube, small lump of ice ; the spirit-lamp. Refer the children back to the process which goes on during the cooling of water from the surface. * Sea w8t*r does not freeze till cooled 4° or 5° below the freezing point of fieetwiter. * (.tccasion may be taken to correct the common error that the pipes aij btirst bv the thaw. 126 OBJECT LESPOXS. The cooler particles sink down, the warmer particles rise to the top. TThy ? I. Boiling. The same process g-oes on when we boil water. The particles below are heated ; they become, therefore, bulk for bulk, lighter and rise to the surface, and the cooler particles sink down, to be in their turn made warmer and lighter. After a time some of the water at the bottom becomes changed to steam. The steam rises in bells or bubbles, which burst as they become cooled in the cooler water above. As heat is still further applied the bells cannot be cooled sufficiently to burst in the water, and so they reach the top, where the steam escapes into the air. Soon the bells ascend to the surface in increasing numbers, creating more and more disturbance, and making the well- known appearance and noive of boiling. II. Convection. Hxp. 135. The appearances above described may be readily demonstrated by boiling water over the flame of a spirit-lamp in a Florence flask. The upward and downward current may be prettily shown by introducing — before applying heat — a Little deeply coloured aniline* solution to the bottom of the flask. This is done by means of a narrow tube used as a pipette. Stop the lower end with the forefinger of the left hand ; fill the tube ; press the forefinger of the right hand on the upper end ; remove the left hand forefinger. The solution is supported in the tube. How ? Thrust the lower end of the tube to the bottom of the flask (Fig. 55) and remove the finger ; the solution flows out and colours the water at the bottom. • A few grains of cochineal thrown into the water will answer the b^ma pnrpoBe. FOURTH STAGE. 127 Place the spirit-lamp under the flame may just ^-) touch the middle of the bottom. Soon the coloured water im me- diately over the flame liecoiiics heated and risos as an upward current through the colourless water. At the same time the cooler liquid at the sides begins to de- scend to take the phice of that which 1 ises, and in a short time the descending currents are made tlie flask, so that the point of ^> v:^^•^^^, manifest to the eye by the colour in the water. In this way heat is conveyed, or carried, to all parts of the liquid : and the process is called conrcii/oii. viz, a carrying of heat. III. Water a had condnctor of heat. E.rp. lo6. The teacher may now iutroduce an iuterestiug ex- periment to show that the water iloes not carry heated paiticles downwards as well as upwards. Flit a little ice at the bottom oi a test-tube and keep it in position by a coil ot wire ' V -"■A 1 III ill 1 1 1 P FiiT. 128 OBJECT LESSONS. Nearly fill the tube with cold water, and apply the flame of the spirit lamp at a little distance from the top (see Fig. 57). The water at the top may be made to boil, while the ice at the bottom remains immelted. If ice is not to be obtained put a little aniline solution at the bottom. W7ty should we apply heat at the bottom of a vessel when we wish to boil water quickly? WTii/ does the water sometimes boil in the spout of a kettle before the main body in the kettle boils ? LESSON VI. DISTILLATION. Abticles for illustration : s small retort, Florence flask, lamp and stand. I. How vapour and steam are condensed. Distillation. ^xp. 137. Place a cold glass over the flame of the spirit- lamp. Some of the vapour produced in burning condenses as water on the cold glass. Breathe on a piece of cold slate. The vapour from the breath condenses as water on the cold slate. £xp. 138. Heat water in a retort, arranged as in Fio-. 58. The steam passes down the long neck into a Florence flask, the latter standing ia a basin of cold water. The heat from the spirit-lamp changes the water iato steam, and the cold water in the basin changes the steam back again to water. This process is called dktillatimi. If ink be placed in the retort instead of clear water, it will be found that oiAj pure wafer is distilled over. The other ingredients of the ink are left behind in the retort. Now put brine or a solution of sugar in the retort and distil. Again only pure water distils over, the salt or sugar is left behind. rorurii mmge. I--.'! "When soa-wator oxaponitos. all the salt is left behind, or the vain-wator would be salt to the taste. Fresh water cun be got out of sea-water by distillation, and many large ships now carry apparatus lor th»5 purpose. O Pistilled water is the purest form of water, but is not pleasant to drink. n. The boiling point of liquids varies. -F.riK lo9. Heat xi'iri's of in'iic in a Florenee tiask. When it hoils insert a thermometer. It will be fovmd that the top of the nierenry stuuis at about 17.2', showing that this liquid, boils and changes to vapour at 170. or about 4(1 lower than water. Ef/n r boils at about i'o' Ir can be boile■:, boils at about OoO'' . ni. Uses of distHlatioii. Distillation is chiefl\- employed to get spirits from malt liquor. ;md br.indy from wine. Those lio.uii^s contain a mixture of alcohol and water. As alcohol Jisals over at .ibout 17J~. arrangements ai-e so made that the mixture shall K. 130 OBJECT LESSOXS. not be heated above ISO"^. At this temperature the spirit distils over, leaving the water behind. As a matter of fact some of the water does distil over with the spirit, and it requires a second distillation to produce proof-spirit, a mix- ture of half water and half alcohol. Distillation is also used to separate the more volatile benzoline from the less volatile par.ifEn oil; and the latter again from the solid paraffin. LESSON VII. EFFECT 01- PRESSURE ON THE BOILING PQiNT OF LIQUIDS. Articles for illustration : Florence flask and spirit-lamp. I. Diminished pressuxe lowers, increased pressure raises, the boiling point of liquids. Exp. 140. Half iill a Florence flask with water. Boil over the spirit-lamp ; the steam will drive out and replace all the air in the fliisk above the water. Remove the lamp. Cork the flask tightly, and invert as shown in Fig. 59. When the boiling ceases let cold water flow from a sponge over the flask, and the water commences to boil again. jS^ow when the heat has been removed and cold water poured on the flask, the temperature of the water inside must be considerably below the usual boiling point of water, 212°. The explanation of the second boiling is simple. The space above the water was filled with steam, the application of cold water condensed it, so that the pressure on the water was diminished. From this we learn that under less pressure than that given by the ordinary atmosphere water boils at a lower FOVETH MAOK. ];u temperature. And the convorso of this is true : if we increase the pressure we raise the boiling point. The same hiw holds good for all liquids. E.rjK 141. Under the receiver of an air-pump see page HO), the pressure on the >Yaterni,iy be diminished almost to nothiirg, and water mav be made to boil at suchtemperatures as 70' or SO', or even lower. The presence of sails in solu- tion raises the boiling point. The teacher can slun\ this by boiling brine or syrup, plieiuij therein a suitable thermometer* to register the temperature. The boilin;; point is raised 10 or IJ" F,.h. ^ II. Practical applications fol- lowing on the facts as now demon- strated. Fia;. 59. 1. In retlning sugar, water has to be dri%"en from the svrup by boiling. Xow the boiling point ot syrup is about -■^0". and at this temperature it is apt to got burned and discoloured. To avoid this the syrup is put into closed vessels, from wbich the air and vapour can be drawn off by a pump. In tins way. by removing pressure the syrup is boiled at 1">0-. and the risk of burning is avoided. '2. In the boiler of a steam-engine the pressure of the steam on the water is very great. If ten times as great as the ordinary pressure of tlie atmosphere, the boiling point rises to ooCf^ Fab. 3. Ti.e pressure of the atmosphere on high moar.tains. as * Vi2.. a thermometer wliicli reiristers temperature? .^bove f 12'\ 132 OBJECT LESSONS. we have seen, is considerably diminished, and water boils at a lower temperature. On the top of Mont Blanc, for instance, water boils at about 160°, and this temperature is not high enough to cook potatoes, or an egg. The potatoes will not get soft, and the egg will not harden. LESSON VIII. STEAM AND THE STEAM-ENGINE. Articles for niustration : diagram of steam-engine. I. Steam is highly elastic ; hence has great expansive force. The teacher may introduce this lesson by eliciting from the scholars any facts about steam — how it is produced, its chief properties, how it resembles air, and so on. He should next direct attention to the most important property of steam — its elasticity or expansile force, the property on which depends its use in the " steam-engine." The rush of steam from the steam-engine gives some notion of the great expansive force of steam. If water is boiled in a vessel closed quite tight with a cork or lid, either the cork or lid will be blown out, or the vessel will burst into pieces. Great iron boilers are some- times burst into thousands of fragments by the expansive power of steam. Under the ordinary pressure of the atmosphere a cubic foot of water when changed to steam occupies 1,700 cubic feet of space. That is, a cubic foot of water will produce sufficient steam under the ordinary pressure of the atmo- sphere — 15 lbs. to the square inch — to fill as nearlv as possible a boiler 12 feet long, 12 feet wide, and 12 feet high. Suppose the top of the boiler could be so arranged as to work up and down like a square piston in a square box, FOURTH STAGE. 133 tlieu at this volume it would remain starionary: that is, there would be a pressure downwards on the lid equal to 15 lbs. on every square inch, or nearly a ton on each square foot of surf'aoe. and there must be also a pressure upwards or expansive force in the steam of exactly the same power. Now if a ton weight be placed on each square foot of the lid the latter will be pressed half-wav down the boiler and the volume of the steam will be one-half its former volume, but its expulsive force is doubled, as we see from the fact that it supports double the weight. The same effect of doubling the expansive force is produced if we put double the amount of steam in our boiler. And, generally, we may say that the more steam we can get into a boiler the greater is the pressure or expansive force of the eoutined gas. If we heat water in a boiler there is no limit to the expansive force of the steam produced except the strength of the Willis of the boiler itself II. The steam-engine. The steam-engine is a machine constructed to utilize the expansive, or el.istie force of sreim. Ihe teacher may illustrate the principle of the steam- entjine by m iking sketches ^Figs. 60 ^ and 61) on a blackboard, a B is a ^ cvlinder in which the piston, p, works "^-T up and down, or backwards and for- wards, but not quite to the ends of the cvlinder; r? and care tubes communi- cating with the air, but fitted witli ^ stop-cocks ; b and d are tubes eomniu- ^ nicating with a boiler K. These tubes are also fitted w4th stop-cocks. Let US suppose all the stop-cocks to be closed, the piston to be at the bottom of the cylinder iFig. (iO), and the boiler T-^ FiiT. CO, 134 OBJECT LESSONS. D Fig. 61 B to be full of compressed steam. Open a and d. The steam rushes in at in a ijrcaft'r r.ifio t/i.rn thf temperature. Suppose we have three cubical boxes of air each o feet in the side, and therefore containing each 12-3 cubic feet of air. AVe will further suppose that the air in each i^^r.fumft'd with moisture, the first at a temperature of o," F,ibr., the second at &2°, and the third at p-J-^. If the air at o2^ contains an ounce of water- vapiour, then the air at ti2" contains t:ro oun cs and the air at ;'J' contairs /'.)»»■ ouiicfs. That is. in tbe three boxes of air. containing in all o75 cubic feet, there are 6t'ri» ouiias of water. Let us ncreo to mix the ;ur in these three boxes. Then we sliall haveoTo cubic iVot of air at 62^ [because 6"J'^ is the average of 30•-^ 6'J-\ and 90-\] How much w ater will ST-i cubic feet of siir contain at 6C" r Cle,irlv^;j- oui.cx^, because 123 cubic feet can hold only two ounces. But we have seen that, before the three boxe* are mixed, thev contain tOi^ether «hvh ouihys, hence when mixed they give 148 OBJECT LESSONS. up one ounce. This owe ounce is squeezed out, because there is no room for it, and it becomes lirst a mist and then rain. It must not be supposed that air is always saturated with water- vapour. This is far from being the case, or rain would be constantly falling at every slight change of temperature. But air at varying temperatures and holding varying amounts of water- vapour is being constantly mixed by winds, and whenever the mixture becomes cooled below its point of saturation, the excess forms mists (clouds) ; on further cooling the tiny particles of mist join together and form rain. But if the mist becomes cooled below '62° Fahr., then it is frozen into snow. II. Claestions on interesting facts coimected with the formation of rain. 1. Why does rain fall in drops ? Because the piirticles attract each other, and those that are near combine and form drops. 2. How is it that the cold night does not ahcays cause rain? Because the air is not always near saturation, and it can thus be chilled, and yet hold its vapour. 3. What is snow ? Condensed vapour frozen by contact with air below 32°. 4. What is hail ? HaU is rain frozen by passing through a stratum of air below 3'2" Ftihr. in its descent. 5. How is sleet foi'tned ? Sleet is formed when snow in its descent passes through a bed of air above 32° Fahr. The snow is partially thawed and falls in a haU- melted state. 6 WTiy is there no snoiv in summer time? Snow is formed ia the upper regions of the atmosphere as well in summer as in winter, but in summer it becomes melted in its descent through warm air. When rain falls in the valleys in Switzerland in summer, the tops and sides of the moun- tains often receive a coating of snow. FOUKTH SXAGB. 149 LESSON XV. SPECIFIC HEAT. Articifs for illnstration : 4-oz. pieces of lead :iTid iron, 4 ozs. of me> cviry, a small tin vessel, a thermometer, and some app;u-aius tor boiling w^ater. I. What is specific lieat ? IZrp. 149. T;tke equ;il weiglifs. s;it 4 ozs., of lend, iron, and mei-cury. and beat them for some time in boiling water. [The mefcury may be held in a test-tube.] The tihree metals will have their tetuperatures raised to 01'3"'\ Xext take three vessels, each containing say 4 ozs. of water at the ordinary temperature of the room, say 5-5~^, and transfer the metals to these vessels. Each of the solids will, of course, raise the temperature of the water ; but they will not raise it equally. The lead will r.iise it leasr. the mercury will raise it a little higher than the lead, and the iron considerably more than either. We inav now add 4 ozs. of w.iter at 21 -"^ to 4 ozs. atoo"-^. and we shall find that the hot water nvises the temperature of the cold fiir more even than the iron. "What do we leiixn from this e:speriment ? TTe learn that ir bodies can hold more Iha^ ^' an ofhrrs. The iron evidently retained more of the heat got from the boiliiiiT water than the lead, for it gave more to the cold water. If lead takes up less heat than the iron, we should expect that it will take /f-^i^ titiw to reach a certain temperature than iron. Av,d this is so. £'_'-;'. 1-30. Take the lead first: put it in a tin vessel and hold it over boiling water till it reaches a temperature of say ISO". [To test the temperature, keep the bulb of a ther- moine:er touching it.] Xote the time. Now test the iron 150 OBJECT LESSONS. in the same way and note the time. As tlie iron requires more heat to raise it to 180°, it takes considerably more time. The teacher may also test the converse by noting the time it takes each metal to cool. The lead holding less heat cools first. Ex]}. 151. We can show that some bodies hold more heat than others in another way. Take an ounce of water at 112°, and an ounce at 40°, we get two ounces at 76°, as may be shown by the thermometer. Now take an ounce of mercury at 112° and an ounce of water at 40° ; we have a mixture of two ounces, but the temperature will be only about 42°. The hot water raised the cold through 36°, while the hot mercury raised it only about 2". Bodies, then, differ in their power of taking in and holding heat, and the amount of heat which a given weight of a body takes in to raise through a given range of temperature is called the specific heat of the body. A pound weight, and one degree are taken as the units. II. The importance of water having a Mgh specific heat. The ocean covers four- fifths of the earth's surface, in some places to the depth of several miles, and this forms an enormous storehouse of heat. It takes up an immense quantity of heat without rising much in temperature, and yields it up again when required, without itself being lowered much in temperature. The great specific heat of water is therefore the chief agent in equalizing the temperature of the globe. FOURTH STAGE. 161 LESSON XVI. LATENT HE\T. AKTTCT.rs for ilhistiwtiou : Floreuce-flajk. spirit-lamp, lurapg of ice, rherniometer. I. What is latent heat ? E.rr. 15C. Take a Florence fla^lc, half fill witli water, and lieat to boiliuir point over the spirit-lamp. Xote the rise of temperature by setiing a thermometer in the flask. The mercury rises steadily in the column till it stands at 21"..''^. At fliis point it is stationary, and no amount of heat further applied under the ordinary pressure of the atmosphere will make the water rise above '21^''^. T\ hat then becomes of the heat we continue to apply during the boiling of water ? TVe answer this question by asking another. What change does the continued applica- tion of heat to water at 01'2'-~ bring about ? Clearly the change in the sfiirc of water from liquid to gas. T/ie Iieai. then, is uft'd up in t/ie ^voccSs of changing tratcr fro'ii the liquid to the gaseous g-^ate. But the sro.nu is no hotter than the water. The heat seems to disippear ; at any rate it has no effect on the thermometer, and hence we call it hiddi n, or htcnt heat. E::-i. 133 Xow half fi.l a ve'isel with cold water, and put in a few lumps of ice. With a thermometer again note the chanire of temperature. The column of mercury sinks gradually to ■I'J'^, where it remains until :ill the ice is melted. Ercn on the ajwUfiifioit of h(.,t. the water shows no increase of temperature until the ice has disappeared. If ice or snow at. s.iy •20\ be placed over a fire, the ther- mometer will show an increase of temperature till the mercury reaches 3'J~, bM tU'.re it will stand so long as ice 152 OBJECT LESSONS. or snow remains ; but wlien all the snow and ice are rneltecl. the temperature gradually rises till it reaches 212°. Here, again, much heat is consumed in the process of changing water from the solid to the liquid state, and the heat used up does not affect the thermometer. It is hidden away or made latent. II. Latent heat of water. The teacher can next give the children an idea of how much heat is used up, or made latent, in changing ice to water, and water to steam. Exp. 154. Mix an ounce of water at 32° with an ounce at 174°, and we have two ounces of water at 103° ; but mix an ounce of pounded ice at 32° with an ounce of water at 174°, and we get two ounces of water at 32"^ ; that is, no less than 142° of heat have been taken from the ounce of water to melt the ice. Of course, a body in falling through 142° of tem- perature must give out just as much heat as it takes in in rising through 142° of temperature. We may say, then, that the amount of heat required to melt a pound of ice is equal to that required to raise a pound of water through 142°, or equal to that required to raise 142 lbs. of water through 1°. Thus the latent heat of water is said to be 142°. III. Latent heat of steam. The children will have noticed, probably, how much longer it takes water to " boil away," viz. change to steam, than it does to raise it to the boiling point from zero — about five times as long. The teacher may show this fact by experi- ment ; but an experiment to show the absolute amount of heat made latent in the change of water to steam will probably have to be described. An ounce of steam at 212° — in other words, an ounce of water changed to steam — if passed into 298 ounces of cold FOUKTH STAGE. 153 water will raise its temperature 1°. That is, the latent heat of steam is 298°. In this case we have recovered the heat which was latent. DiiFerent bodies vary very much in the amount of heat they make latent on passing from solid to liquid, or liquid to gas. Thus the latent heat of alcohol is less than one-half that of water, while that of ether is less than one-sixth. IV. The advantages we derive from the high latent heat ol water and steam. 1. It takes a considerable quantity of heat to melt ice. and hence it takes a considerable time to complete the chanLre. If it were not so the winter ice and the snow of the mountains and high valleys would melt too quickly, produc- ing overwhelming tonents and Hoods. 2. Similarly in the case of steam, if it were generated too quickly, we should be much more liable to dangerous explosions. The teacher may with advantage stiU further enlarge on the special properties of water as tending to prevent sudden cliangcs of innperature. LESSON XVII. COOLING BODIES AND FREEZING MIXTURES. Articles tor illustratiou : fiii.ill quantity of etlier, snow or ice, am- monium nitrate and chloride, and thermometer. This lesson consists of interesting applications of principles enunciated in former lessons ; and, given the facts as shown by experiment, the reasons may be elicited by questioning. I. Cooling bodies. E^p. lob. Poiu- a little ether on the palm of the hand ; 154 OBJECT I.ESSOKS. it quietly evaporates and produces a painful sensation of cold.* " In what state was the ether when I poured it into Tom's hand ? " In a liquid state. " In what state is it now it has gone away into the air ? " In the state of vapour. " To change a liquid to a gas or vapour, what is neces- sary?" Seat. " "Where does the liquid ether obtain its heat ? " From the hnnd. "Then what makes the hand feel cold?" The heat is taken away to change the liquid ether to vapour of ether. On this experiment the teacher may deal with the following : — 1. Why we put aromatic vineg-Lir and water on the head when it feels hot and feverish. 2. How perspiration cools the body. 3. Why ladies use fans in hot rooms. 4. Why a windy day feels colder than a still one when the temperature, as shown by the thermometer, is the same. 5. How a shower of rain cools the atmosphere. 6. Wliy sprinkling a floor with water cools a room. II. Freezing mtstores. Exp. 156. Make a mixture of three-pnrts by weight of snow or pounded ice, and one part of crushed common salt. The two solids liqiiefy, and if a small bottle of water is placed in the mixture the water will become frozen, even if the mixture is placed iu front of the fire.t If tested with the thermometer the temperature will be found many decrees below freezing point. * Put a drop or two of water on half an ounce of carbon disnlphicie in a sh:illnw vessel. P!acejn current of air. The water changes to ice. t Hake a mixiuio rf two pnrts by weight of pulverised ammoniuui nitrate and one part of ammoniam chloride, and dissolve in three parts of wat^r. Stir the mixture with a test-tube containing a liitle water. The water in the test-tube will be Iruzen. FOURTH STAGE 155 " 111 what sfnfe were the snow and salt before mixing P " Li thf soUd state. " T^'hat change was brought about on mixing f " They were changed to a liquid. " T^'hat was necessary to produce the change ? " Seat. "Whence could the bodies obtain their heatP" Only from tlicDiselres and the vesxel in tchich ii/aced. " How can you show that the heat was abstracted from the bodies themselves ? " Because they became much colder. " If you put your finger first in the freezing mixture and then into some snow at about 82°, which will feel the warmer, and why P " The snow, because none of iti heat has been taken airay. On this experiment the teacher may ask : — 1. JJliy people should not throw salt on the pavement in tVosTv weather to melt the snow or ice. 2. l'7jy the air often feels cold and chilly when a thaw sets in. 3. IV/iy the temperature often rises after a fall of »uow. LESSON XVIII. SPECIFIC GRAVITY OF SOLIDS, Akttcles for illustratioii : balance, and speciinens of same size for wcicliiiig, a short tube of lead with solid cylinder to tit exactly, or an eqiuvaleut The teacher will refer to the lessons on gravity and buoj- aucv of liquids as an introduction to this lesson, and then tell the class that it is very convenient for us to be able to compare the weights of equal volumes of different bodies. L Standai-d of comparison. To be able to compare the weights of bodies we must make some one body the sfandu/d for cumpali^Ja, 156 OBJECT LESSONS. Take pieces of, say, wood, cork, and lead of equal bulk, and ask the children to compare their weights. They will say at once that lead is heavier than wood, and wood than cork, but how much heavier they cannot say. Distilled water at a temperature of 39°* is the standard of comparison used in this country. II. How to compare the weight of any solid with the weigh tof an equal hulk of water. To show this roughly, but, at the same time, very clearly,, the teacher must arrange to measure a bulk of water equal to the bulk of the solid selected for experiment. For instance, a half inch lead tube two or three inches long, firmly closed at one end with a piece of hard wood. A solid cylinder of lead to fill this tube exactly may be made by pouring in molten lead, or a small earthenware vessel may be filled with melted sealing wax, or a solid glass rod may be found to fit with tolerable exactness into a glass tube. Example 163. Suppose we take lead for the experiment. Drop the solid cylinder of lead by means of a piece of thread gently down into a tumbler filled with water to the brim, and collect the displaced water which runs over into a vessel below. Pour the collected water into the lead tube ; it very nearly fills it. It would quite fill it were it not that we lost some on the outside of the tumbler and on the basin below. Now I weigh this water. Say it weighs 1 oz. Now weigh the lead, first in the ordinary way. It will weigh a little over 11 ozs. Now weigh it in water. It weighs a little over 10 ozs. That is, it loses a weight of 1 oz., or the weight of the water displaced. [For method of weighing in water (see Fig. 63). a is a weight equal to the scale-pan which has been removed.] * At this temperature water is at its maximiim density ; that is, it is heaviei at this temperature than at any other. FOURTH STAOE. 157 From these experiments we learn — 1. That when placed in water a solid displaces a volume of water equal to its own volume. 2. That a solid loses weight when weighed in water. Fig. 6i. 3. That the weight of water displaced is equal to the difference between the weight of the solid in air and in water. In the case of the lead the weight of the lead in air was a little over 11 ozs., and the weight of a volume of water equal to that of the lead was 1 oz. Clearly therefore lead is a little over eleven times the weight of water. And this is called //.v own wcigJd compared icif/i iratcr, or its .specific gradty. If glass had been used instead of lead, we should h;ive found its specific gravity to be 2i. That is, glass is two and a half times as heavy as water. Platinum, the heaviest of metals, is twenty-two times, and gold nineteen times, as heavy as water. Hard oak is a little heavier than water, the other common English hard woods a little lighter than water. Cork has a specific gravity of j. If the children are sufficiently advanced the teacher may from the above experiments deduce the rule for finding the specific ij-ravity of solids heavier than water. Diride tht. icright ill air hy the loss of iceight in water. The quotient ts the speei_fie gravity. SXTPPLEMETiTTART LESSONS ON PROPOETION. In Stage V. many of the lessons, especially those on machines, require a clear understanding oi proportion. If the Stage coincides with Standard VI., as is most likely, it may be desirable to rationalise the proportion that comes under the head of arithmetic. A sense of proportion is also one of the most important developments of human faculty, florals (science of what is due) depend largely on it. Esthetics are scarcely anything else than a fine sense of proportion, e.g., in architecture, music, &c. The comfort of social life depends on " give and take," and this is taught only by an inherent sense of proportion. For these reasons the subject ought to have a more prominent place in school teaching. And as in its most elementary form proportion appeals to the eye and to other senses, its apprehension may be greatly assisted by object lessons. LESSON I. Articles for ilhastration : Two shades of red and two shades of Hue colours ; blocks of wood, or substitute, weighing respectively 6 lbs., 3 lbs.. 2 lbs., and 1 lb. . = s r J 1. Comparison of Colours. Set before the class two patches of some colour — red, for instance — one light and one dark. Mark them a and b. SITPPT.E5IENTART LESSONS. 15P " T\Tiat colour is the piece marked aP" Red. "And the piece marked B ? " Red. " Do you see any difFerence in the shade of colour in the two pieces ?" Tea, that marked b u darker than the one marl.i'd a. Nw.t take two shades of another colour, say blue, and mark tL eiu c and D. " What diderence do you note in these two colours which I have marked c and D ? " d is darker than o. '■ Well, now can you tell me how much darker b is than A, or D than c ? " Xo. " y.0, because you have no means of exactly measuring ^ colours." '' Can yon tell me whether D is as much darker than c as B is darker than a ? Xo, and for the same reason — you have no means of exactly measuring colours. If you could say that B was twice as dark as a, and that t> was twice as dark as c, then you could answer, Tes, d is as much darker than c as B is darker than a." " We will now take auuther sort of comparison which you can measure." II. Comparison of Weight. Take four blocks of wood, weighing respectively 6 lbs., 3 lbs., 2 lbs. and 1 lb. [Of course, weights of any more con- venient substances may be substituted. ] Direct a boy to compare as best he can, by holding in his hand, the weights of the larger pieces, and then of the smaller pieces. Xow weigh the blocks, and lead the children to compare their weights. Thus, the weight of the first block is twice the weight of the second, and the weight of the third block is twice the weight of the fourth. ill. JJatio. " What difference or relation is theie between the first 160 OBJECT LESSONS. block and the second as to their weights?" The first is ttcice (or 2 times) tJie second. * " The number 2 then represents the relation between 6 lbs. and 3 lbs., and we call 2 the ratio of 6 lbs. to 3 lbs." " Now what is the ratio of 2 lbs. to 1 lb. ? Clearly 2, because the first is twice the second. That is, the ratio of 6 lbs. to 3 lbs. is equal to the ratio of 2 lbs. to 1 lb. The teacher may now explain (1) that to find the ratio of two numbers we always divide the first number by the second. The quotient is the ratio. (2) That relation can exist only between things of the same kind. There can be no ratio between lbs. and feet, or between cats and shillings.* Questions such as the following will fix these ideas in the minds of the scholars. 1. What ratio is there between — (1) 12 lbs. and 6 lbs. ? Am. 2 (2) 6 lbs. and 12 lbs. ? „ i (3) 3d. and 9d. ? „ i (4) 8d. and 2d. ? „ 4 (5) £10 and £1 ? „ 10 (6) £2 and £9 ? „ 2 2. What is the ratio of — (1) 5 pks. to 2 pks. Am. (2) 19 yds. to 57 yds. (3) 21 horses to 7 horses (4) 16 feet to 5 feet (5) ^ inch to 5 inch (C) 3 cows to £6 3 34 Impossible. Seo note, page 162. SUPPLEJlEXrARY LESSONS. 161 LKSSON II. I. Fnrtlier Illustrations of ratio. Draw squares on the blackboard, the first, A, 2 feet in the ^ide, and divide it into 6-inch squares ; the second, b, and A I D u the third, c, each 1 foot in the side, and divide into 6-inch squares ; and the third a square 6 inches in the side. The scholars will see at a glance that the square a is four times the area of the square b ; and also that the square c is four times the area of the square d. That is, the ratio of a to B is 4, and the ratio of c to d is 4. In other words, we may say that the ratio of a to b is the same as the ratio of c to d, or there is an equality of ratios. Take another example. Draw three equal squares, and then nine of same sizf arran"in>r them as in Fiffs. a and b. In this example clearly the ratio of a to B is |- K^ow draw two lines and divide as in l' and D. or 162 OBJECT LESSONS. 12 In the second example the ratio of c to D is -ya" OJ" A.nr 1 «• That is, the ratios in each case are the same. Here is again an equality of ratios. II. Proportion. Now, when there is equality of ratios, the numbers which represent area, or weight, or length, or anything else, are said to be in proportion. If 10 men can dig 15 acres in a certain time, 5 men, working at the same rate will dig 7-i acres in the same time ; the quantities of work done are in proportion to the number of men employed. The ratio of 15 acres and Tj acres is 2, and the ratio of 10 men and 5 men is 2. This may be written — 15 acres is to TJ acres as 10 men is to 5 men. Or in short — Acres Acres Men Men 15 : 7i :: 10 : 5 Again, if 7 men earn £3 3s. (viz. 9s. each) in a given time, 6 men should earn £2 14s. (viz. 9s. each) in the same time. The ratio of bas. to 54s. is |-f = |-, and the ratio of 7 men to 6 men is ^. 8. 8. Jfeu il^n or 63 : 54 :: 7 : 6 Now suppose the 6 men receive only £1 7s. (or 4s. 6d. each) for their work, this would be unfair, it would be out of proportion ; work and pay should be on the same proportion. The teacher may show this as follows : — The ratio of 68s. to 27s. is \ or 2\, aud the ratio of 7 men (O 6 men is \ or 1^. STJPPLEMENTAItT LESSONS. 163 There is no eq^uality of ratios, and hence there is no pro- portioiu LESSON III. I. Terms. " What is the ratio of 3 to ? " f or li. " What is the ratio of 6 to 4 ? " f or 1|. The ratios being equal, we have this proportion — - 3 : -2 :: 6 : 4 Tell the children that the numbers which form the pro- portion are called terms, and they are named Is;", '2iid, ord, and ith, in the order in which they are placed. Thus — 1st term : 2nd term :: 3rd term : 4th term. The 1st and 4th are called ejrh-ciiie terms, and the 2nd and 3rd are the mccui terms. IL The product of the extremes equals the product of the means. The teaeber may show this by taking any number of actual examples, or by some such method as the follow- ing :— Draw lines a, b, c, d. I i i i A I I I B III'' D Thus A represents 3 units, B 2, c 6, and d 4. Then a : b : : c : D, because 3 : 2 : ; 6 : 4. Xext draw two parallelograms with sides A and d and B c respectively. 164 OBJECT LESSON& D B 1 The parallelogram A D is equal in area to the parallelo- gram B c. Note that the length of the sides a and d are the two extremes in the above proportion, and b and c the two means. Show that these figures are equal in area, each containing 12 equal squares. The parallelogram formed by the lines which represent the extrevies has the same area as the parallelogram formed by the lines which represent the means. 3 X 4 = 12 and 6 X 2 = 12 Where proportion exists the product of the ejctremes is equal to the product of the means. An example. If 24 yds. of cloth cost 96s., what must I pay for 19 yds. it the same rate ? Here the first ratio is — 24 24 yds. : 19 yds. = |q And the second ratio is — 96 96s. : a s. =^ — . , 24 96 , A.nd rn ^ "T") o'" *''** terms would not be in proportion. SUPPLKMENTABT LESSOKS, 165 We may write the proportion — 24 : 19 :: 96 : a; Or 19 : 24 :: a; : 96 And 24 a; = 96 X 19 . , _ 96 X 19 _ ,p ..X 76s. in. Given tliree terms of a proportion, to find the foartli term. Take the proportion 3 : 4 : : 6 : 8. ^1.) To find tile first term. Put x to represent it. Then a; : 4 : : 6 : 8 „ 8^ = 24 X = 3 (2.) To find second term — 3 : J- :: G : S 6 a; = 24 a- = 4 (3.) To find third term— 3 : 4 :: a; : 8 4 a- = 24 a; = 6 (4.) To find fourth term — 3 : 4 :: 6 : .:; 3 a- = 24 X = S LESSON IV. The teacher may further illustrate the idea of proportion in a variety of -n-ays. The foUowinar are suiiLi^cstions; — 166 OBJECT LESSONS. (1.) Rates are paid in proportion to the yearly value of the house. Value of Value of large house J small house '. '. large rate ; small rate £100 : £20 :: £5 : £1 (2.) The value of men's -work being equal, wages are paid in proportion to time worked. longer time : smaller time '. '. larger wage : smaller wage 20 dys. : 5 dys. : : £5 : £li (3.) We say men are ^cell or ill proportioned. What does this mean ? We have an ideal in our minds which pleases the eye, and with this we compare. Say, for instance, that a 6-foot man has an arm which measures 32 inches, and this to the eye is a pleasing ratio (72 ins. ; 32 ins). Then a man measuring 5 feet 3 inches (or 63 ins.), should have an arm 28 inches long to be in the same pleasing ratio : — For 72 ins. : 32 ins. :: 63 ins. : 28 ins. Suppose the arm of second man to measure 30 inches, then we should say it is too long for the body or the body too short for the arm ; the lengths of the body and the arms are not well proportioned. (4.) This kind of comparison is of constant recurrence. (a.) Size of clothing must be proportioned to the size of the wearer. (b.) Work assigned must be proportioned to the strengh of the worker. (c.) The strength of difEerent parts of the bodies of animals is proportioned to the work they have to do. The neck of the elephant, for instance, is proportioned to the size and weight of the huad it has to carry, and the work the trunk has to perform. SUPPLEMEXTART LESSOXS. 167 (d.) The empire of man over the brute force of the lower animals is proportioned not to his strength, but to the knowledge be possesses of their re- spective constitutions. (e.) In architecture there must be a just proportion between the parts. (,/'.) In sculpture, and in painting, there must be a just proportion of the several parts to one another and the whole. If further elucidation is necessary, the proportional com- passes will be a good subject for another lesson. FIFTH STAGE. FIFTH STAGE. LESSON I. FORCE. ARTicirs for iUn=tr:Ttipii : a g'ass tube and pith ball andapiece of sit, or a magnet and iron tilings. I. Definition of force. Fjtlk 164. Rub a dry glass rod bristly 'witb a ^flrm silt handterchief, and then present one end to a pith* ball suspended by meaits of a silt thread. The ball is at first drawn towards the rod, but after touchiag it is pushed away. H.rp. 165. Or, srrew a few iron filings on a sheet of paper placed over a bar, or horseshoe, magnet. The filings arrange themselves in lines diverging from the ends of the ma gnet. ^v^cw what causes the pith l^all, or the iron filin gs to move ? To this question we can give no s;itisf.ictory answer. TTe know that the objects move, and we are quite certaia there must be a cause ; but there our knowledge ceases. This cause we call &/oree. When a body "jores we tnow there must be a cause, and when a body in tnofioi) is brought to rest we know there must be a cause. When there is a change in the motion of a body there must be a cause. When a body is held in a particular jcsiticii we know there must be a cause. • Cm from the pith of a branch of the eldei-ota. 172 OBJECT LESSONS. We attribute motion, and rest, and change, and position to some force. Force w that irhich can produce, change, or destroy motion. II. Kinds of forces previoTisly described. Having demonstrated in this simple way what we mean when we speak of force, the teacher will assist the children to recall to their minds such forces as have been described in preceding lessons. These are : — 1. Force of cohesion. 2. Force of adhesion. 3. Force of capillary attraction. 4. Force of gravity. A few questions will serve to refresh the memory on the chief points connected with cohesion, adhesion, and capillary attraction ; but before proceeding to deal with another force — the chemical force — it will be well now to explain the phenomenon of gravitation a little more fully. III. Attraction of gravitation. It is a law of nature, so far as we know, that all bodies, great and small, attract each other ; and, if they are free to move, will move towards each other with increasing swift- ness until they meet. Thus (Fig. 6o) A and b are balls of equal size and Fig. 65. weight. Hence they attract with equal force, and would meet at c, a point midway between them, if there were no preventing cause. But the fact is the earth attracts the balls, and being so many times larger attracts with so much greater force that FIFTH STAGE. 173 the balls have no power to move except towards the earth. And this is the case generally. It is not that the earth simply draws all bodies on its surface to itself; but that all bodies pull the earth towards themselves just as the earth pulls them. Only that the earth is so many millions of times larger thivn the largest body on its surface, that the effect of the pull of the latter cannot be felt or measured. It is as though an elephant were pulUng at one end of a rope and a fly at the other. [It may be interesting to the children to learn the rapidity or ivlociti/ of a body moving towards the earth. It moves through 16 feet in tie first seeond : for every suc- ceeding second it moves with a greater velocity. In the second second it travels 3 X 16=: 48 feet, in the third second 5 X 16 = SO feet, in the fourth second" X 16 = Hi feet, and so on. If a stone be dropped from the top of a tower and it occupies two seconds in falling, the height is 2 x ~ X 16. If three seconds 3 X 3 X 16. If foui- seconds 4 x 4 X 16, and so ou.J LESSON II. CHEMICAL ATTRACTION. Articles for ilhiftration : the articles will, of course, depend on the experiments selected. The first experiment is the most striking, but i( requires a little c;ue. H.rp. 166. Place "flowers" of sulphur in a small flask, and drop in a few bright copper shavings. Heat the mix- ture over the spirit-lamp; but place the whole apparatus in a p.'i.n or tub with a little water at the bottom in case the flask should break. Heat gently. The sulphur melts, then blackens and boils. The copper now becomes red hot, when the lamp 174 OBJECT LESSOX^S. may be removed. The copper and sulphur unite together to forai an entirely new substance, giving off iatense light and heat in the pro- cess. Exp. 167. On a small bit of phosphorus* place a few grains of iodine. The two combine and burn with a smoky flame. Exjj. 16S. Pass carbonic acid g^s through lime - water (see Lesson XIII., page 60). The gas combines, with the lime in the water to make chalk, which sinks to the bottom as a white powder. Exp. 1&). Put a small piece of the metal sodium on water in a broad shallow vessel. The metal becomes spherical in shape and runs about over the water but gets less and less until it finally disappears. The metal takes some- thing from the water, of which we shall learn more in a future lesson ; and forms with it a new substance which dissolves in the water. Exp 170. 3Iix ammonia gas with hydrochloric acid gas (see Lesson VII , page 49) ; a new substance is formed quite different from either of the gases. From one or more of these experiments, or from some similar experiments, the teacher will lead the children to see that some substances have such an attraction for each other that when placed together they unite to form a separate and distinct substance. Here, then, we have another force, which is called chemical affinity, or chemical attraction. When placed close together under favourable circumstances many substances unite to form others. In the first experiment heat had to be applied to start the combination of the sulphur and copper, and we may say * Phosphorus should always be kept under water. When a small piece baa t J he cut off hold th^ stick with wet hlottiug-paper. FIFTH STA&E. 175 generally that heat brings about and promotes the chemical combination of sabstances. Sometimes, however, heat will sepnrate the substances which are joined to tbrm another substunL^e. JS.rjK 171, Heat, a little of the red oxide of mercury in a test-tube. A gas is given off, whose presence is detected by the re-kindling of a glowing chip of wood placed in the tube, and liijuid mercury is left behind. LKSSON III. ELEMENTS AND COMPOUNDS. Articies for illustration: dioxide of maiig^uicje, hyib.HUoric .-jcid, copper toil, or powdtaed aiitimouy. I. Elements and compounds compared with letters and words. The children will most readily grasp the meaning ot e/tmeiitg and coiiijwuiids in the language of chemistry by a comparison with the /oit.cr. 182 OBJECT LESSONS. Fxp. 177. Fill the bottle with water, and over the jmen- matic trough allow hydrogen to expel two-thirds of the water, and oxygen the remaining one-third. The bottle will then contain a mixture of hydrogen and oxygen in the pro- portion of two to one. "Wrap a duster round the bottle, remove from the water, and apply quickly a lighted taper. A smart explosion follows. The gases combine, and form water.* LESSON YI. COMBUSTION. Articles for illustration : mat of iron, and charcoal or other articles to show comhustion. I. Meaning of comhustion. We saw in the last l?sson how freely oxygen and hydro- gen unite, and that in the act of combination they give ofiF heat and light. Bodies which, when they combine, give off light and heat are said to undergo comhustion. Or to put it in another way, we give the name combustion to the nnion of two or moi3 substances when in the act of combination they give off light and heat. TVe say that hydrogen burns, but it burns only Trhen it can combine with oxygen, as in the air. It would not burn in a bottle of carbonic acid gas for instance. Oxygen does not burn in the air, because it cannot combine with itself, or with the nitrogen mixed with it in the air. But oxygen causes combustion in an atmosphere of hydrogen, just as hydrogen bums in oxygen. In nature, however, oxygen is found free everywhere, whilst hydrogen is never found uncombined, and so it comes about that when we speak of combustion in common lan- guage, we always mean the combination of some body or * If the teacher can command the apparatus for decomposmt; wnter, he will he able still hett r to show the compositiou of water. FIFiH STAGE. 183 otlier with (tsypren. Oxygen, therefore, is said to support comhiistioix Sometimes in chemistry the word comlnisrion is used in a wider sense. Thus the oxygen of the air combines with iron, and forms a reddish-brown powder which we call rnsf, and the iron is said to he slowly i'Oi:siimed, althouLrh it gives off no light and no appreciable heat. [Show the rust of iron and compare its properties with those of its elements, iron and oxygen.] Again, the oxygen of the air taken into the body through the lungs combines with the waste matter, and in the act of combination gives oft" heat. This conibi -ffuvi of the wastt matter is the source of all the heat of the body. n. The common prodnets of comhustion. TTe have said that charcoal, or carhoii, is an element. What do we mean when we say that charcoal burns? VTe mean that it combines with the oxygen of the air. and that in the act of combination it gives oft" heat and light. And what new compound does it form ? If we burn charcoal in oxygen over lime-water we shall find that the water becomes milky, showing that the compound is the carbonic acid lt.is de- scribed in a former lesson. You may remember that carbonic acid gas is harmful to breathe in large quantities, and some- times people have committed suicide by shutting themselves in a close room, and breathing the fumes of burning charcoal. A larire part of our coal, and wood, and oil, and coal gas consists of c^irbon, and hence wherever these are burned car- bonic acid gas is made. N\ hen hydrogen was burned we saw that water was made, and a large part of our fuel and lights being composed of hvdrogen in combination with carbon, or with carbon and oxviren. whenever we have a tire or a light from oil or gas we are manufacturing water. Ciirbotifc add (iiis and tcafrr. thou, are the two chief products of combustion. 184 OBJECT LESSONS. LESSON VII. THE CHEMISTRY OF A CANDLE. Articles for illustration : a can Jle, and stem of a " clay " tobacco-pipe. I. The candle. Refer to Second Stage, Lesson XV., page 64, on candles. Question as to what substances ,are used in the manufac- ture of candles. Tell the children that all fats and oils are made up of various compounds of carbon and hydrogen, or of carbon, hydrogen, and a smaller quantity of oxygen. The common fats classed under the name of tallow contain the three plements. Paraffin and paraffin oil contain no oxygen. The wick is made up of carbon, hydrogen, ; '), and oxygen, with a little earthy matter. II, How a candle ttims. Light the candle ; call attention to the cup of melted tallow, then to the ascent of the liquid fat up the wick by capillary attraction. The children may see the flow upwards. When the liquid fat reaches the flame it is changed by the heat to gas. It is the gas which burns. Show this by putting one end of the stem of a ''clay" pipe into the centre of the flame (Fig. 71). A portion of tho gas escapes through the pipe and may be ignited at the other end. III. Strnctnreof thefiame. Xow look at a sieady candle-flame* very carefully sir'e- ' The candle may be placedin a wide cbinmey-glass, but of course open to the iiir below. Fig. 71. FIFTH STAGE. 1S5 w:iys. Tn tlie inside a dark zone is easily detected. This is simply a zone, as we liave shown, of unburnt tallow-gas. Xext, and surrounding this central zone, is the very bright or /'i)//i/ioi's zone. Here, for the most part, the hydrogen of the gns combines with the oxygen of the air. This chemical union produces an intense heat, which causes the tiny p:!rtioles of carbon to glow, and in fiict produce the light. Outside this fi(j/if-jiroifNriii(j zone there is a more abundant supply of oxygen, and combustion is complete. Thi» outside zone is therefore very hot. biit yields less light. The candle-flame then consists of three pirts: a dark ceiitr.il zone of gas to which the oxygen of the air cannot penetrate, and which therefore is not burning; a second or light-producing zone enveloping the tivst, where some oxygen penetrates, and where the particles of carbon are raised to white heat before themselves undergoing complete combustion ; and a third, or heat zone, again enveloping the luminous zone, where combustion is complete. We can show the threefold structure of the flame in another wa V. Press a sheet of white paper, held horizontally, into the flame of a candle almost down to the wick. Retain in that position for a second or rwo. Remove and note the eflect on the paper. A black ring of carbon, in the shape of fine soot, is shown ; outside this there is another rini: of lighter shade where less carbon is deposited, and within the ring a p;^ ;j light deposit of soot is shown. This deposit of a dark and two light rings of carbon is easily explained. The dark ring corresponds to the lumin- ous zone where there is abirndonce of carbon at a white heat. The outer lic-ht ring corresponds to the heat zone, where combustion is more complete and consequently there is less carbon to deposit. The c»irbon idi/thi the dark ring is deposited as we press 186 OBJECT LESSORS. the paper down through the flame. That the inner zone deposits no carbon may be shown by directing the jet from the pipe (Fig. 72) on to a sheet of paper. LESSON VIII. ELECTRIC AND MAGNETIC FORCES. Articles for illustration: strips of copper and zinc, piece of copper wire, sulphuric acid, iron filings, a magnet, a thread of silk untwisted and a se>9Tii£' needle. I. Electric force. Exp. 178. Take a strip of zinc plate about 4 inches long and an inch in width and place in a glass (Fig. 73) containing dilute sulphuric acid. Direct the children to notice the result. There is formed a collection of bubbles on the surface of the zinc. These break away, rise to the surface of the liquid, and are dispelled in the air. Then other bubbles take their places. Xow what causes these bubbles, and of what gas do they consist ? Refer back to the lesson on hydrogen. How is hydrogen prepared ? These bubbles then are hydrogen gas. We can collect and burn them. Next put a strip of copper of similar size into the dilute acid and without touching the zinc. Ave bubbles formed on the copper ? Xo, only on the zinc. Now lean the strips of zinc and copper against each other as in Fig. 74. and note the result. Torrents of bubbles rise from the copper, and but very few from the zinc. It will be found after awhile that the zinc is worn away, but that the copper is left intact, notwithstanding that the bubbles rise from the copper. Fi.s. 73. Fig. 74. FIFTH STAGE. 187 The same effect follows when the plates are connected hy a wire instead of being placed in contact. Break the wire, and hubbies no longer rise from the copper. AVe may conclude from these expe- riments that there must be some con- nection between the metals to bring .ibout the particular action we have noted. I'nless the wires are connected the particular action does not occur ; hence it seems thar some influence is exerted by the metals upon one another through the wire. £.rp. 179. Suspend a magnetized sewing needle* bv a fine untwisted silk thread. The needle will point practically north and south. Xow carry the wire connecting the plates under and parallel to the needle as in Fig. ?(>. The needle turns on its axis and tends to place itself at a right angle to the wire. Here, then, we hare another force which we have not before considered. It is called the r/iyfrw force, and is closely allied to another force referred to in Lesson I. of this stage — the mairnetic force. II. Electric aaid magnetic forces counected. E.rp. ISO. Take a magnet and plunge it into iron filings. Xote the result. Xow wrap a piece of paper round a piece of thick iron wire — a six-inch French nail will answer very well — leaving the ends free, and then wind around it twenty or thirty • To make :i magnet of the needle, dmw it several times across one end of a ina^utt t'r.ui end to end and a;w:i_vs in the same diuciion, not backw.-irds and forwards. 18S OBJECT lESiSOKS. tnms of copper ■wire, keeping the coils from toncliing eact other. Connect the ends of the iron wire with the zinc and copper plates, and plunge one end of the wire into iron filings. The wire has becomes a magnet which attracts the filings just as the magnet did. The electric force in the wire imparts to the iron another force — a magnetic force — precisely similar to the force exercised by the magnet. When the contact between the plates is broken the wire ceases to act as a magnet. Its force is gone. It is sufficient for our purpose here to show that there are two other forces of nature very closely allied beyond those already described. It is of course in the discretion of the teacher to pursue the subject further as opportunity may ofier. LESSON IX. CENTRE OF GRAVITY. Articles for illnstration : nater-lottle, couple :f forks, a few wheat- straws without flaw, and some blocks of wood. The teacher should introduce the subject of this lesson by a reference to Lesson I. on gravity, viz. pressure down- wards, or weight. Place say a pound weight in the hand of one of the scholars. He experiences a pressure downwards. What is this pressure ? A pound. Xow with what force must he press upwards to keep the weight from falling ? Clearly one pound. L To Und the centre of gravity. Erp. 181. Xow balance a slate horizontally on the thumb of the left hand. What force does the thumb exert in an upward direction ? A force equal to the pressure of the slate downwards, viz. its weight. FIFTH STAGE. ISO In the o:ise of the slate eveiy pa'ticle of it presses down- ward in a perpindicuhir direction (Fiir ~7). The pressure iipwai-ds is collected in one point, the top of the thumb, and this point supports the whole weight of the slate. "VTe can imagine the whole weight of the slate to be collected at the point supported by the thumb, for when that point is supported the whole slate is supported. If the thumb be placed on any other point the slate is not ba/aiiccd, and falls to the ground. We may call the point where the whole weight of the slate is supported \\ie centre of iceigJit, or as it is more commonly called, the centre of grarity. In the case of the slate, which is of regular geometrical form, we can find the point by drawing lines diagonally from corner ro corner. The point where the lines cross is the centre of grarity. In this case we haye not considered the thickness of the slate ; but --appose the slate to haye a thickm. S'- of half an inch, then the exact centre of treig/it will be a quarier of an inch from the surtaeo. hut in a yertical line aboye the point sup- ported by the thumb. If. therefore, we wish to support aociv body, we must be careful to apply the support directly under or aboye the centre of graa'ty. Another method of tinding the centre of gravity of a body is by ^uspe^lsion. Take the slate once more. Suspend it from any point, say a corner. Draw a perpendicular line, found by improvising a plumb-line of a piece of tine twine and a weii^ht. Suspend from another point, say a soooiul corner, and draw a second perpendicular line. The point lyt) OBJECT LESSONS. where the lines cross will be the centre of grayity ; and if a fine hole were drilled through the slate at this point it Kg. 78. might be suspended in a horizontal position by means of a piece of string (Fig. 78). A bottle of water may be supported by a single bent straw (Fig. 79). The straw is bent before being placed in the bottle, so that when the bottle is lifted the centre of gravity is displaced and brought directly under the point of suspension. II. When the centre of gravity of any body is snpported, that hody cannot fall. Take two blocks of wood cut as in Fig. 8u. In (1) the line of direction A B falls within the base of support; hence the centre of gravity is sup- ported and the block does not fall. In (2 ] the line of direction c d falls without the base of support, and the block cannot stand in the position indi- cated. A picture of the "leaning tower of Pisa" might here be exhibited. And it might here be explained that the reason why the tower has stood :iafely for hundreds of years is that, as in the case of block (1) Fig. 80, a perpendicular line Irom the centre of gravity falls within the base. Fiff. 79. FIFTH STACtE. 191 The teiH'ber should now assist the children in pointing out how men and animtils are eontiuuallv sbifting the position of their centres of gravity to bring them within their bases ^ ^ of svipport. A man in carrying a weight on his back h\ins forward, a nurse carrying a baby leans backward ; a man carrying a bucket in one hand leans toward the other, and stretches out the other arm to preserve his balance. It must be borne in mind that the base of support is not necessarily limited to that part of the under surface of a body which rest on its support. Thus the base of support of a man resting on two feet includes not oulv the space actually covered bv the feet, but also tbe space between the feet. One other point with reference to the centre of gravity remains to be considered, viz. that the lower I the centre of grarifu Jiis the more staUe ig the bodi/ ; that is, the less easily can the body be overturned. It is not easy to balance a lead pencil on the point of the'tinger, but if weights be at- tached as in Fig. SI the centre of gravity is brought below the point of si.pport. and the pencil is supported without difficulty. In ships and boats the centre of gravity is brought as low as possible by putting heavy ballast in tlie bottom. H.j I 1>"J. A curious and interesting ex- Fi'- SI. periment to further illusirate this fact may be arranged as in Fig. ^'-. Fix two forks in a cork, and into the bottom of the cork insert a sewimr needle. On the neck of a bottle place a coin 192 OBJECT LE^SOXS. — a half-cro^vTi or a penny. Balance the forks as shown in the figure. The contrivance may be made to revolve with- out destroying the equilibrium. The centre of gravity here is somewhere in an exact line with the needle but con- siderably below it. This line we have called the line of direction. The teacher may now ask such test questions as the following : — 1 . \\ by does a person in rising from a chair bend forward ? 2. Why is it more difficult to over- throw a man when he is standing witli his feet some distance apart than when his feet are placed close together ? 3. Is there any advantage in turning out the toes when we walk ? 4. A man stands with one side close to a perpendicular wall; why cannot he hold up the other leg from the ground ? 5. Why cannot a mau standing with his back and feet close to a wall stoop to pick up unything in front '' 6. How far may a wall be made to lean and yet stand securely '^ To make the following lessons on Simj'/r Machines or tjie Ifcc/iioiiciil Powers effective, the teacher will need working models. Their being rough and commonplaue will not letract from their value as working illustrations. LESSON X. LEVERS AND THEIR USES. I. The teacher should introduce the subject of the simple FUTH STAiiE. 103 maeliiius by q^lo^rionillg the children on such of them a? they luiist have seen in their everyday rambles. The pulley \ised for raising buildiuir materials on to the ?c;iif'old ; the ladder up and down which ea^l;s are rolled, or pushed into or from \vaj:gous ; levers used lor lifting bodies too heavy for the unaided arms, and to on. Fi^-. S4. I. First order of lever. The model lever* should nest be introduced, and its parts deseribed. The pin on vrhieh the lever turns is ealled the/tucrmn. v. TaeKxly to be raised is ealltxl the m.'a/if, w , and the force ap- plied at the other end of the lever to raise the weight is called the noinr. v. The parrt of the lever on eacb side of the fulcrum are called the (fr/A V. Under the teacber's guidance, the children should discover for theiiisches all the facts useful for them to know jboi:t tbe lover. 1. Place equal weights, one at tbe lud of each arm. Xote tbe rcstilt. Tluv b.il.;r.ce. What advantage is tb.ere in a lever so arr,.nircd r Suiiposc the weight to be oO lbs., aiul we have to raise it one foot. 30 lbs. woidd be heavy for a bov to lift. But if tasteticd to one end oi tbe lever he mii;l.t fit on tbe other, and so raise the wcigbr without exerfioi.. • Thr l^veriTfly oor.>;st of ;i Kir of stiff wcixl :."l'cv.t 4 ffot "]_ ■ir. l.iiirh ilirov.ch which sivtr..! holes .ire driiltd, ns in Fiir. S4. iiie ft.r.id r.iiv ccaisift of s itKf made of inch Ji .1 bosi-d 1,') inches In- s or 0. and aii upright made of 4-inch quar- it';-;-.ir. A irrixive m:!s! V out in I'e qi:ai;iri-i: at Tip lo :)H"W o! fri'O nioiior. lo ilio Ivir. Fiir. >". a. A h -Ir- is tniled ;^ ;hro.:i:.'. forthoiiu inwhi.h the ievcr tn-.i.s. Fiir. S^. b, y;^ 194 OBJECT LESSONS. The advantage is in changing the direction of the force. In pushing or pulling doumwards we are assisted hy the weight of our bodies. 2. B,emove the weight, take out the pin and shift the lever one foot to the right, the pin to go through the hole marked 2 in the Fig. One arm will now be 1 foot long, and the other 3 feet. Xow make the weight 3 lbs., and attach 1 lb. to the end of the long- arm. A^ain note the result. 1 lb. at the end of the long arm rather overbalances 3 lbs. at the end of the short arm. If the lever had no weight it would exactly balance. 3. Arrange the lever so that the long arm shall measure 3 feet 6 inches, and the short arm 6 inches. It will be found that 1 lb. at the end of the long arm wiU overbalance 7 lbs. at the end of the short arm. From these experiments the children will readily grasp this principle, that the longer the "power-arm " is in comparison tcith the " weight-arm," the greater is the weight we can raise with a given power. The teacher may also go a step beyond this, and show that — neglecting the weight of the lever — the weight multiplied info its distance from the fulcrum is equal to the power multiplied into its distance from the fulcrum. Or weight : power : : kngth of power arm : length of weight arm. II. Examples. Common examples of this kind of lever may now be brought under review. 1. Ordinary scales for weighing. In this machine the arms are of equal length, and a pound on one side balances a pound on the other. 2. The child's see-saw. Kote, if one child is heavier than the other, how the arms have to be arranged. 3. The common steel-yard (Fig. 85), another machine for FIFTH STAGE. lOJ woicliiiiir g-oi.d#. is a lever of tlie kind described, but with one arm longer than the other. ()iio7\ 196 OBJECT LESSOXS. and the power p at the other. If a pound weight be placed on the end of the lever at p it is quite clear that, to keep the lever in its horizontal position, we must pull upwards with a force of 1 lb. in addition to the force required to support the lever. In this Now shift the weight to the centre of the bar. □ t m 1 Fi". 88. Fi>. 89. position the upward force at p must be half a pound, as the fulcrum supports the other half. This may be tested by passing a line from P over a small pulley suspended from above, and affixing a half-pound weight * (Fig. 8S). Again, shift the weight to the distance of one foot from the fulcrum (Fig. 89). What upward force must be exerted at P to support the weight in this position ? Suppose for the moment the power to be applied at the centre of the lever at p' ; in that position, omitting the weight of the lever, the upward force must be half a pound. But a force at p equal to a quarter of a pound acting in an opposite direction, will balance a force at p' of half a pound. Hence a quarter pound force at p will support lib. at a dis- tance of one foot from the fulcrum. Allowing a little far the weight of the lever, experiment will show f\ie truth of this statement. • Some extra weight will te required to support tlie lever itself. FIFTH STAGE. 197 From the above experinieiits the teiicher will ngniu be able to deduoo — 1. Thai fhr lo/nji r f/ic poin'r-ann is in proportion to the iccight -orii), the greater ii< the weight ire can ra/i: disfance from the fiilerum i^s equal to the j'ower muffipfied hi/ its di,\taihr Jroni tin fnicrum. II. Examples. Coiumon example* of tliis lever : — 1. The wheelb ivrow The wheel is the fulcrum, the load iu the barrow the weight, aud the mau liftiuir the handles the pow or. ■J. A boat o;u-. The water is the fulcrum, the boat the weight, while the power is applied by the baud. 3. A choppiuir knife turning on a fidcrum .it one end is another example. 4. yiitci-ackers aud cork-sqiioezors are double levers of this kind. The children m;iv now be told that when a lever is so arr.ini:oil that the fulcrum is at one onl. the power at the other, aud the weight iu the mi>!iilo. it is said to be of the fit iV lid (■■()■<,-■ or order. FiiT- i^O shows how a lover of the second class is some- times used. Fi-. 90. LEsso>; XII. LE\ERS A\iO THEIR USES, I. LeVvH- of the Third Order. "We now come to a lever having a different arrangement air.un. It is called the lever of the third eiass or order. 198 OBJECT LESSO^'S. The fulcrum is at one end of the lever as in the second kind ; but in this case the weight is at the other end, and the power is applied somewhere between the fulcrum and the weight (Fig. 91). It will be found on trial, viz. by passing the string by ■which the power is applied over a puUey, that if the power /^ p' p F r o O O S\ 1 A J wpi Fig. 91. is applied in the centre of the lever, it will require a 2 lb. weight to support 1 lb. at the end of the lever. The teacher may show that this must be the ca>e from the lever of the second kind. Suppose the weight ^v of 1 lb. to be arranged to pull the lever upwards, then it will balance a weight of 2 lbs. at p. Or, which is the same thing, a power equal to 2 lbs. in weight applied at p in one direction ■will balance a weight of 1 lb. applied at w in the opposite direction. In the same way the teacher may show that it will require a power equal to 4 lbs applied at p' to support a weight of 1 lb. at v\-. The second and third kinds of lever are identical, except that the power in one case takes the place of the "veight in the other. In the lever of the third kind it will be seen that the power is always greater than the weight. TThere, then, is the advantage ? The advantage is that the weight is moved through a greater space, and of course at greater speed, than the power. This may be shown Viy experiment. In this case also fJir potcer multiplied info its distance from thf fulcrum /•-. equal to the ircight multiplied info its distance froi.'i the fulcruin. FIFTH STAGE. 199 II. Examples. Lo\ ors of this kiud are less common than the others. A good illust ration will be ro direct a boy to hold a weiirht in a long-handled shovel or in a spaile. One hand, usually the right, acts as the fulcrum, the other hand furnishes the power which lifts or balances the weight.* A man raising a board, a pole, or a ladder with one end pressed against the bottom of a wall or held by a second person is another example which may be illustrated in the schoolroom. [Xote, when the raisintr commences the lever is of the second kind, and so remain,- tmtil the man gets below the centre of gravity of the article he is raising.] In the mechanism of the human body there are manj' examples of levers of the third kind. The arm may be taken as the most simple tor iUustration. The socket of ths elbow ioint terms the fulcrum, the biceps muscle is the power, and the weight is the forearm and anything sup- ported by it. A pair of tonirs is an example of a doiiok lever of tins kind. * A fishiiiir-rod hold withlxith hands lorui? a -imilu- example. 200 OBJECT LESS0X3. LESSON XTII. THE PULLEY.* I. Single Fixed PuUey. From the lever of the first kind to the pulley is an easy transition. Fasten a short lever (see foot-note), to work on a screw as a pivot on the side of the pulley-stand (Fig. 94) Show how the lever Tvorks. The arms are of equal lengths and the Teight and power must be equal. There will be just as hard a pull in one cord as in the other This pull or strain is usually described as the tension. Xow substitute for the lever a disc of deal board having a diameter equal to the whole length of the lever (Fig. 95). Fig, 94. It turns on the screw as a fulcrum, as in the case of the lever. Fasten the cords to the disc at a and b. This machine ia nothing more nor less than a lever of the first kind. * The side of a deal table or an old box stood on end and with the lid removed, will serve as a pulley-stand. Insert a couple of screws on which to hang the pulleys or fasten the cord. FITTH STAGE. •201 Fi.^. 95. Xoxt take a longer cord and pass it over the puller. The necessity for the groove round the circumferouce is at once apparent. By this apparatus \\ o can raise the weight as high as the pulley is tixed, but as the anus of the le\ ir are of equal length the power must equal the weight. That is, if wo want to raise 1 cwt. we must pull down ■svith a force of 1 cwt. This revolving lever wiih arms of equal 1- iigth -.s called a piiIU;/. Sbow the schol.irs that the advantages of this pulley are (U dmrfioii in the application of the power, and f^^ we can raise the weight to any desired j^^^|^^^^^=^=:; ^ height. II. A Fixed and Movahle Pulley. The ttarher will now fit up a couple of pulleys.* one fixed and one movable as in Fig. Pci. and then proceed to show the advan- tage of this system of p. alleys. S;;ppose the weight w to be S lbs. The cord A b passes roimd the movable pulley and supports the weii^ht : hence the torsion or >Train iu each part of the cord is the same. What is tbi'i tension r To find this we Fi-. se- tter bach to a lever of the .^eee-.u! kind. * Blo.'s I u.^i yj m.4y be pu-oh..>fa .it qnit. 99. at B. Bv trial find out what weight suspended at the end of the long arm f f of the lever will support a given weight, w, at the end of the short arm a f. At present our machine is a lever of the first kind. Given that our wheel is arranged to work horizontally, and the groove widened to take a number of loils of the cord, and the cord lengthened to the extent required, we have the wheel and axle a* used on board ship for raising the anchor (Fig. 100). * See Lesson XVIII. 204 OBJECT LESSO.VS. Show the scholars how this is practical! 3' a lai'ge wheel and a small one, the circumference of the larger wheel being the Fig. 100. path made by the end of the spokes, and the part of the axle which receives the cord the smaller wheel. We may use a wheel with a groove instead of the spokes, and then, placed in a perpendicular direction (Fig. 10 L), we see the machine as used in another way. It would not be difficult for the teacher to illustrate this by a rough model* as shown in Fig. 102 and to show that if the radius of the larger wheal is 3 inches and of the smaller 1 inch, a weight of 1 lb. placed at r will balance a weight of 3 lbs. placed at w. Note that the cord round the large wheel or axle is wound in a contrary direction to that round the small axle « Ons large and one small cotton-reel fa>tpne'l t.-'CPtlier answer very well. FIFTH ST.AGE. 205 Sometimes instead of the large axle This is the case in the common irind/ass for raising water out of ■wells. (Sec Fig. 103.) The teacher will show here again that the centre of the axle is the fulcrum, the radius of the axle the short arm, and the radius of the circle desciibeJ by the handle rhe long arm of the lever, and irom these calculate the power requisite to raise a given weight. handle is used. Fie;. 103, LESSON XV. THE INCLINED PLANE, The teacher may introduce this lesson by some questions on the way in which brewers' men raise the barrels into their waggons, and on the comparative amount of force required to carry a load of equal weight up a hill with a gentle slope, or upi another with a steep slope, walking at the same pace in each case. To cairv a load up the gentle ascent will be the easier task. Why, and by how much? To answer these questions in the most satisfactory way, the teacher must construct an incliued plane and affix a pulley at the top (Fig. 104K It will he found on trial that a weight of, say 5 lbs., acting downwards from the pulley will support a weight of 10 lbs. on the inclined plane when (1) the string from the weight to the pulley is parallel to the plane, and [2), when the length of the plane .\ r. is double the height a c. We should have expected this to be the case from the 206 ■ OBJECT LESSOR'S. principle that " what is gained in weight is lost in space and time." Here the weight has to travel over twice the length it would have to travel over supposing it to be lifted perpendi- cularly from c to A, When the length of the plane is three times the height, Fig. lot. the power required to balance the weight will be one-third the weight ; when the plane is four times the height, the power required is one-fourth the weight, and so un. In actual practice, to raise a weight on the inclined plane the power must be somewhat greater than the proportion here given, because there is friction to overcome. It will be sufficient if the scholars thoroughly master the fact that the longer we make the inclined plane the easier it is to raise a given weight. When very heavy casks have to be rolled up an inclined plane another contrivance is sometimes adopted. Two ropes are fastened at the top of the plme, then laid along the plane to the bottom and passed round the cask and doubled back to the top of the plane. This raav be imitated in a small inclined plane by passing a couple of pieces of twine round a piece of lead tube. In this case the teacher will show that the power has to move through twice the distance, and that consequently twice the weight can be raised by a given power. The apparatus constitutes in fact a single movable pulley, and FIFTH STAGE. 207 tbe te'isiou in each cord is half that of the tension in a single cord as in Fig-. 104. Tiro ro}ies are used for convenience ; they keep the cask in proper position for rolling, but no further advantage is g.uned over the use of one rope. LESSON XVI. THE WEDGE. Take a piece of wood cut in the shape of an inclined plane, and show how it can be used for raising a weight on the table bv pushing the plane under the weight instead of pushing or pulling the weight up the inclined plane (Fig. FiiT. 105. V 11. 106. Fie 107. 103 ; in fact, making the inclined plane a movable instead of a t'j-id machine. SLXondlv t.ike anoiher inclined plane of the same shape and size as the former; place tlie two base to base (Fig. 106), and us? them to r.iise the weight just in the same way as with the single inclined plane. Tell the children that the double inclined plane is called a wediTO. Show its use for splitting wood oy a sketch on the blackboard v^ig- l^^'^ 208 OBJECT LESSO^"^;. The power applied in the case of the wedge is not a simple pull or a push, but a sharp blow with a hummer or mallet, the force of which we cannot measure ; ' neither in most cases can we measure the force of resistance to the entry of the wedge. Hence, all the teacher need to impress on the children is that we gain a great mechanical advan- tage by using this machine, as we do in using others. The teacher will lastly refer to common forms of the wedge and their uses, such, as a knife, a nail, the prongs of a digging fork, a ploughshare, a needle, a pin, &c. LESSON XVII. THE SCREW. The screw, like the wedge, is a movable inclined plane ; but in this case the inclined plane is wrapped round a cylinder. This can be shown very readily. Take a piece of white flexible cardboard. Cut from it a long and low inclined plane. Colour the edge of the plane, and wrap round a cylinder (Fig. lOS). ijl [The size of the cylinder must depend to some eiitent on the length of the inclined plane, but the whole should be as large as possible.] The children will be able to see from this experi- ment the likeness of the coloured line formed by th" coloured edge of the plane to the thread of the screw. Next take a large screw of some kind (Fig. 109) iig. 108. — wood or iron — and let the children trace the easv ascent of some supposed living insect round and round the * Let p = power, and r resist;iiicp, i^ length of half the hack of the wtd^t.'. ^^"ergth 01 \ved-t.' : then p ; k ^ ^ A ; /. FlFril SiAGK. ?09 cylinder, but up and up the iueliued plane from the bot- tom to the top. Eefer to spiral staircases constructed on a similar plan. It IS not. often that the inclined plane wound round a cylinder is used as a lixed machine lor raising bodies ; on Fitr 103. the contrt\ry. it is nearly dways used as a movable machine for penetration, or for squeezing and holdiuir toirctlier. In '• presses " for pressing very hard, long and powerful levers are employed to turn the screw, Fiir. 110 represents .J copying-press with two lexers. Tr.o pressure in a machine of this s^rr is very ereat. Bv lookiniT at one turn of the screw unrolled we shall iret some idea of the power (,Fig. Ill ;. 210 OBJECT LESSOXS. Let the height of the inclined plane, or, which is the same thing, the distance between one thread and the next, be one-eighth of an inch, and the length of the inclined plane, viz. the circumference of the cylinder, be three inches or Fig. Ul. twenty-four eighths *; then the weight (in this case the pressure downward) will be twenty-four times the power. The children will understand that when we use a lever to turn the screw, the force with which the screw presses down- wards will be immensely increased, and they may be told, just to illustrate the power of presses, that the machine is now practically an inclined plane whose height is the distance between the threads and whose length is the cir- cumference of the circle described by the lever. Let this circumference be 6 feet, viz. 72 inches, or 576 eighths of inches, and the distance between the threads one- eighth of an inch ; then a pressure of 10 lbs. at the end of the lever would produce, if there were no friction, a pressure below the screw of 6760 lbs., or more than 2| tons. Prac- tically the pressure is much less. The common screw for joining wood or iron work, the corkscrew, and gimlet, are all examples of the " screw." LESSON XVIII. FRICTION. In the preceding lessons on the simple machines the term friction has been once or twice used. What is friction ? And what effect has friction on bodies moviug one over another ? FIFTH STAGE. 211 Take a block of rougb wood and call on one of (he scholars to press it along over a rough board. The boy has to exert considerable force to slide the block along. Xow let him slide a polished block along a polished table. The force required is much less. Over smooth ice or a sheet of glass it would be less still. The irsistaiicc which the moving block meets with from the surface on which it moves is L-al\edfn'efion. Boys cannot slide any distance on a road, however level, but they can make long slides on the ice. Whv ? The friction is less on the ice. It is difficult to walk on ice ; we are liable to slip and fall because the friction is -light. But strew ashes or sand over the ice, the friction is increased and we can walk at ease. In frosty weather sand is thrown over the streets to keep the horses from falling. In taking heavy loads in waggons down steep hills, the driver fastens an iron shoe on one wheel to keep it from turning round. The friction between the shoe and the road is great. It prevents the waggon from moving so freelv and eases the horse. Sometimes " breaks " are applied to carriage wheels. The friction between the break and the wheel hinders the wheel from turning so freely. When masons want to move large blocks of stone, they Kg. 112 put rollers under them to lessen the friction (Fig. IT^i The teacher may illustrate this by putting a heavy body on two or three " round " rulers. 212 OBJECT LESSONSw It is the friction between the road and the wheel which makes the wheel turn. Take the railway train, for instance. The steam moves the piston, and the piston, by means of complicated machinery, moves the large " driving wheels ; " but if there were no friction between the wheel and the iron rail the wheel would simply turn round and rot roll for- ward. The wheels would not bite, as people say. Sometimes in frosty weather it is difficult to start or stop the train because the I'ails are so slippery — that is, there is little friction — and sand is thrown on the metals to increase the friction. The wheel, of course, turns on an axle, and here we want no friction, because all friction here tends to hinder motion, and so the axles are kept well greased or oiled. So of carriages and waggons, that part of the axle on which the wheel turns is kept well greased to lessen the friction. Friction is not confined to solid substances. Rocks over which water constantly flows are worn away by friction. It is by Iriction that the wind raises the waves on the water, and it is by friction that the air causes a corn-field to assume a wavy motion. Wherever there is motion on the earth it is more or less retarded by friction. INDEX. Absorbent, 9. Adhesii'D. 77. 17'2. --idhesi\ e. me.inincr of, 17. Siib?i-;uioes. 17. Air, lesson on, 4S. Prcs^ueof. 50. Kl,!-- icity of, 5-2. A mixture of g-ases, 173. Aiv-pump, le-:^ n on. 111. Alcohol. 16. Alum, IFi. Mmc^yheve, weight of, 94- Pi^ssure of, iU, A^ected by he;it. UO. Compositioii of, itvO. B Bplloons, es. Barometer. 99, Construction of, 100, Benzine, 16. BeiL-Liline, 63. r^lottmir-piper filter, 12. iM'dies hirhr. hexry. S. Boihuir of water, 1-5. Bnoks, '24. Brittle, '20. S2. Camphor, 16. Candles. 64- Chenii? ryof,lS4. Common dip, 64. :Monld. 64. CapQHiT attr ction, 79, 172. Caabon o nou], lesson on, 60, 183. Cement IS. Ceuti-e of criiTitT. ISS. Chalk, -.0 ■ Charcoal filter, 12. Chemicid attraction, 17^. Expenments on, ! 74 China --'4. Clay. '^-j. Clothing, 137. Clouds. 43. Coal p-)s. 56. Cohesion. 75, ir2. Colours, comp irisi 'O of, 159. Combustion, is-;;. Common products of. 18S. Condnction of heat. 136. Condiu^-.ors, 136. Good and bad, 137. Contraction, 117. Convection, 12'^ Cooling cf bodies, 153. Copper -wire, 20. Ooik, i2. Dew, 40, 145. Diimond, 19. Bistillation. 12S. Ductile metals. 26 82. E Earthenware, 24. Earth filter, 12. Elastic, meaning- of, 21, i^2. Electric force, 1S6. And mapiietic, 1 c<7 Elements and compoundSj 17i Common, 176 Evaporation 40. Expansion, 117. Filter, a blottinir-p.iper, 12. A cbm-nal. 12. E-mh. 1 2. fponc-e. 11. I'se .-.f. 13. Firedanip 5" Fr e-eiici'-.e. the 103. Flexible, 2(i. S2. Eosr. 41. FLiice. definition of. 171. Force-pump, 106. Frceziuir mixtuies. 151. Fieezniir "f water, 124. Fr.oii'ii. 210. Fusion. 25. '^as. lesson on, 54, 76. Glass, 6. tilue. 17. (iold, 19, 26. Gnvity. force ul. S3, 172. Centi-e of, l-^S- Gravitatiou, atti-action of. 17i Gunpowder, 31. Hail. 4$. 147. Hard and soft, IS. Hardness. 61. ' He.it cause of motion in nir. 143, Heavy and lig"ht, meaning ui, s. Hoar firost, lie. Hvdrogen, IH'. Ice, 45, 46. Inlia-rubber, 22 -23. Iron, S. Latent heat, 152. 214 INDEX. Latent heat of water, 152. Of steam, 152. Lead, 26. Lesson on, 27. Alloys of, 29. Lever, ihe. and. its uses, 192. First order, 193. Second order, 195. Third order, 197. Lifting pnmp, 1 .1. Light and he&Ty, 8. Lime. 16. Liqnefaction, 119. Liquids. 8, 55, 76, lia Buoyancy of, P5 , Pressure in, 1 , S7. Pres,sure in, II„ 90. Lucifer matches, 31. U JIalleable, 26, 82. Mercury, 17. Lesson on, 40. Jfetals, 19, 25. Mis% 44. -Molecule.", 73. Of w Iter, 85. Mortar, 18. N Naphtha, 17, 69. Nitrogen, 179. Non-absorbents, lesson on, 13. O Oxygen, 178. Paraffin oil, lesson on, 62 Particles, minute, 3. Pewter, 3'^. Plane, the inclined, 205. Plaster of Paris, 17. Plastic, 23. Porous, 9. Bodies, 11. Pressure, effect of, on boiling-point of liquids, 130. " Propel ties " of bodies, 5. Shape, size, colour, 6. Proportion, lessons on, 158—167. Terms of. 163. Pulley, the single fixed, 200. Fixed and movable, -iOl. Three or more, 202. Pump, the common, 103. Putty, 14 Radiation, 188. Eadiators, good and bad, 1C9. Eain, 44, 147. Eatio, 159. Illustrations of, 161. Terms of, 163. .dnshlight, 64. Salt, 10, 14, 15, 25, 87. Manufacture of, 15. Saltpetre, 16. Screw, the, 208. Siphon, the, 108. Sleet, 148. Snow, 45, 46, 148. Soap-bubbles, 6-'). Whattheyleirh,66. Soft and hard, IS. Solids, propei ties of, 8. 76, 81, llC Specific gravity of, 155. Soluble, 14. Solvents, 15. Specific heat, 149. Sponge, 6. 7, 9, 2a. Sponge-filter, II. States of matter, 75. Steam, 44, 132. Steam-engine, the. 133. Steel, 19. Stoves and open fii-es, 143. Sulphur, lesson on, 30. Substances soluble in water, H,. Sugar, 25. 37. Manufactu-e of, 15. Syringe, the, 101. T Tar, lesson on, 58. Tenacious metals, 27, 82. Thermometer, the, 120. Mercm-ial, 121. Tough, 20. Tm-pentine, 10. Twine, 10. V Vaporization, 119. Vapoui', 40. TV ■Water, 15. Lesson on, 85. A-solvent, 16, 87 . Finds, its level, .?6. Compound of two gases, ISO. Distilled, 89. Wedge, the, 207. Weight, lesson on, 7, 83, &i. Comparigon of, 159, Wh'lebone, 22. Whfel and axle, 2L3. Winds, 144. Wood, a THE END. SCl£^'C£. Shaler's First Book in Geology. 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For grammar and high schools. 140 pages. 60 cts. Woodbum and Hodgin's The American Commonwealth. Contains several orations from Webster and Burke, with analyses, historical and explanatory notes, and studies of the men and periods. 5S6 pages. Si. 50. S^?ii by tTuii^ post paid on receipt of prices. See also our list of books in History. D. C. HEATH & CO., PUBLISHERS, BOSTON. NEW YORK. CHICAGO. EDUCATION. Compayr^'S History of Pedagogy. " The best and most comprehensive history of Education in English." — Dr. G. S. Hall. S^i-75. Compayr6*S Lectures on Teaching. "The best book in existence on the theory and practice of education." — Supt. MacAlister, Philadelphia. ?i.75- Compayr6's Psychology Applied to Education. A dear and concise statement of doctrine aud application on the science and art of teaching. 90 cts. De Garmo's Essentials of Method. A practical exposition of methods with illustra- tive outlines of common school studies. 65 cts. De GarmO^S Lindner's Psychology. 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PestalOZZi's Leonard and Gertrude, "if we except 'Emile' only, no more im- portant educational book has appeared for a century and a half than ' Leonard ani Ger- trude.' " — TJie Nation. (,0 cts. RadestOCk'S Habit in Education. " it will prove a rare ' find ' to teachers who are seeking to ground themselves in the philosophy of their art." — E. H. Russell, Worces- ter Normal School. 75 cts. Richter's Levana ; or, The Doctrine of Education. "A spirited and scholarly book." — Prof. VV. H. Payne. S1.40. Rosmini'S Method in Education. " The most important pedagogical work ever written.'' — Thomas Davidson. $1.50. Rousseau's Emile. " Perhaps the most influential book ever written on the subject of Education." — R. H. Quick, go cts. Methods of Teaching Modern Languages. Papers on the value and on methods of teaching German and French, by prominent instructors. 90 cts. Sanford's Laboratory Course in Physiological Psychology. The course includes experiments upon the Dermal Senses. Static and Kinsesihetic Senses, Taste, Smell, Hearing, Vision, Psychophysic. In Press. Lange's Apperception : A monograph on Psychology and Pedagogy. Trans- lated by the members of the Herbart Clubj under the direction of President Charles DeGarmo, of Swarthmore College. Si. 00, Herbart's Science of Education. Translated by Mr. and Mrs. Felken with a pref- ace by Oscar Browning. $1.00. Tracy's Psychology of Childhood. This is the first ^^-w^r-z/ treatise covering in a scientific manner the whole field of child psychology. Octavo. Paper. 75 cts. Sent by ?naii, postpaid, on receipt of price. D. C. HEATH & CO., PUBLISHERS, BOSTON. NEW YORK. CHICAGO.