EXPERIMENTS 
 
 ON SOLAR LIGHT, 
 
 By JOHN W. DRAPER. 
 
 Professor of Chemistry, Hampden Sidney College, Va. 
 
 FROM THE JOURNAL OF THE FRANKLIN INSTITUTE. 
 
C ONTENTS. 
 
 1, Action of Absorbent media. 6, Absorption of the different rays of light and distorted 
 spectra- 16, Absorption of radient heat, instrumental arrangement fox measuring. 24, 
 -Action of vapour of iodine and nitrous acid. 28, Absorption of chemical rays. 29, Bodies 
 nearly opaque to them. 33, Thermal disturbance of gaseous mixture, and penetration of 
 their dimensions. 38, Decomposition of carbonic acid by the sun’s light. 39, By radient 
 heat. 40. Power of capillary action exalted by heat. 
 
 43, Action of vegetable leaves on carbonic acid. 44, Analysis of gas evolved by non-lu- 
 minous heat. 46, Analysis of gas under ordinary circumstances. 47, Plants evolve nitro¬ 
 gen, and not oxygen gas, in the sunshine. 52, Effect of stopping the chemical rays. 54, 
 Identity of the primary and resulting volume of the gas. 68, Non-oxygenation of phosphorus. 
 62, Decomposition of salts of silver. 64, Fundamental trial. 66, Discovery of the diffrac¬ 
 tion of the chemical rays. 
 
 68, List of metallic salts changed by light. 69, Analysis of the dark chloride of silver. 
 73, Action of moonlight and artificial lights. 
 
 74, Perihelion motion of matter. 76, Doubtful if it be always the same. 77, Modes of 
 making these experiments. 80, Dew of water. 81, Dew of mercury. 82, Iodine. 83, 
 Chloride' of gold. 84, Non deposition on a glass plate. 86, Current action. 88, Action 
 of terrestrial flames. 91, Temperature of sides of ajar. 92, Modification of light by reflex¬ 
 ion. 93, Action of a metal screen. 95, Protecting action of a metallic ring. 102, Action 
 of non-conducting bodies. 104, 106, Electricity obtained from the solar ray. 110, Ex¬ 
 planations founded on that hypothesis. 112, Determination which class of rays causes 
 this perihelion motion. 115, Effects of light on vegetation. 
 
EXPERIMENTS OX SOLAR LIGHT, 
 
 1. The effect of absorbent media upon the colorific rajs of light, has 
 been, as was predicted by an eminent writer on Optics, of singular service 
 in developing new views of this subtle agent, and giving us a more precise 
 knowledge of the complex constitution of the Solar beam. Hitherto, the 
 action of these media, upon the calorific and chemical rays, has not been 
 thoroughly investigated, nor are there, so far as I know, any experiments 
 on record, exhibiting this matter in its full importance. 
 
 2- We have been accustomed to regard the chemical properties of the 
 Solar ray spectrum, as due to the violet ray,—as something coherent to it. 
 A similar opinion was formerly maintained, respecting the calorific consti- 
 tion of the red ray. The position to which we are brought by advanced 
 investigation, has long ago established the separate existence of heat 
 making rays, and the experiments here communicated give much weight to 
 the doctrine, that the chemical rays have also a separate existence. It is 
 true it cannot yet be proved, though analogy and probability are favourable 
 to the idea, that there are sub-divisions both of the chemical and calorific 
 rays, similar to those of which our senses give evidence in the colorific ray, 
 each of which is endued with distinct powers of its own. 
 
 3. How complex and compounded is the constitution of the solar beam; 
 a ray of heat, composed perhaps of three or more rays of different refrangi- 
 bility; a ray of light, composed of three simpler rays; a ray endued with 
 chemical energy, and of a similar composition to the former, as analogy 
 would lead us to suspect. Again, each of these elementary rays is compos¬ 
 ed of particles, one-half of which have their planes of polarization at right 
 angles to the other. All these elements taken together, constitute a beam 
 of the same light. Emanations from the sun, after they have undergone 
 the absorptive action of the atmosphere of that great luminary, and of that 
 of the earth, still reach us in abundance, accompanying his light, and tra¬ 
 versing the great vacuum, perhaps as far as his attraction is felt. 
 
 4. If we take a coloured medium, of any kind, and transmit through it a 
 beam of the sun’s light, we find, on examination, that certain of the rays ex¬ 
 citing vision are absorbed, that the light which passes through is not homo¬ 
 geneous, for it is capable of decomposition by the prism; it is a compound 
 coloured ray, consisting of all the rays, complementary to those which the 
 medium has absorbed. Nor is ihis absorbing effect confined to the rays pro¬ 
 ducing vision, the rays of heat suffer in like manner, sometimes those which 
 are more refrangible are wanting, sometimes those which are of less, or of 
 medium refrangibility are absent. Often, at the same time, do the chemi¬ 
 cal rays sustain a similar attack. There are solutions and media, trans¬ 
 parent to light and nearly opaque to heat; there are others, transparent to 
 light and to heat, and opaque to the chemical ray. It is from these facts, 
 that we are able to establish the separate existence of three genera of rays, 
 in the sunbeam, each of which is essentially distinct in its properties, and 
 different in its mode of action, to the others. Our eye can detect, in the 
 rays exciting vision, difference of constitution, because we are able to per¬ 
 ceive a difference of colour. Had we specific organs for indicating differ- 
 
 A 
 
2 
 
 ence in the heat making, or chemical, rajs, perhaps we might find in them 
 a similar constitution. 
 
 5. It is between three and four years since, that the investigation, which 
 forms the subject of these papers, was first commenced, under the form of 
 an examination of the properties of the chemical ray. In tv/o of the num¬ 
 bers of the Journal of the Franklin Institute, V. XV, p. 79, and p. 155, 
 some of the earlier results are recorded, and among them the extraordinary 
 fact, that the crystallization of camphor, which has long been known to take 
 place on the enlightened sides of vessels exposed to the sun, occurs with 
 very great rapidity, if the giass in which it is tried, be exhausted of air. 
 In tracing out this fact, to ascertain its cause, a field of abundant discovery, 
 and, no common interest, has been entered. I do not here present a record 
 of the facts as they were successively developed by an analysis of the 
 phenomena; but place them in that order which appears to me the best to 
 obtain a true estimate of their bearing. 
 
 6. Into a darkened chamber, the shutter of which is seen in section at 
 a a Fig. 1. Plate XI. a beam of the sun’s light may be made to pass hori¬ 
 zontally, by means of a mirror of silvered glass c. The mirror which I 
 use is one belonging to a solar microscope, and by turning the milled 
 screws e e, it can be brought into any position required to throw a beam 
 horizontally into the room, no matter what may be the place of the sun. A 
 brass tube/, belonging to the same instrument, and two inches in diame¬ 
 ter, can be screwed into the position figured, if desirable; there is also a 
 lens, g, which may occasionally be fixed at g , its focus is nine inches, its 
 diameter about two inches, and the diameter of the Sun’s image of an 
 inch. 
 
 7. A piece of sheet lead about a quarter of an inch thick, is to be cut 
 into the form of a horse-shoe, of such magnitude that a circle one inch 
 diameter might be inscribed in it. Upon this lead, two pieces of very 
 pure and transparent crown glass are cemented, so as to form a trough, 
 for containing a variety of liquids. It is well to accommodate this trough 
 with a strong foot, or basis a «, and several such troughs may be provided. 
 Fig. 2, c c c the leaden horse-shoe, b b the glass plates. 
 
 8. A thin metallic plate, three or four inches square, is also to be pro¬ 
 vided, having a longitudinal slit about one inch long, and inch wide, in 
 it. It is convenient that this, too, should be furnished with a pediment, 
 Fig. 3, a a the slit. 
 
 9. The lens g , Fig. 1. having been removed; by turning the screws, a 
 beam of light is to be thrown horizontally into the room, the screen Fig. 3 
 is then to be placed before the brass tube /, so that the slit in it may allow 
 a narrow streak of light to pass. The trough Fig. 2 is then placed behind, 
 in such a position that half the light which comes through the slit in the 
 screen, may pass through the liquid contained in the trough, and the other 
 half pass by its side unintercepted. This arrangement is shewn in Fig. 8. 
 Behind the trough is placed a flint glass prism a, Fig. 4, and further still 
 a white pasteboard screen e, ot suitable dimensions, a being the screen, b 
 the trough. 
 
 10. The action of this arrangement is as follows. The beam of light 
 cast by the mirror into the room, is entirely intercepted, except the small 
 portion which passed through the slit, in the metallic screen. A part of 
 this passes through the trough, and a part on one side of it, the middle part 
 being obstructed by the leaden horse-shoe. Two beams of light, therefore, 
 fall on the prism, one of which has passed through the trough, and one 
 
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 FZate JL 
 
 Experiments on Solar Erir/ht. 
 
Digitized by the Internet Archive 
 in 2018.with funding from 
 Getty Research Institute 
 
 https://archive.org/details/experimentsonsolOOdrap 
 
3 
 
 M'hich has not, and they are separated from each other by a dark interval. 
 The prism decomposes both, and there falls on the pasteboard screen, 
 two spectra side by side, and close enough for a very accurate examina¬ 
 tion. One of them has been acted on by the fluid in the trough, the other 
 is undisturbed. In my arrangement the spectrum a happens to be the na¬ 
 tural one, and b the disturbed one, Fig. 10. 
 
 11. Let us now take an example, as an illustration of the use of this 
 apparatus. Fill the trough with distilled water, and let the mirror throw a 
 horizontal beam. Two spectra are seen on the screen e. Fig. 6, close to 
 each other, side by side, with a dark interval between them. They con¬ 
 tain, as may be perceived, all the seven colours of Newton, nor does the 
 one differ in any wise from the other as in Fig. 11. 
 
 12. Having poured the water out of the trough, fill it with a strong, but 
 clear, solution of the chromate of potassa; on looking at the spectra on the 
 screen, a is still found of its natural appearance, but by the side of it there 
 is a distorted spectrum, formed by the light that has passed through the 
 trough; the blue, the indigo, and the violet rays,are wanting,as is seen in Fig. 
 12, these colours have then been absorbed, by the solution of chromate of 
 potassa. If this solution be poured out, and one of sulphate of copper and 
 ammonia poured into the trough, another kind of spectrum is produced, 
 where the red, and much of the yellow light, is wanting, see Fig. 13. If a 
 strong solution of brazil wood is used, the disturbed spectrum will be found 
 to have lost its violet, indigo, blue, green, yellow, and a great part of its 
 orange rays, as represented in Fig. 14. 
 
 13. By having the two spectra side by side, and close to one another, 
 they are placed under circumstances most convenient for making a perfect 
 comparative estimate of the light which is lost. In this manner the fol¬ 
 lowing table has been constructed. The specific gravity of the solutions 
 is not given, as it is not supposed that any direct connexion exists between 
 the density of a solution and its absorptive power. Much more depends 
 on the shade of colour. 
 
 Table of Colorific Rays absorbed by solutions. 
 
 Name. 
 
 Kays absorbed. 
 
 Bichromate of potassa, 
 
 blue, violet. 
 
 Prussiate of potassa, 
 
 extreme red, extreme violet, yellow. 
 
 Sulphate of copper. 
 
 extreme red. 
 
 Chloride of gold. 
 
 violet. 
 
 Chloride of platinum, 
 
 extreme violet. 
 
 Sulp. copper and ammonia, 
 
 red, yellow. 
 
 Solution of tannin, 
 
 violet, indigo blue, orange and a part of 
 green. 
 
 Solution of Litmus, . 
 
 orange, yellow, green, extreme violet. 
 
 Chromate of potassa, 
 
 extreme red, blue, violet. 
 
 Linseed oil, 
 
 violet, indigo blue. 
 
 Hydro-sulphate of lime, 
 
 violet, blue. 
 
 Decoc. logwood in alum water, 
 
 orange, yellow, blue, and green. 
 
 Decoc. of brazil wood. 
 
 Cochineal in cream of tartar solution, 
 
 violet, indigo blue, green, yellow, orange. 
 
 yellow and part blue. 
 
 14. Some remarkable phenomena may be produced, by taking double 
 solutions-, a beam which has passed through a stratum of solution of sulphate 
 of copper and ammonia, and then through a decoction of brazil wood, be¬ 
 comes almost totally extinct. On looking through such solutions, separately, 
 at the noontide Sun, he appears with overpowering effulgence, but on using 
 
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4 
 
 them together, only a very faint trace of a dirty olive green light, indicates 
 his position. The sulphate of copper and ammonia, absorbs the red rays, 
 and the Brazil wood decoction, nearly all the remainder. 
 
 15. Already have some of these phenomena of absorption, in the hands 
 of Sir D. Brewster, disclosed important facts respecting the colorific rays. 
 The colour of the sky, and of the clouds, and of the sea has also been 
 long attributed to an action of this kind, exercised by thick masses of air, 
 or vapour, or water. 
 
 16. But this action is not alone confined to the rays producing vision, it 
 extends to the other elementary constituents of the spectrum. Whilst the 
 trough b Fig. 4, is filled with a solution of sulphate of copper and ammonia, 
 if the prism and the metallic screws be removed, and a very delicate ther¬ 
 mometer be 4 plunged in the ray, a new phenomenon is discovered; the ray is 
 found to be, to a great extent, deprived of the power of exciting heat, and 
 the thermometer shows little disposition to rise. How is this? is it because 
 the red making ray is gone, that the sunbeam has lost its power of exciting 
 a sensation of warmth? It was at one time supposed, that as the violet 
 ray had the power of determining chemical change, so the red ray possessed 
 the power of exciting calorific impressions. 
 
 17. Fill the trough next with a strong decoction of Brazil wood, analyse 
 the light which passes through it, by the prism, (sect. 9) and it will be 
 found that all the rays have been absorbed except the red. Now, in such 
 a beam, if the red ray possess inherent caloric, the thermometer should rise 
 as much, or nearly as much, as if it were in the direct solar ray; if the colour 
 passes in all its integrity, so too should the caloric; but place the thermo¬ 
 meter in the beam, and it does not rise. Nay, throw a concentrated column 
 of such light upon it, by a convex lens, and it is still unmoved. We are 
 therefore forced to conclude, that the rays exciting heat, are independent 
 of those exciting vision; that neither the red nor the yellow, nor the blue, 
 possesses inherent caloric; and moreover, that substances may be transparent 
 to red, to yellow, or to blue light, or to all, and yet more or less opaque to 
 the rays of heat. 
 
 18. It is not alone among watery solutions, or alcoholic tinctures, that 
 we find abundant instances of this kind of action, the mineral kingdom 
 furnishes many. A very thin lamina of pitch, is transparent to red light, 
 but almost opaque to the rays of heat. I have examined a variety of bodies, 
 gaseous, liquid and solid, and shall here point out the method which has 
 been followed in obtaining the results contained in the following table. 
 
 19. The mirror being placed upon the shutter as in sect. 6, a plano-convex 
 lens is to be screwed into the tube, so as to bring the rays to a focus, on 
 one of the balls of a very delicate differential thermometer, the motion of 
 the fluid is rapid, and the instrument soon attains a position of equilibrium: 
 this gives the heat of the sunbeam as concentrated by the lens. To find 
 the effect of any liquid medium in absorbing these rays, the trough filled 
 with the substance under trial, is placed at the extremity of the brass tube, 
 in a position as at c Fig. 5. The cone of rays converging from the lens a, 
 on the ball b , is subjected to its action, but because the trough has plain 
 and parallel surfaces, the rays still pass on, and form an imageon the focal 
 ball of the thermometer. The total effect, as given by the expansion of the 
 gas in the instrument, and which has formed the basis of the following table, 
 is not however an exact estimate of the action of the liquid solution. In 
 the instrument which I am in the habit of using, the convex lens is of flint 
 glass, and the plates of the trough of Boston crown glass; there are there- 
 
5 
 
 fore at least two disturbances, the absorbing action of the former, and still 
 more powerful effect of the latter. It has been considered, from the ex¬ 
 periments of Melloni , that the power of absorption, was inversely as the 
 power ot refraction, but whether an extended train of investigations, will 
 corroborate this supposition, remains to be seen. In the following experi¬ 
 ments, the instrumental arrangement being always identical, a comparison 
 may be instituted of the action of any two of the solutions; but the abso¬ 
 lute action of each cannot be determined, except after allowing for the 
 additional effect of the flint glass lens, and the crown glass plates. 
 
 In the practical operations, it will be found very useful, to blacken the 
 focal ball of the thermometer, as seen at e Fig. 5, which serves to give a 
 larger scale of thermometric expansion. It is also requsite to cover the 
 thermometer, with a very thin case of pure and transparent glass, which 
 serves not only to prevent the disturbance of currents, but also, of the 
 heat radiated from other bodies in the vicinity; this introduces however the 
 absorptive action of a third plate of glass; b d is the thermometer, and e e 
 the glass cover, Fig. 5. 
 
 21. By these arrangements it was found that a thin stratum of pitch en¬ 
 closed between two plates of crown glass, and which transmitted a homo¬ 
 geneous red light, absorbing all the other colours of the spectrum, allowed 
 only nineteen rays of heat to pass through it, of every hundred that fell 
 upon it. 
 
 22. A solution of the sulphate of copper and ammonia that absorbs the 
 red and the yellow light, being operated upon in like manner, was found 
 to transmit twenty rays, for every hundred that fell upon it. 
 
 23. There is however considerable difficulty in obtaining these numeri¬ 
 cal results with accuracy, arising partly from the difficulty of obtaining speci¬ 
 mens of exactly the same composition, but more especially, owing to 
 changes taking place in their colour. In process of time, most vegetable 
 solutions undergo spontaneous changes, and no longer give the same results. 
 But, where the same sample is operated on, under the same circnmslances, 
 repeated experiment assures me, that this arrangement gives comparable in¬ 
 dications. 
 
 24. Vapours and gases, may also be put under trial. The vapour of 
 iodine, whose spectrum is remarkable as containing only the extreme rays, 
 and wanting those of medium refrangibility, Fig. 15, absorbs two-thirds of 
 the heat that impinges on it. The vapour of nitrous acid, which stops the 
 violet, blue, indigo, and yellow light, Fig. 16, has a similar effect on the heat. 
 To experiment upon these bodies, a cubical bottle, Fig. 17, is very conve¬ 
 nient to generate the vapour in, and also to transmit the light through; it 
 will then replace the trough of section 7. Nitrous acid vapour is best 
 made for these purposes from Nitrate of Lead. 
 
 25. Having prepared a variety of solutions for the purpose of experi¬ 
 ment, and using for each the same trough, thoroughly cleansed after each 
 trial, the following table will give an estimate of the results obtained, it is 
 arranged according to the power of each solution, the first on the list being 
 the most energetic. 
 
6 
 
 Table of the Thermo-absorptive power of Solutions. 
 
 Decoction of logwood in alum water 
 
 Muriate of cobalt, 
 
 Solution sulph. copper and ammonia, 
 
 Bichromate of potassa, 
 
 Litmus water, 
 
 Hydro-sulphate of lime, 
 
 Decoction Brazil wood, 
 
 Muriate of iron, 
 
 Decoction cochineal, 
 
 Oil of turpentine, 
 
 Solution tannin, 
 
 Prussiate of potassa, 
 
 Solution chloride chromium, 
 
 Sulphate of copper, 
 
 Tincture turmeric, 
 
 Chloride of platinum, 
 
 Tincture saffron, 
 
 Chloride of gold, 
 
 Ink diluted, 
 
 Oil of bergamot, 
 
 Sulphocyanate of iron, 
 
 Linseed oil, 
 
 Hydro-sulphate ammonia. 
 
 Nitrous ether, 
 
 Water. 
 
 26. Still more powerful effects are produced, by making binary or ter¬ 
 nary arrangements. If, for instance, a beam of the Sun falls upon a very 
 thin transparent stratum of pitch, and then passes through a solution of 
 sulphate of copper and ammonia, or through linseed oil, not more than one- 
 fortieth part of the caloric is transmitted. 
 
 27. A question here naturally arises, what becomes of the heat thus lost; 
 does it enter into such combination with these media, so as to be detected in 
 them by the thermometer, as sensible heat? One of the pupils of this school, 
 Mr. Good, examined the amount of sensible heat, which these solutions 
 acquire on being exposed to the solar ray. It is a question of much diffi¬ 
 culty, there are so many disturbing causes in operation, the general results 
 of such experiments have not yet furnished actual proof, that the heat 
 missing is to be found in the fluid solutions. I would not however be un¬ 
 derstood to deny that such is the case, only, that at present, our informa¬ 
 tion does not warrant such a conclusion. It might be supposed that these 
 solutions do not act by a proper absorptive power, but merely offer that 
 kind of obstacle to the transmission of heat that turbid media do to light. 
 Not only, however, does direct experiment discountenance this, but the 
 analogy of their action on the chemical ray, renders it extremely improba¬ 
 ble,, an action which I proceed to develop, Fig 5 being still consulted. 
 
 28. Having removed the differential thermometer and its case, and pro¬ 
 duced a cone of light converging from the lens, where light passes through a 
 solution of sulphate of copper and ammonia, contained in the trough; if now, 
 we hold in the focus a piece of bibulous paper, imbued with chloride of silver, 
 although little or no heat is transmitted through the solution, yet an ex¬ 
 tremely dark spot is produced, characteristic of the blackening of that sub¬ 
 stance, by the solar rays. Though, therefore, the double salt transmits the 
 ray of heat with difficulty, the ray of chemical action passes with great faci¬ 
 lity. If a trough, containing a strong solution of bichromate of potassa, be 
 now substituted, a far larger quantity of light will pass, and vastly more 
 heat, but a paper imbued with chloride of silver, being held in the focus, no 
 chemical change whatever goes on , the chloride retaining its usual white¬ 
 ness. 
 
 29. I placed a piece of paper, imbued with chloride of silver, in a cubi¬ 
 cal box, one of whose sides was formed of a pair of glass plates, With a so¬ 
 lution of bichromate of potassa between them; it was exposed for many 
 days to the sun’s light, and only assumed a faint bluish stain, whilst a simi¬ 
 lar piece exposed to the direct rays, was fully blackened in fifteen minutes. 
 
7 
 
 So powerful is the action of this salt, that when a stratum of it, not more 
 than the hundredth part of an inch thick, was included between two plate 
 glasses, it stopped the decomposition of chloride of silver. It was after a 
 long examination of a great variety of substances, that I first became ac¬ 
 quainted with the great absorptive power of the chromates of potassa. In 
 my earlier experiments, I had made use of the chloride of platinium and 
 the chloride of gold, both of which have an analogous action. The solutions, 
 which I have recognised as possessing this power in the most eminent de¬ 
 gree, are 
 
 Bichromate of potassa. 
 
 Chromate of potassa. 
 
 Yellow hydro-sulphuret of ammonia. 
 
 Hydro-sulphate of lime. 
 
 Muriate of iron. 
 
 Chloride of gold. 
 
 Chloride of platinum. 
 
 Coloured vegetable solutions. 
 
 It is remarkable that all the mineral solutions, on this list are yellow, the 
 absorptive power however is by no means connected with that colour, for 
 the yellow tint is a compound one; all the rays of homogeneous light, are 
 absorbed by one or other of the bodies, on the foregoing list. 
 
 30. It is interesting to know whether these absorptions be really the 
 abstraction of something from the Solarray, or merely some change impressed 
 upon it. If light consisted merely of tremblings, pulses, or undulations, or 
 any other kind of motion of a homogeneous elastic medium, in virtue of 
 which it is competent to excite sensations of heat, and effect chemical 
 change, we might explain the action of these media, as the result of some 
 change occurring to that motion, either in direction or degree. We might 
 suppose too that when a ray had been deprived of its power, by passage 
 through one medium, it might have it restored, in a greater or lesser degree, 
 by being transmitted through another. I have not found, in thus comparing 
 together nearly three hundred media, any indications of such a result; and 
 therefore suppose, that something of a material character has been abstract¬ 
 ed from the ray, that it is really a loss, and not a change. 
 
 31. Being thus possessed of the means of depriving the beams of the sun 
 of their heat and their chemical force, I have proceeded to examine a va¬ 
 riety of questions of interest. A great many changes in the constitution of 
 bodies, on their exposure to light, are recorded in the books of chemistry 
 and physics, but they are there imputed to light, in the aggregate, without 
 any reference to its compound character, we shall find there are changes 
 due to the colorific ray, changes due to the calorific ray, and changes due 
 to the chemical ray. 
 
 32. One of the most important and extensive functions exercised by 
 radiant matter from the sun, is the decomposition of carbonic acid by 
 vegetable leaves, and the elimination of oxygen gas. Vegetable physiology 
 looks to chemistry for information, but hitherto the chemist has not pos¬ 
 sessed the means of perfectly developing the matter, and unfolding its 
 mystery. Its intrinsic importance entitling it to investigation, I shall not 
 oiler any apology, for passing from the direct object of this paper, to the men¬ 
 tion of some facts, necessary to the thorough understanding of the matter. 
 
 S3. It would appear, that there is a particular kind of combination to 
 which attention has hardly yet been drawn, distinct from what are 
 understood by chemical combination and mechanical mixture. A pint of 
 
8 
 
 alcohol, and a pint of water, being mixed together, the result will measure 
 somewhat less than a quart, and the same might be indicated, of a variety 
 of other liquids. No instance I believe is yet on record, of a like penetra¬ 
 tion of dimensions, being observed in the case of gases; if it exist at all, 
 it exists to a very small amount, and the change of volume which these 
 bodies readily experience, by alteration of temperature and pressure, ren¬ 
 ders so minute an effect, very difficult of detection. It has been supposed, 
 judging from analogy, that the constituent gases of the atmosphere, the 
 uniting volume of which is always constant, are held together in this man¬ 
 ner, or that the whole volume is condensed and retained by some force of 
 compression. There are some experiments which indirectly prove this: 
 sound passes along different media with a different velocity; if a can¬ 
 non were therefore discharged at a distance, it should impress the ear with 
 two distinct sounds; the one coming along the particles of nitrogen, should 
 arrive first, and shortly after be followed by one passing along the oxygen, 
 the intensity of these sounds, not being the same, on account of the superior 
 density of the nitrogen; that is, if the particles of oxygen and nitrogen 
 transmitted sounds independent of one another, or in other words if they 
 were not in a state of condensation, the molecules of the one pressing on 
 the molecules of the other. But it is well known, from observations made 
 directly on this point, that instead of there being any reduplication of the 
 sound, it comes clear, distinct and alone,—we have therefore to infer, that 
 these two gases are held together in a state of compression. 
 
 34. In making experimental investigations of this matter, two different 
 courses may be followed; first, we may measure the resulting volume, after 
 the mixture of known volumes of the gases under trial; or, secondly, we 
 may ascertain whether any thermal disturbance takes place, during the act 
 of their uniting, the latter is the mode I have followed in my researches. 
 
 35. Take a cylindrical glass, A Fig. 6, Plate XI. two inches in diameter, 
 and four in height, close its upper extremity with a flat piece of wood, by 
 means of cement, in the centre of it cement a stop cock a of large bore, 
 and at a suitable distance from that centre, make two holes the one to have 
 a piece of bent tube b, cemented into it to serve as a gauge, the other to have 
 a piece of copper wire c bent into the shape c Fig. 7, passed through it air 
 tight, by means of a cork x imbued with tallow. The other extremity of 
 the cylindrical glass, is likewise to be closed by a flat piece of wood, larger 
 than the former, for the purpose of bearing a little cup d, containing colour¬ 
 ed water, into which the gauge tube may dip, and in its centre it is to be 
 perforated to admit of an arrangement as in Fig. 9, Plate XI. where d is a 
 perforated cupping glass, having a stop-cock b mounted on it, whose further 
 extremity opens into a glass pipe c, which terminates in a hole,in the centre of 
 a flat copper circle «, three-quarters of an inch in diameter, this arrangement 
 is to be cemented, air-tight, into the flat piece of wood, that closes the 
 lower extremity of the cylindrical glass, as is seen in Fig. 6. Moreover, 
 beneath the cupping glass, there is a glass reservoir g , of suitable dimen¬ 
 sions, filled with water. The object of this arrangement, is to fill a soap 
 bubble with any gas, to expose it to atmospheric air, to burst it at will, and 
 to mark any thermal expansion of the two gases, by the indications of the 
 gauge; the mode in which this is accomplished, will be described in the fol¬ 
 lowing illustration. 
 
 36. The whole apparatus having stood for some time in a quiet room, 
 along with the gases to be tried, until they have all acquired an uniform 
 temperature, close the lower cock, fill the cupping glass with hydrogen 
 
9 
 
 gas,and raise the reservoir g, so that the level of the water maybe 
 near the top of the cupping glass. The upper cock being open, convey 
 through its bore, by means of a glass tube, of smaller diameter, a little soap 
 water, which is to be deposited on the copper circle, in its centre, over 
 where the glass pipe e opens, the tube is then withdrawn. Next open 
 slowly the lower cock, and as the gas is expelled from the cupping glass, 
 by the pressure of the water in the reservoir, it expands a bubble in the 
 large cylinder, the displaced atmospheric air, passing out through the upper 
 cock. When this bubble has attained the dimensions desired, close both 
 cocks, and observe if the liquid in the gauge be stationary; if so, turn the 
 wire c on its axis, so as to bring its crooked extremity, which is within the 
 cylinder, in contact with the bubble; it bursts, there is a thermal disturbance, 
 and an expansion of the two gases, for the fluid in the gauge instantly 
 falls, and as the gases cool, it slowly returns to its former position. If a 
 bubble of atmospheric air be employed, instead of a bubble of hydrogen, 
 these effects will not ensue. We therefore conclude, that when hydrogen 
 gas is mixed with atmospheric air, the temperature suddenly rises, and 
 therefore that it is probable, that the volume of the mixture, is less than the 
 sum of the volume of its integrant constituents. 
 
 37. If a soap bubble, filled with hydrogen, be burst in an atmosphere of 
 nitrogen gas, which may be effected by using a more complex arrangement, 
 than that indicated in the preceding section, there is also a thermal expan¬ 
 sion, indicating that the constituents of ammoniacal gas, even without chemi¬ 
 cally uniting with one another, exercise an attraction for, ora pressure on, 
 each other, a kind of capillary action. These compounds, for they form a 
 distinct class of bodies, a class by no means of small extent, require a dis¬ 
 tinct name. I have suggested that of capillary compounds; because they 
 exist under, and can be decomposed by the force of capillary attraction. 
 An example will here illustrate what is meant. Oxygen and hydrogen 
 gases may be mingled with each other in the proportion of one to two, the 
 result existing in a compressed state, and forming a capillary compound; 
 the contact of flame, or the passage of an electric spark, changes it into 
 aqueous gas, a chemical compound; in the former state, decomposition is 
 readily effected by capillary attraction; in the latter it cannot produce 
 such a result. The general law of these decompositions by tissues without 
 pores of sensible size, was announced by me in the Journal of the Franklin 
 Institute, Vol. XVII p. 1, &c., it is a very simple one, showing that a capillary 
 equilibrium is gained, only when the composition of gaseous media on each 
 side of a barrier, is chemically the same. This was proved by exposing 
 extremely thin soap bubbles filled with different gases, to different gaseous 
 atmospheres, and then measuring and analysing the media, within the bub¬ 
 ble and without. This law is applicable not only where a barrier sepa¬ 
 rates a gas from a gas, but also when one of the gases is held in solution by 
 water; and in the energy with which the media endeavour to attain an 
 equilibrium, is to be found not only one of the causes of the decomposition 
 of carbonic acid by the light of the Sun, but also a very fruitful source of 
 erroneous experimenting. Having made reference to this matter, I proceed 
 to detail the steps which have been taken, to lay bare the nrystery of this 
 decomposition. 
 
 38. Take four globular vessels, such as a, Fig. 18, Plate XI. three or 
 four inches in diameter, with necks a couple of inches long; fill them with 
 spring water, and put a bunch of pine leaves in it, immerse the end of the 
 neck beneath the surface of the mercury, contained in a cup b. Let one 
 
 B 
 
10 
 
 of these vessels designated A, be exposed to the sun’s direct ray; a second 
 B, to the light which has passed through a solution of bichromate of po- 
 tassa; a third C, to the light which has passed through a solution of sul¬ 
 phate of copper and ammonia; and a fourth D, in a dark place. It will be 
 found that in the course of a few hours, A has eliminated most gas, B 
 somewhat less, and C and D none at all; this is a very instructive experi¬ 
 ment; we find from A and C, that the sun’s rays have the power of elimi¬ 
 nating gas from its solutions; from B we learn that the absence of the 
 chemical rays does not affect the apparent result, but that if the calorific 
 rays are obstructed, it ceases to go on. 
 
 39. A variety of experiments having thus convinced me, that the mere 
 evolution of gas, is neither due to the rays of light, nor to the chemical rays, 
 I have attempted to produce a like effect with the calorific rays, emitted 
 from a common fire; rays, in whieh the light was altogether disproportioned 
 to the heat, and the chemical power totally wanting. The arrangement is 
 as follows; in the focus of a concave speculum of brass eighteen inches in 
 diameter, I placed one of the glass globes of the preceding section, so that 
 it might receive the rays emitted from a common wood fire, converged 
 on it by the mirror. The fire was burning without flame, being what is 
 technically called a dead fire, and the distance of the mirror eight feet. In 
 a few moments, gas was copiously liberated, more copiously than if it 
 had even been exposed to the solar ray. In Fig. 20 this arrangement is 
 depicted, a is the concave mirror, b the glass mattrass, filled with spring 
 water and containing a bunch of pine leaves, c a cup of mercury into which 
 its neck might dip. 
 
 40. I shall have occasion to remark hereafter, that when a beam of light 
 falls upon any surface in contact with a medium, it causes that surface to 
 exert an apparent pressure on the medium, capable at times of producing 
 singular effects; it is therefore probable, that to this action we are to attribute 
 the evolution of gas by vegetable leaves, spun glass, raw silk, &c. The per¬ 
 colation of liquids and gases, through tissues in obedience to the laws of 
 capillary attraction, should also, on these principles, be controlled by the 
 action of a solar beam. If we arrange two champagne glasses, with their 
 footstalks cut off, and capped with a thin lamina of Indian rubber, their 
 various apertures dipping into cups of water, so that they may be in all 
 respects as like each other as possible, and fill them with protoxide of nitro¬ 
 gen, we shall find, that one of them exposed to the sunbeam, will throw off 
 its gas much quicker than the other, shut up in the dark. Or, if one of 
 them be exposed to an atmosphere, much warmer than the other, the liquid 
 confining the gas in it, rises far more rapidly. It has been remarked to 
 me, by some chemist, that the experiment of which I gave an account, in 
 the Journal of the Franklin Institute, Vol. XVII, p. 177, of the passage of hy¬ 
 drogen gas through a thin film, without pores of sensible size, is not uni¬ 
 formly attended with success. In examining the causes of failure, I have 
 been able to trace them, entirely to this source; at a certain temperature, 
 the effect is scarcely perceptible, but as the thermometer rises, it becomes 
 more and more marked. The same observations may be made of ammo- 
 niacal vapour. There are temperatures at which these permeations are im¬ 
 perceptible, but at 75° Fall, they take place with great rapidity. 
 
 41. Rays of radiant heat, whether of the Sun or of terrestrial fire, pressing 
 on the surface of an obstacle, cause it to exert an increased action, resem¬ 
 bling a force of attraction or pressure, on any medium with which it inay be 
 in contact. A few fibres of unspun silk, being immersed in water contain- 
 
11 
 
 ing the elements of atmospheric air in solution, and exposed to the sun¬ 
 shine, became speedily covered with bubbles of gas. The exact chemical 
 constitution of these bubbles, is determined by a variety of circumstances, 
 the velocity of evolution, by the solvent action of the water, which is greater 
 for one gas than another, and by the presence or absence of the chemical 
 rays. I shall here be excused for remarking a circumstance which ap¬ 
 pears to me indicative of a proneness even in capillary compounds, to ex¬ 
 hibit tendencies of combination by multipule volumes. Atmospheric air 
 contains oxygen and nitrogen, in the proportion of 1 to 4; the gas expelled 
 from spring water, contains the same element in the proportion of 1 to 2; 
 and the gas given off by pine leaves from water, holding carbonic acid in 
 solution, contains the same elements in the proportion of 2 to 1. 
 
 42. The chemical rays emitted from the Sun, are not, therefore, the cause 
 of the evolution of gas from liquids by fibres, or by vegetable leaves, for it 
 takes place in their absence; the blue, the indigo, and the violet rays, have 
 nothing to do with it for the same reason; and the green, yellow, orange, 
 and red, are not the cause of it, for though they are present, it refuses to 
 go on. To the calorific ray, we are therefore to impute it; it happens, not 
 by the action of any kind of light, acting as a mere stimulus on plants, for 
 when the light is nearly absent, it goes on with undiminished energy. 
 
 43. Action of vegetable leaves on carbonic acid. —The evolu¬ 
 tion of gas, depending, therefore, on the rays of heat, we are next led to 
 inquire, whether the chemical rays affect the operation in any manner. 
 To understand this, 1 exposed a quantity of boiled water, which had been 
 suffered to cool in vacuo, to carbonic acid gas, of which it absorbed a cer¬ 
 tain amount. A portion of this water was placed in the focus of the brass 
 mirror, 39, and was there acted upon, by the non-luminous rays; its tem¬ 
 perature never exceeded 40° Fah. In a short time the pine leaves com¬ 
 menced giving off’ gas very copiously, and continued to do so; but it was 
 found on trial, that nearly the whole of it was absorbed by lime water, and, 
 that no decomposition had occurred. Therefore, though rays of non-lumin¬ 
 ous heat are competent to cause the evolving of gas, they are not able to 
 cause decomposition. 
 
 44. The record of an analysis, will place this effect in its true light; care 
 being taken that the water should be impregnated with pure carbonic acid 
 ! gas, and the leaves recent, when a sufficient quantity was evolved, 39 mea¬ 
 sures were taken, of which caustic potassa absorbed 34. Hydrogen gas 
 being then added, a diminution to the amount of 4 volumes was produced 
 by a platinum ball, the remaining gas was proved to be nitrogen. The com¬ 
 position of this gas was therefore, 
 
 Carbonic acid, 
 
 34.00 
 
 Nitrogen, 
 
 3.67 
 
 Oxygen, 
 
 1.33 
 
 
 39.00 
 
 It is proper to observe that a change very evidently takes place in the 
 structure of the vegetable leaves, their colour becoming of a dirty brown, 
 and all their greenness lost. Whether it is a change of their acting tissue, 
 which hinders decomposition, or whether there is some peculiarity in the 
 constitution of non-luminous heat, which incapacitates it from producing 
 those effects which result from caloric radiating from highly incandescent 
 bodies, I shall proceed to discuss. 
 
 45. Let us first consider what is the action of an ordinary unchanged 
 
12 
 
 sunbeam, on carbonic acid in solution, and in contact with vegetable matter. 
 A wide distinction is here to be made, between common spring water, such 
 as pump water, and water charged with carbonic acid only, the former, con¬ 
 tains a compound of oxygen and nitrogen, isometric with protoxide of nitro¬ 
 gen; but the protoxide is a chemical compound, having its two volumes 
 of nitrogen compressed into one, whilst this is a capillary compound, 
 existing with an almost insensible condensation. The process of evolving gas 
 from spring water, and from carbonated water, is essentially different, the 
 former taking place by an exaltation of temperature, occasioned by the im¬ 
 pinging of radiant heat, no kind of decomposition at all going on; but the 
 latter is accompanied by a true decomposition, due to the presence of vege¬ 
 table matter. 
 
 46. This case will be better understood by an analysis of the gas given 
 off* from carbonated water. A certain volume of water, had its carbonic 
 acid and all other gaseous impurity expelled, by long continued boiling; it 
 was then rapidly cooled by refrigeratory measures, and impregnated with 
 pure carbonic acid gas; being introduced into a mattrass, (plate I. fig. 16,) 
 with a bunch of pine leaves, the neck of the mattrass dipping under the 
 surface of some mercury contained in a cup, so as to cut off communica¬ 
 tion with the atmosphere, it was exposed to the sun, the day being very 
 favourable, clear, and hot: 47.50 measures of the gas evolved, were taken; 
 a piece of caustic potassa absorbed 3.50 measures of carbonic acid, the 
 remainder being 44.00 measures; 90 measures of hydrogen were added 
 thereto, making the full volume 134.00 measures; a platinum ball reduced 
 this to 67.00; indicating 22.33 of oxygen, there remaining of nitrogen 21.67. 
 The composition of this gas was, therefore, 
 
 Oxygen, . . 22.33 
 
 Nitrogen, . . 21.67 
 
 Carbonic acid, . 3.50 
 
 47.50 
 
 To prove that the remainder here spoken of was really nitrogen, one hun¬ 
 dred volumes of the original gas were taken, and the electric spark passed 
 through it; there was no diminution in the volume, nor any carbonic acid 
 gas generated; it could not therefore be carbonic oxide, hydrogen gas, nor 
 any of the carburets of hydrogen; it possessed, moreover, all the negative 
 qualities of nitrogen. 
 
 47. But, the solution was composed of carbonic acid and water; great 
 care having been taken to cut off all access from the atmosphere during its 
 preparation, and also during its exposure to the sun, for fear of a capillary 
 interchange of the carbonic acid with the gaseous elements of atmospheric 
 air. None such had occurred. From what source then came the large 
 amount ot nitrogen gas evolved? the only elements within the mattrass were 
 carbon, hydrogen, and oxygen, yet here a large amount of nitrogen was 
 found, which could have come from no other source than the pine leaves. 
 
 48. This fact, furnishes a clue to the mystery of the decomposition of 
 carbonic acid, by vegetable matter. A compound expressed by the formula 
 c -+• 20, is exposed under the circumstances detailed; at the completion of 
 the experiment, the constitution of the gaseous elements is expressed by 
 C + N. We therefore find, that the vegetable leaves had absorbed one 
 volume, whose composition is expressed by c 4- 6, and had given in ex¬ 
 change one volume whose symbol is N. Or, to speak without symbols, in 
 this experiment the pine leaves absorbed one measure of carbonic oxide, 
 
13 
 
 and gave in exchange for it one measure of nitrogen, and the resulting gas 
 contained therefore, half its volume of nitrogen, and half of oxygen, united 
 without sensible condensation. 
 
 49. Hitherto it has been supposed by chemists, that when vegetable 
 leaves were placed in carbonated water, they absorbed the carbon and caus¬ 
 ed the oxygen to be evolved. Vegetable physiologists, botanists, and others, 
 have raised a great many theories upon this fact, which, however, a long 
 course of experiments assures me are without any foundation. There is no 
 truth in the idea, that plants absorb carbonic acid, and assimilate carbon, 
 and evolve oxygen. On the contrary, they actually evolve nitrogen, and 
 the decomposition of carbonic acid, though remotely brought about by the 
 action of the solar ray, is mainly due to the complex play of affinities of 
 the elementary constituents of the plants. 
 
 50. I will here give another example in point, substantiating the same 
 fact under different circumstances. Carbonated water that had been 
 exposed with due care to the sun for two days being provided, 25.75 mea¬ 
 sures of the resulting gas were taken, and found to contain 1.25 of carbonic 
 acid; for caustic potassa diminished them to 24-50. Next, 31.50 measures 
 of hydrogen were added, making in all 56.00, and a platinum ball being 
 introduced, there remained 7.50 indicating 16.16 volumes of oxygen, and 
 8.34 of nitrogen, the composition of the gas being therefore, 
 
 Oxygen, 
 
 • 
 
 16.16 
 
 Nitrogen, 
 
 . 
 
 8.34 
 
 Carbonic acid, 
 
 • 
 
 1.25 
 
 
 
 25.75 
 
 Allowing for unavoidable errors of manipulation, the formula will stand 
 26 -f N; the leaves therefore had absorbed a compound of oxygen and car¬ 
 bon, whose composition is expressed by the formula, 3. vap. C + 0 = 1. 
 that is, condensed into one volume, and had rendered up one volume of 
 nitrogen in return, being of course the same amount; the resulting gas was 
 therefore one-third nitrogen, and two-thirds oxygen, united without sensi¬ 
 ble condensation. 
 
 51. If any further proof was required, that the evolution of nitrogen by 
 the plant, is an essential part of this decomposition, it is furnished by the 
 results of an experiment, in which spun glass was used to replace the pine 
 leaves. This arrangement, though exposed to the sun under the most fa¬ 
 vourable circumstances, will not evolve any gas, but on passing into it a 
 leaf, no matter how small, decomposition at once commences, because the 
 requisite quantity of nitrogen is given off. 
 
 52. A box «, 6, e, c, of a cubical shape, fig. 1, plate I., and nearly 12 
 inches in each of its dimensions, had one of its sides taken out, and replaced 
 by a trough k k of suitable size, consisting of two glass plates, cemented at 
 a distance of | inch from each other. This trough was filled with a solution 
 of bi-chromate of potassa; one of the sides of the box was hung on hinges 
 e e, as a door, for the sake of obtaining access to the interior. Within this 
 little chamber, a mattrass filled with carbonated water, and enclosing a 
 bunch of pine leaves, its neck dipping beneath the surface of some water in 
 a cup, was shut up, and exposed to the sun’s rays, which, passing through 
 the trough, impinged upon it. In a short time, air bubbles were copiously 
 given off, and when a sufficient quantity was obtained for analysis, its con- 
 
14 
 
 stitution was determined. The following is selected from a number of 
 analyses, being probably the most correct, and very nearly the mean. 
 
 Carbonic acid, 
 
 16.00 
 
 Oxygen, 
 
 8.16 
 
 Nitrogen, 
 
 4.84 
 
 
 29.00 
 
 We here remark the existence of a far larger proportion of carbonic acid, 
 but the relative proportion of the oxygen and nitrogen is still observed with 
 tolerable accuracy, the deviation may be satisfactorily referred to disturb¬ 
 ing causes. The greater amount of carbonic acid, as compared with sec¬ 
 tions 46 and 50, may likewise be due to the higher temperature of the ar¬ 
 rangement when shut up in a close box, where currents of air, or other 
 cooling agents, could not have free access to it. Or, it may hereafter be 
 found, that there are chemical rays of different colours, as it were; or, more 
 strictly of different refrangibility, and absorbability, and that those which find 
 a passage through bi-chromate of potassa, can cause the decomposition of 
 carbonic acid, though they cannot blacken chloride of silver. The doctrine 
 that chemical rays are nothing more than undulations of an elastic medium, 
 whose waves vary in breadth, I shall endeavour to support; each of these 
 kind of waves, being competent to bring about changes peculiar to itself; 
 or, adopting another hypothesis, that they are particles whose axes perform 
 certain oscillatory movements. Not in this place, however, to anticipate 
 what I have to offer on these matters; I shall continue to use the term chemi¬ 
 cal rays, as expressing those which blacken chloride of silver; and these, I 
 say, are not engaged in the decomposition of carbonic acid. 
 
 53. From the first observations made on the decomposition of carbonic 
 acid by Priestley, this subject has afforded much scope for chemical specu¬ 
 lation. Count Itumford examined it successfully, but wanting means of 
 accurate gaseous analysis, and above all not understanding the doctrine and 
 laws of interchange through tissues, his conclusions are devoid of that de¬ 
 gree of precision, which the advance of chemistry, in all its departments, 
 enables us to attain. The conclusion to which these earlier philosophers 
 came was, that plants had the power of absorbing carbonic acid, and ren¬ 
 dering oxygen in return, by elaboration from their vessels; arid this they 
 regarded as the great means employed by nature to maintain the integrity 
 of the composition of the atmosphere. A similar view has been taken of this 
 subject by almost every philosopher, who has since examined it. Professor 
 Burnet, to accommodate the theory to the observed facts, infers that plants 
 exercise two functions, the one of breathing, the other of digestion, the latter 
 only occurring during the stimulant action of the sunshine. This pheno¬ 
 menon is, however, unquestionably, one depending on the exalted capillary 
 action of a tissue when radiant matter impinges on it; and the evolution of 
 nitrogen or of some other gaseous or vaporous matter is, therefore, an es¬ 
 sential part of the process. 
 
 54. The calculations of analyses made in the foregoing sections, involve 
 the principle, that the volume of gas which remains after action is complete, 
 is exactly the same as the volume of carbonic acid first operated on. The 
 best method of proving this, is to take a tube, the diameter of which may 
 be half an inch or upwards, which is graduated to inches and decimal parts. 
 Fill it with water, from which all gaseous matter has been expelled by long 
 continued boiling, place a few vegetable leaves in it, carefully removing 
 any bubbles of air which may be attached to them, invert the tube in a 
 
our. Frwik.J/i.sti/utr Pol. JX. 
 
 Tlate 2 
 

 
 
15 
 
 vessel of water, and pass into it, as quickly as possible, a measured quantity 
 of pure carbonic acid, and transfer it to a mercurial trough. This arrange¬ 
 ment is seen in fig. 5, plate I. Conduct the experiment, first, in a cool 
 dark place, absorption will rapidly go on, and in a short time all, or the 
 greater part of, the carbonic acid will disappear, a column of mercury e e, 
 rising in the tube to replace the gas. It is to be remarked, that it is not 
 always easy to procure the entire absorption of all the gas, a little bubble 
 remaining in the upper part of the tube, containing the impurities that may 
 have existed in the gas, and also any remains of the carbonic acid, for the 
 amount absorbed depends upon several circumstances, as to the relative pro¬ 
 portion of the volume of gas to the volume of water, the height of the mer¬ 
 curial column suspended in the tube, the temperature of the arrangement, 
 &c. Then, on exposure to the solar rays, gas is copiously given off, the 
 quantity continually decreasing until any further exposure ceases to evolve 
 any more. On making the usual corrections for temperature and pressure, 
 the aggregate of evolved gas will be found precisely the same as the volume 
 first operated on. 
 
 55. Precisely I say, but this is with certain restrictions. Sometimes 
 the volume is increased by an amount varying from .10 downwards; due 
 chiefly to a certain amount of gas given off from the leaves extraneously; 
 and partly to the capillary action of the whole system upon the elements 
 of atmospheric air, which are transferred by slow degrees to the water 
 operated upon, should there be a film of that fluid between the mercury 
 and the sides of the glass tube; but, by making allowance for these disturb¬ 
 ing actions, the proportion of equality will be found to be rigidly observed 
 by the absorbed and the evolved gases. 
 
 56. We find, therefore, that the evolution and decomposition of carbonic 
 acid by the solar ray, are due to that part of it exciting heat; that the 
 chemical ray has no direct agency in the matter; it may bring about changes 
 which to a certain extent complicate the phenomenon, but that it does not 
 produce the abstraction of any compound of oxygen and carbon, from car¬ 
 bonic acid. Apart from the agencies exercised by the elements of the plant, 
 agencies which are unquestionably of the least importance, the decomposi¬ 
 tion is remotely brought about by the action of radiant matter. But non- 
 luminous heat, though capable of evolving gas, produces no change of its 
 constitution; (section 44,) shall we then suppose that there is a difference 
 in point of quality, between the heat given off'by the bodies below ignition, 
 and the heat of incandescent matter? Or, does the light itself aid decom¬ 
 position. An experiment may be made, which appears to me, to bear 
 directly on the answer which should be given to this query. Let a beam 
 of light, fig. 3, plate I. two inches in diameter, pass through an aperture 
 in the shutter A B, and fall upon any medium, c ri, which should absorb a 
 certain number of the rays ofheat, as bichromate of potassa, which may be 
 so diluted, as to absorb exactly 50 rays, out of every 100. Having, by 
 means of a good thermometer, g, measured this, let the beam of light pass 
 through a second trough e f, containing the same solution of the same 
 strength, and its temperature again be taken, it will appear that the ray 
 instead of losing half its heat, will contain nearly all of it, or in other words 
 the second trough exerts no action on the passing beam. In an experi¬ 
 ment, tried after the manner here indicated, the thermometer having shown 
 a loss of 50 rays by the action of the first trough, fell only to 47, or gave a 
 loss of 3 rays only, as the action of the second trough, an action to be re¬ 
 ferred, undoubtedly, to a degree of turbidness, which does exist to a 
 
16 
 
 small extent, in the clearest solutions; and also to the reflective and 
 scattering action of the surfaces of the troughs. Now, the very same thing 
 takes place in the case of light. A beam that has passed through a green 
 or any other coloured glass, loses a large amount of its intensity, but if it 
 pass through a second plate of the same tint, the second loss is entirely 
 disproportionate to the former, and the reason of this is very apparent, for 
 if the second plate had been of a different colour, the ray might have been 
 much more affected, or even entirely extinguished. Delaroche made an 
 identical observation in the case of non-luminous heat, for he proved, that 
 a plate of glass obstructed a large portion of the rays falling on it, but that 
 a second plate allowed these rays to pass with far less loss. Now these 
 experiments would lead us to conclude, that there are essential differences 
 in radiant heat, analogous to the differences in light. The rays of heat 
 given off’by a cannister of hot water may be, to use an expressive solecism, 
 violet heat, and a piece of transparent glass may be able to transmit green 
 heat only; hence, in using two plates, the absorptive action of the first has the 
 largest share in producing the phenomenon, the second transmitting nearly 
 all which passed the first, an action identical with that, of coloured glass 
 on light. Bodies, as their temperature rises, emit more and more rays 
 capable of passing through glass, simply because they become of that class 
 over which the medium does not exercise an absorptive power. 
 
 57. The general conclusion which we are to draw from these researches 
 is, that the decomposition of carbonic acid gas by vegetable matter, is a 
 very complex phenomenon, due to the combined action of three forces, 1st 
 the decomposing action of a tissue, 2nd to the impinging of radiant heat, 
 3d to chemical affinity, it being probable that any of these alone would be 
 incompetent to produce this result. And, in the case of gas, such as oxygen 
 being evolved from spring water, we are to refer the change to the ready 
 decomposition of capillary compounds, compounds essentially distinct from 
 chemical, and which can suffer decomposition by the force of capillary 
 action. The colorific and the chemical rays have no influence in this lat¬ 
 ter case. 
 
 58. Non-oxygenation of Phosphorus. It is stated in the books, that 
 Ritter in making observations on the slow combustion of phosphorus at 
 common temperature, found that it emitted white fumes in the movable 
 
 1 red lay of the solar spectrum, but in the movable violet, phosphorus in a 
 
 0 state ot oxygenation, was instantly extinguished. As a similar action is 
 alleged to take place when the sun’s rays shine on ignited carbon, it be¬ 
 comes desirable to understand the mode of action,—the original experiment 
 of Ritter, was therefore repeated with a view to ascertain its accuracy. A 
 cylinder of phosphorus, a b fig. 6. plate I., an inch long, and about one- 
 sixth ot an inch in diameter, was shielded from the action of aerial currents, 
 by a glass jar. In front of the jar, an equiangular prism of flint glass was 
 placed, so that the rays of a decomposed beam of light coming in through 
 the shutter c c/, could successively be thrown on the phosphorus which was 
 placed horizontally in the jar; the beam of light also came nearly horizon¬ 
 tally into the room, reflected by the arrangement already described. Situ¬ 
 ated thus, by turning the prism on its axis, any ray could be made to cover 
 the phosphorus, the temperature in the shade being 72° Fall., a fine sheet 
 of metaphospiioric acid, mingled with vapour of phosphorus, so thin as to 
 be almost imperceptible except in certain positions, was observed to be 
 rising from the cylinder, sometimes it would form a fine nebulous cloud, 
 which hung for a moment on the phosphorus, and then rise gracefully in 
 
17 
 
 curled wreaths. The extreme mobility of this cloud was remarkable, 
 even the warmth of the observer, by causing currents within the jar, would 
 affect it, if the hand approached as at A, fig. 7, plate I., the phosphoric 
 vapour came to the side of the vessel as it were to meet it, and then re¬ 
 bounded and circulated along the top of the jar. The size, position, and 
 shape of this cloud when enveloped in the red light of the prism were de¬ 
 liberately marked, its motions were merely more capricious than when in 
 the shade. And now, by turning the prism, the extreme violet ray was 
 brought upon it, but neither did its motion, or magnitude, or figure, appear 
 in any wise changed. 
 
 59. The impression conveyed by Ritter’s experiment is, that the chemi¬ 
 cal rays possess the faculty of hindering oxygenation. The negative con¬ 
 clusion here arrived at, might be due to local circumstances, and be 
 referred to the action of the prism, as to its composition, to the state of the 
 atmosphere, &c.; but no better success attended a variety of trials made on 
 different days, and with prisms of crown glass, turpentine, and water. 
 Trial was therefore made of absorbing media, a beam of light being made 
 to pass at one time through a solution of sulphate of copper and ammonia, 
 and at another through bichromate of potassa, but the condition of the phos¬ 
 phorescent cloud, was found to be too rough an estimate of the real action. 
 A cylinder of glass A B, fig. IS, plate I., .75 inch in diameter and 3 
 inches long, was therefore fitted at its upper end with a stop-cock er, its 
 lower extremity was closed air tight with a cork, through which an invert¬ 
 ed syphon b b passed, each of its limbs being four inches long and the bore 
 being Ath of an inch; the outer limb was fitted with a scale. After having 
 opened the cock a, a stick of dry phosphorus e was suspended in the cylin¬ 
 der, which was made very clean and dry, and the syphon being filled with 
 water, was firmly seated in its place and the cock closed. Now as the 
 phosphorus oxydized, the metaphosphoric acid was removed by the water 
 present, and the level falling in the laternal limb, indicated what quantity 
 of oxygen was consumed, and therefore the rate of combustion of the phos¬ 
 phorus. This was expected to give a more accurate estimate of any 
 changes occurring in the phenomenon, and was accordingly applied to de¬ 
 tect them. 
 
 60. A column of different coloured light, being made to pass at different 
 times through the cylinder, so as to impinge on the phosphorus, attempts 
 were made to ascertain the rate of combustion for each, as also for the white 
 light of the sun, and in the shade. In each insulated experiment, the 
 fluid in the gauge sunk with great regularity, more rapidly at first, but then 
 more slowly, but the same regularity was not observed in different trials. 
 At one time the phosphorus would consume with more than double the 
 rapidity that it did at another, though to all appearance under identical 
 circumstances. If the slow combustion of phosphorus be at all affected by 
 the action of solar light, it is certainly not to that extent which Ritter sup¬ 
 posed. So far from extinguishing, the violet rays do not exert any control 
 over it, or if they do, it is to so small an extent that the most delicate ar¬ 
 rangements fail to detect it. 
 
 61. It is possible however that atmospheric temperature may exert an 
 influence on the result. During the trials here made, a thermometer in the 
 shade has ranged from 70° Fah. to 82° Fall. At these points, the affinity of 
 the combustible material for oxygen may be so exalted, that the action of 
 any weaker force becomes masked. It is not stated what were the tem¬ 
 peratures at which the alleged results were obtained. But it is most pro- 
 
18 
 
 bable that the presence of extraneous matter has been the cause of all these 
 variations. It is well known that certain compounds of hydrogen and car¬ 
 bon, in extremely minute quantity, will entirely put a stop to the oxidation 
 of phosphorus, and during the course of these trials, I have had abundant 
 reason to notice errors arising from this cause. By simply wiping out the 
 cylinder with a linen cloth, which contained an almost imperceptible trace 
 of spirits of turpentine, an erroneous result like that of Ritter was at once 
 obtained. 
 
 62. Decomposition of the Salts of Silver, Several of the salts of 
 silver, undergo a remarkable change when exposed to the rays of light. 
 The bromide, the chloride, and the nitrate, being very good examples; 
 these, which are all white, become of a dark colour approaching almost to 
 black, when exposed to the violet rays; it is stated that the bromide is most 
 readily affected, yielding a brownish black colour. 
 
 63. II a piece of paper be soaked in a solution of nitrate of silver, and 
 then dipped into a solution of bromide of potassium, it affords a very ad¬ 
 vantageous means of facilitating these experiments. The chloride may oc¬ 
 casionally be substituted for the bromide of silver. 
 
 64. A beam of light, fig. 2 a a, entered a dark chamber horizontally, and 
 was obstructed in its course by a plane metallic screen 6, having a hole half 
 an inch in diameter in it. The beam c, which passed through this aperture, 
 fell upon a flint glass equiangular prism cl, and was decomposed by it, the 
 spectrum ef being received on the table, this spectrum was about three 
 inches long. And now a piece of paper, imbued with bromide of silver, 
 was placed to receive it, with the intention of ascertaining how far the dis¬ 
 coloration would extend. In the course of five minutes, a very marked 
 change had taken place, and on examination it was found that the deepest 
 tint had been occasioned where the violet blended with the indigo rays; 
 beyond this, even in the dark space beyond the spectrum, there was a stain, 
 as also as far in the spectrum as where the green light merged into the yel¬ 
 low, an effect represented in fig. 14, a a,bb, being the spectrum, during this 
 experiment the spectrum was kept stationary. Again, a column of light 
 three inches in diameter, converging from a convex lens a a fig. 15, was 
 intercepted by a screen of pasteboard b b, which had a circular aperture in 
 it, halt an inch in diameter, this screen was placed at such a distance from 
 the focus, that the circular section of the cone of light was half an inch in 
 diameter, and therefore passed exactly through the aperture; a piece of the 
 prepared bromide paper, was then fastened on the back of the screen, so 
 as to receive the condensed rays which passed the aperture. In a few 
 moments a black spot appeared about the central parts of the paper, and 
 at the end of the experiment there was an intensely black circle surround¬ 
 ed by a brown ring-like penumbra, as in fig. 8; the diameter of the black 
 spot being three-quarters less than that of the aperture through which the 
 light passed. 
 
 65. Diffraction of Chemical rays. Under certain circumstances, 
 two serial vibrations, each of which, if separately striking the organs of hear¬ 
 ing, would produce a musical sound, may so interfere with each other as to 
 produce an unmelodious rattling, or even silence. Also, two rays of light 
 whose paths bear a certain relation to one another, instead of increasing 
 each other’s intensity, may have a directly opposite effect, and neutralizing 
 each other, produce darkness. It becomes therefore a question, not only of 
 mere curiosity, but one whose bearings are important, to find if the chemi¬ 
 cal rays emitted from the sun, when placed under similar circumstances, 
 
19 
 
 exhibit similar phenomena. For then analogy would lead us to know that 
 it is possible for two rays of heat to be so situated with regard to one 
 another, that instead of exalting the temperature of the body on which 
 they fell, to lower it, or in other words to produce actual cold. 
 
 66. In my early trials for the solution of this question I met with many 
 disappointments, but at last I fell upon an arrangement which yielded posi¬ 
 tive information. It is however an experiment requiring careful manipulation. 
 A horizontal beam of light being projected into a room by the apparatus 
 heretofore so often referred to, at extremity e e, fig. 12, plate I. of the brass 
 tube, a double convex lens of gfiort focus was screwed, this brought the 
 rays to a point at a distance of three-quarters of an inch from the lens, here 
 they were obstructed by a metallic screen b b , having a round hole c, one- 
 eighth of an inch in diameter, perforated in it. This screen, revolved about 
 a vertical axis on a pillar d, so that it could be brought to any angle with 
 the incident rays. The rays passing through the round hole c, were re¬ 
 ceived on a white screen g g, at a distance of six inches. When the screen 
 b b received the incident rays perpendicularly to its surface, then, of course, 
 the image thrown on the screen g g was circular, but if the screen b b was 
 made to receive these rays at an acute angle, then the image was lenticular. 
 Under the last condition, the phenomena of diffraction is represented in 
 fig. 9, plate I. where a a is the screen, b b the lenticular image cast on 
 it, which is of bright white light, except at its central part c, where there 
 is a dark image produced by the interference of the passing rays. 
 
 67. If in such an arrangement the chemical rays do not interfere with each 
 other so as to neutralize their effects, chemical effects should be produced 
 in every part of the image, even including its centra! part c; but if, on the 
 other hand, these rays are obedient to the same laws as the rays of light, 
 then in the central parts of the image no chemical effects should ensue; 
 the problem is therefore reduced to the finding how any compound, change¬ 
 able by these rays, will comport itself on the central and peripheral parts 
 of such an image. 
 
 68. In place of the screen g g, a substitute was used consisting of two 
 thin plates of mica, with a lamina of bromide of silver included between 
 them, these were mounted in a little ivory frame abed, fig. 4, just in the 
 manner that objects are usually mounted for the use of the microscope, and 
 the lenticular image cast upon the bromide. After an exposure of five 
 minutes, during which care was taken to keep the sun’s place perfectly im¬ 
 movable, and also to avoid all local tremor, which might make the image 
 traverse on the bromide, the result was very apparent, being as represented 
 in fig. 11, of the natural size, the peripheral parts being of a deep brown, 
 and the centre yellowish white. Viewed through a lens, the boundary line 
 was not so sharp and distinct, but seemed to merge by insensible gradation 
 into the unaffected part, as in fig. 10. The conclusion to be drawn from 
 this result, possesses no common interest. For the same cogent reasoning, 
 which applies in the proof, that light consists of undulations of an elastic 
 medium, applies here also. 
 
 68. The chemical rays, thus closely attending the luminous rays, and 
 being like them subject to the forces bringing about reflexion, refraction, and 
 inflexion, it would become a matter worthy of inquiry, to find whether 
 there be any different classes of these rays, analogous to the different co¬ 
 loured rays of light, or the unequally refrangible and absorbable rays of 
 heat. The salts of silver, are only one of a class over which the chemi¬ 
 cal rays exert an action. The following list contains, I believe, all 
 
20 
 
 the metallic salts, at present known, in the constitution of which changes 
 are brought about by exposure to the sun. 
 
 Chloride of manganese, 
 Sulphocyrate of iron, 
 Sulphate of nickel, 
 Carbonate of lead, 
 Carbonate of nickel, 
 Nitrate of bismuth, 
 Chloride of uranium. 
 Sulphate of uranium, 
 Nitrate of uranium, 
 Chloride of copper, 
 
 Iodide of mercury, 
 
 Chloride of mercury, 
 
 Bichloride of mercury, 
 
 Chloride of silver, 
 
 Bromide of silver, 
 
 Sulphocyrate of silver, 
 
 Nitrate of silver, 
 
 Bromate of silver, 
 
 Chloride of gold, 
 
 Chloride of osmium and potassium. 
 
 Besides which, there are two others whose constitution is not well known; 
 one prepared from an alcoholic solution of the double chloride of platinum 
 and sodium, by the action of chloride of potassium, and the other in a simi¬ 
 lar manner from the cyanide of platinium. 
 
 69. The changes which these bodies experience, are of different kinds, 
 some become black and some bleach; some, as the sulphate of nickel, un¬ 
 dergo change of crystalline arrangement. If we are to take the chloride 
 of silver as a type of those bodies in this list, which undergo partial reduc¬ 
 tion, it will be found probable, that the change impressed on them is only 
 superficial, as analysis wili show. But we cannot tell with certainty, 
 whether a perfect reduction of some of these compounds takes place, or 
 whether it is a subsalt of a dark grey colour that results. By taking ad¬ 
 vantage of the property which chloride of silver possesses, of subsiding 
 very slowly from neutral solutions, so as to make them assume a milky 
 consistency, we may present it in a state extremely favourable to the action 
 of the solar ray. For if a thick mass alone be exposed, the central parts 
 will not undergo the same change as the exterior, being shielded by them 
 from the sun. A milky solution like this will, after an exposure for a cer¬ 
 tain time, become quite clear, the chloride precipitating, owing to the 
 liquid becoming acidulous. Mechanical agitation being then resorted to, 
 to expose fresh surfaces of the precipitate to the sun, very frequently dur¬ 
 ing a period of eight or ten days, and care being taken to suffer no dust or 
 other impurity to enter the vessel, it will be found that the powder has 
 become of a reddish grey, interspersed with little particles of unchanged 
 white chloride; these, from their superior density, will have precipitated more 
 readily than the grey particles; washing and decantation will therefore 
 readily effect a perfect separation of them. One hundred grains of the 
 dark chloride thus treated, will yield an analysis 79.3 of metallic silver; 
 that quantity contains therefore 20.7 of chlorine, it has lost then by expo¬ 
 sure 5.3 grains of chlorine, of the quantity originally contained in it. 
 
 70. Other analyses of the same sample, furnished results not widely vary¬ 
 ing from this, but such is not the case with analyses of different samples, 
 these give sometimes more, sometimes less, chlorine, they prove that the 
 chloride of silver as darkened by light, is not a definite compound, but 
 rather a mechanical mixture; that the change of composition is chiefly con¬ 
 fined to the surface, and does not affect the interior of the particles to any 
 extent; it is true, that microscopic observation shows them to have an uni¬ 
 form consistency and colour, but of course reveals nothing of their internal 
 character. An error is frequently made by writers who describe the 
 changes happening in this partial reduction; it is not, as they say, hydro¬ 
 chloric acid which is evolved when the chloride is under water, but it is 
 
21 
 
 chlorine, as is made very evident by the strong disagreeable odour of that 
 gas when he experiment is conducted in close vessels. 
 
 71. In addition to the list given above of substances changed by the 
 chemical rays, there are some others which exhibit their energy in a very mark¬ 
 ed manner. Chlorine and hydrogen unite together with an explosion; car¬ 
 bon and chlorine are also made thus to unite, in producing the per-chloride 
 of carbon: all kinds of vegetable colours are bleached; hydrodide of carbon 
 and chloro-carbonic acid are always made by the action of solar radiant 
 matter. 
 
 72. It has been stated by some chemists, that whilst the violet extremity 
 of the solar spectrum blackened cloride of silver, there are other parts of it 
 which would bleach the salt so blackened; but it is not so, for neither does 
 any part of a very dispersed spectrum, nor the rays which have passed 
 through a variety of absorbing media, exert such an action. These experi¬ 
 ments I tried repeatedly, under all the conditions of variation of tempera¬ 
 ture and brilliancy of the solar rays, but no observation led to the infer¬ 
 ence that there was any change of colour, or any sign of an approaching 
 change, even after the lapse of a whole month. Indeed, it would seem that 
 the state of this case does not justify any such expectation; when the chemi¬ 
 cal rays have disunited the chlorine, it is gone and lost forever to the silver, 
 being scattered abroad in the atmosphere; if therefore, the substance ever 
 regains a white colour, chlorine must have been purposely furnished from 
 other sources, or the white substance said to result, is some compound of 
 unknown ingredients. 
 
 73. The light of the moon is a remarkable example of luminous rays 
 existing without either calorific or chemical rays; the most delicate ther¬ 
 mometric arrangements have hitherto failed to show any rise of temperature 
 in the moonshine. A piece of paper, imbued with chloride of silver, may 
 also be exposed to the rays of the full moon, converging from a glass, and it 
 will not exhibit any change; this I proved, by placing such a paper in a 
 situation where for a whole night the rays of the moon could reach it. And 
 the same observation applies to terrestrial flames. In none of these has the 
 existence of the chemical rays been detected. Chloride of silver, after 
 being exposed for eight hours to the bright flame of an argand lamp, con¬ 
 verged by a lens, retained its whiteness. The same effect was witnessed, 
 when the flame of alcohol tinged red by strontian was employed, or the yellow 
 flame produced by chloride of sodium, and the green of Boracic acid; in 
 these cases the periods of exposure did not exceed half an hour. 
 
 74. Of the Perihelion motion of matter. Probably the most re¬ 
 markable effect exhibited by the solar rays, is the motion they produce in 
 media endued with much mobility. For many years it has been known, 
 that camphor exposed in a bottle to the rays of the sun, formed a crystal¬ 
 lization on that side of the vessel nearest the luminary; but the action is so 
 slow, and requires such a length of time for its completion, that no successful 
 investigation has been made as to the nature of the forces in operation. 
 Some philosophers have assumed, upon insufficient grounds however, that 
 the crystallization was effected on the most illuminated side, merely be¬ 
 cause it was the coldest, as we know that vapours are always deposited on 
 that part of a surface whose temperature is the lowest. 
 
 75. About three years ago, I published a series of observations on this 
 point, in the Journal of the Franklinlnstitute, Vol. XV., p. 156. Having 
 found from some theoretical considerations, that the crystallization of cam¬ 
 phor took place in vacuo, with a rapidity convenient for experimental in- 
 
22 
 
 vestigafion, I was led to make an extended inquiry into the whole matter. 
 The results so obtained, are now given to the public, they appear to me to 
 be so singular and important, and to conceal some secret respecting the physi¬ 
 cal constitution of the sun’s rays, that I cannot doubt they will lead to a rich 
 harvest of discovery. 
 
 76. The sun’s rays have the power of causing vapours to pass to the 
 perihelion side of vessels, in which they are confined, but, as it would ap¬ 
 pear, not at all seasons of the year. For example, I have a certain glass 
 fitted up for making these observations, and in this vessel, during the months 
 of December, January, and part of February, 1836—37, a deposit was 
 uniformly made towards the sun; during the months of March, April and 
 part of May next following, although every part of the arrangement remain¬ 
 ed to all appearance, the same, yet the camphor was deposited on the side 
 furthest from the sun. From May until the present date, the deposit is 
 again towards the sun. It does not appear that any immediate cause can 
 be assigned for this waywardness. Does it exist in the sun’s light? or in 
 changes affecting the earth’s atmosphere? or in imperceptible changes in 
 the instrument with which the observation is made? as respects the latter, 
 I think a negative answer may be given without any hesitation; but beyond 
 a mere expression of the fact that these anomalous circumstances do oc¬ 
 casionally occur, I would not be understood to speak decisively; if periodic 
 changes like this do occur, which is doubtful, they have not been watched 
 for a sufficient length of time, nor have I made sufficient variations in my 
 trials to be able to refer them to any distinct cause. A large bottle con¬ 
 taining camphor, which has been deposited therein for more than a year 
 under ordinary atmospheric pressures, has uniformly showed a crystalliza¬ 
 tion towards the light. 
 
 77. For making these experiments properly, it is necessary to possess an 
 air pump receiver ground so true as to be able to maintain a vacuum for 
 several hours, or even days. A less perfect jar may be made to answer, 
 by fastening it down to the pump plate with cap cement, it will however 
 be liable to leak when the cement becomes warm by exposure to the 
 sun. For many of these trials, a barometer tube is sufficient. Those who 
 are provided with a good pump and jars accompanied with their proper 
 transfer plates, will have no difficulty whatever. 
 
 78. Upon the plate of the pump, or one of the transferers a a Fig. 1, Plate 
 II., place some camphor in a watch glass c, supported by a stand; over this 
 place a bell-jar, and exhaust until the difference of level of the syphon guage 
 amounts to half an inch or less, the further the rarifaction is pushed the 
 better; remove the arrangement into the sunshine. In the course of five 
 minutes, if the atmosphere be clear and the sun bright, small crystalline 
 specks will be found on the side nearest to the sun, these continually in¬ 
 crease in size, and at the end of two hours, many beautiful stellated cry¬ 
 stals, from one-eighth to half an inch in diameter, will be found on that side, 
 but on the other parts of the glass, only a few straggling ones here and 
 there. This appearance, is represented in Fig. 2. Sometimes, as is the 
 case in a result which I keep by me, the whole side next the sun is covered 
 with a lamina of camphor, the other side containing none at all. 
 
 79. Or, having made a torricellian vacuum, in a tube upwards of 33 
 inches long and five-eighths wide, pass into it a piece of camphor, which 
 will rise into the void. This arrangement, like the former, when kept in 
 the dark shows no crystallization, even though so kept for more than four 
 months, but on bringing the^vacuum into a beam of the sun, crystallization 
 
Jou/'. Frank. Institute Vol. XX 
 
 Plate U. 
 

 
23 
 
 rapidly goes on, and at the end of a quarter of an hour the appearance is 
 such as represented in Fig. 4. It is not important that the temperature of 
 the sunbeam, or of the atmosphere, should be high; this is an experiment 
 which will succeed at temperatures varying from 120° Fah. to 60° Fah., 
 and probably at much lower degrees, for it is readily performed in the 
 depth of winter. 
 
 80. It is in no wise a phenomenon connected with the process of crys¬ 
 tallization. Take a jar twelve inches high, and four in diameter, quite 
 clean and dry, place it over a glass of Water b, fig. 5, and expose it to the 
 sunshine. In this experiment, it is not required that there should be a 
 vacuum within the jar. In the course of an hour or two, there will be a 
 copious dew at a, and on further exposure drops of water will trickle down 
 the side of the glass, but on the opposite side not the least cloudiness will 
 be found. 
 
 81. Barometers, hung up in such a position that the sun’s rays can have 
 access to them, exhibit an analogous appearance on the side nearest the 
 light, being studded with metallic globules. 
 
 82. In any of these experiments iodine may be substituted for camphor, 
 provided mercury is not present, nor any other substance on which this 
 material acts; the most advantageous method of using iodine is by heating 
 it in a suitable vessel, and when the vessel is quite full of vapour, present¬ 
 ing it to the sun’s ray, deposition goes on, on the perihelion side, as the con¬ 
 densation takes place. 
 
 83. Nor is it requisite in obtaining these results, that the material should 
 be either gaseous or vaporous. The rays of light, have the property, as was 
 found by Count Ruinford, of decomposing an aqueous solution of chloride 
 of gold; on making this experiment in a test tube one-third of an inch in 
 diameter, as a Fig. 6, small spangles of metallic gold will be seen, by re¬ 
 flected light, on the side towards the sun b , by transmitted light it appears 
 of a pale green tint, as is the colour of gold leaf. Here we find, that under 
 certain circumstances, solutions will deposit metallic matter, in obedience 
 to the same laws which cause the crystallization of camphor, and the de- 
 posite of aqueous dew. 
 
 84. A few pieces of camphor were laid on the plate of an air pump, and 
 a circle of glass two inches in diameter, a Fig. 7, was supported on a pedes¬ 
 tal in the midst of them, the upper part of the glass being four or five inches 
 above the pump plate; it was then covered with a jar, and exhaustion per¬ 
 formed. On exposure to the sun for a suitable length of time, numerous 
 crystals were found on the jar, but none on the circular plate, although it 
 had received the full beams of that luminary. This experiment was made 
 with a view of determining what peculiar condition a glass surface was 
 placed in by exposure to the light; for experimental purposes the rounded 
 form of the glass receivers, being very unsuitable, it was not therefore 
 without surprise, I observed that, however long the plate was continued 
 in the beams of light, no crystallization would ensue. A flat surface, 
 however, being essential to the trains of experiment pursued, trials were 
 repeatedly made, by various changes in the arrangement, to cause a de¬ 
 position of camphor upon such a crown glass plate; but though in five days I 
 could procure starry crystals upon the bell jar of more than half an inch in 
 diameter, in no instance was a solitary one found on the glass plate. 
 
 85. Two circumstances may determine the precipitation of camphor 
 crystals on a surface; 1st, Degradation of temperature; 2d, Increase of 
 pressure. To the former we cannot look for an explanation in the case be- 
 
24 
 
 fore us, for there is an actual increase of temperature in every part, and 
 more especially n that side of the vessel which is nest to the sun. Why 
 then does this condensation take place on the hottest surface, the side 
 nearest to the sun? we cannot admit, that the rays of heat have any active 
 part in bringing about the phenomenon. On the other hand, they ought 
 rather to exert a contrary effect, antagonizing the powers that solicit the 
 camphor crystals to form, and driving them to the coldest surface. We 
 are therefore reduced to the supposition, that when the light of the sun im¬ 
 pinges on a surface of glass, it places that surface in such a condition, that 
 it exerts a pressure on the adjacent medium, immediately followed by a 
 condensation of that medium. The state of the force here spoken of, ap¬ 
 plies to the glass surface alone; it is not an action between the solar ray 
 and the powers that effect crystallization, seeing that it equally takes place 
 in the deposite of aqueous or mercurial dew, and even of solid gold from a 
 solution of its chloride. In other words, if a ray of the sun be incident on 
 a surface of glass, it develops a force of attraction on that surface. 
 
 86 . A gaseous medium, having its temperature disturbed at any point, 
 has a current determined in it. In a chamber, such as the bell of an air 
 pump, this current circulates round the walls, ascending on the hot and 
 descending on the cool side; it might be supposed, that to this circumstance 
 was due the fact of no crystals being found on the plate of glass, sect. 84. 
 The condensation cannot however be attributed to this cause; for if so, a 
 lamp, or any other source of heat, would be equally effectual, it will how¬ 
 ever be hereafter shown, that terrestrial flames tend to remove these 
 depositions from the side nearest to them, and cause them to be accumu¬ 
 lated in the colder regions. 
 
 87. Beneath a receiver, a Fig. 3, a cubical bottle b, having flat sides, was 
 placed, and in the bottle a few pieces of camphor, the mouth of the bottle 
 was about half an inch in diameter, and was left open, the pressure of the 
 atmosphere being reduced to I 3 inches of mercury. Temperature of the 
 ray 57° Fall. On examination after the lapse of one hour and twenty-five 
 minutes, no crystals whatever could be found on the receiver, and but a 
 few sparsely scattered on the sides of the cubical phial. Now there can be 
 no doubt that the whole receiver was full of camphor vapour, and it does 
 not appear, that any reason can be assigned for the anomaly of its non¬ 
 crystallization. 
 
 88 . Will artificial light produce analogous results? To ascertain this, I took 
 a glass globe about one inch and a half in diameter, with a neck four inches 
 long, fitted it with a stop-cock, and introduced within it a drop of 
 water. The vapour of this water exhibited extreme mobility; the light from 
 the clouds caused its immediate deposition. A further advantage was 
 gained by the use of this apparatus, for by heating the globe uniformly, 
 until all the moisture on its surface was vapourized, and then allowing it 
 to cool, the particles of water readily obey the forces that solicit them. 
 This glass globe, supported vertically on an appropriate stand a, Fig. 8 , 
 plate II., was placed at a distance of nine or ten inches from a brightly 
 burning argand lamp A; to protect it from accidental currents of air, and 
 from irregularities of radiation from other sources, the whole arrangement 
 was covered by a bell b c, open at both ends, and about fifteen inches high. 
 It appeared at first that a thin dew lined the inside of the whole globe, in¬ 
 stead of being confined to one part, but after a certain space of time, the 
 heat which passed from the lamp through the protecting glass, disturbed 
 the results, the dew being driven to the coldest parts. To get rid of the 
 
25 
 
 effects of this heat, at a distance of about three feet from the lamp A, Fig. • 
 9, a double convex glass lens c, inches in diameter, was placed, which 
 brought the rajs to a focus at a distance of five or six feet, where stood the 
 glass globe a , covered with its protecting jar. The globe had been pre¬ 
 viously slightly warmed, so as to expel all the dew from its surface, and 
 give it an uniform temperature; in several trials it was found, that there 
 were no evidences that the bright flame of an argand lamp exerted any 
 force soliciting the vapour of water to move towards one part of the glass, 
 rather than another. 
 
 89. I took the arrangement of section 80, and shut it up in a dark closet, 
 having previously made the jar perfectly clean and dry; it remained there 
 for several days, that it might be found whether those little irregularities 
 of temperature which occur in such confined chambers, would cause this 
 dew to pass to one side of the glass rather than another; it did not appear 
 that such was the case, for the glass was as free from moisture when taken 
 out, as when shut up. And now, this arrangement being placed in the 
 window, where the sun was brightly shining, exhibited on its perihelion 
 surface in the course of three and a half minutes, a pearly dew; and in six 
 minutes drops of water were trickling down that side. 
 
 90. But it is not essential to the success of this last experiment, that the 
 solar ray itself should impinge on the vessel. The temperature in the shade 
 being 94° Fall., I placed the receiver with its cup of water in a window 
 having a northern exposure, and found that the dew readily made its ap¬ 
 pearance on that side which was towards the light. 
 
 91. It has been suggested by some who have seen these experiments per¬ 
 formed, that when a glass vessel is exposed to the sun, that part of the glass 
 which is nearest to him, may actually be the coldest ; such an opinion it is 
 evident rests on no sufficient grounds; for the sake however of those who 
 see force in this objection, the following experiment was made. A jar a g, 
 Fig. 10, was taken, of such dimensions that it could receive the differential 
 thermometer c d b, the balls of which b and c, touched the opposite sides, 
 and in the dark the liquid stood at zero, but on bringing it into the sun¬ 
 shine, if the side a was exposed, then the ball c was warmest, and if the 
 side g then the ball b was warmest, as was indicated by the motion of the 
 liquid. Hence we know, that in all cases where crystals of camphor, dew 
 of water, &c. are deposited on the side next the sun, they are so deposited 
 in spite of an energetic force, which tends to remove them. 
 
 92. Light which has suffered reflexion at certain angles, appears to have 
 undergone a remarkable modification, being no longer able to put the 
 glass into such a condition that it can cause motion towards the sun. It 
 is not to be inferred that any connexion is here traced between this dis¬ 
 turbance of the condition of light, and the change impressed on it by polar¬ 
 ization. A beam of the sun falling on a plate of glass, and being reflected 
 at an angle of 45°, may be intercepted by any of the arrangements of sec¬ 
 tions 78, 79, as by the barometer tube. It will be found that the crystal¬ 
 lization proceeds with considerable rapidity, not however on the perihelion 
 side of the vessel, but on the opposite side. It is probable that this result 
 is not dependent on the polarization of light, inasmuch as it takes place 
 equally well at all the angles, less and greater than the maximum angle of 
 polarization of glass. A ray of the sun cannot be made to disappear en¬ 
 tirely, as is well known, by any disposition whatever of two reflecting glass 
 plates, though the pale light shed by the clouds, may be very nearly brought 
 to that condition. But light, even that of the sun, having once undergone 
 
 D 
 
26 
 
 reflexion, has received some determinate impress, which disables it entirely 
 from causing camphor to crystallize on the perihelion side of vessels. 
 
 93. Another very remarkable phenomenon, is exhibited by the following 
 arrangement. Take a receiver a , Fig. 11, twelve or fifteen inches high, and 
 three or four in diameter, place it as usual upon the transfer plate, with its 
 proper charge of camphor c. Then cover it with a tin cylinder ef of suffi¬ 
 cient dimensions, to the end that all the light may be shut out, except at 
 one point g where there is a hole, half or three-quarters of an inch in dia¬ 
 meter. Under favourable circumstances, as a serene sky and bright sun, 
 let the arrangement be exposed so that a column of light may pass through 
 the aperture g, into the glass, it may or it may not finally fall on the cam¬ 
 phor at c. It would of course be expected, that a collection of crystals would 
 form on the inner surface of the glass, corresponding to the aperture g. 
 But on trial it is not so; for however bright the sun may shine, or however 
 favourable other circumstances may be, not a solitary crystal will make its 
 appearance, either there or on any other part of the vessel, provided its 
 temperature has been pretty uniform. On an exceedingly calm and serene 
 day in July, 1835, when every circumstance seemed propitious, I made 
 trial of this matter, and because the jar that I was using was not ground 
 sufficiently true to fit the transfer plate accurately, it had been fixed there¬ 
 on with common cap cement, and on exposure to the sun, the temperature 
 of the whole arrangement rose so high, that the cement was in almost a 
 semifluid condition; it was one of those days when the eye cannot behold 
 the sky, or look on the ground, without pain, yet not one crystal could be 
 made to appear opposite to the whole. But on taking ofif the metallic 
 screen, and exposing the jar, in a little more than a minute, small specks 
 were observable on the glass, and in a quarter ot an hour, its perihelion 
 side was densely coated with crystals. How are we to explain this? Bo 
 the edges of the aperture g impress any change on the passing light? Or is 
 the glass surface placed in such a condition, that it can no longer solicit 
 the deposit of crystals, we shall see hereafter that there are circumstances 
 yet more remarkable, which put us in possession of an explanation. 
 
 94. For the proper understanding of the rationale of these experiments, 
 it is required to know, whether it be essential that the solar ray should im¬ 
 pinge on the camphor or not; or whether the action is spent on the vapour 
 only. A tube was therefore taken of suitable dimensions, in the lower part 
 of which a fragment of camphor was deposited, and screened as much as 
 possible from the rays of the sun, whilst the upper part of the tube was 
 freely exposed. Crystals formed without difficulty, at a distance of three 
 or four inches, or even a foot, from the camphor, but there appeared to be a 
 limit beyond which they did not readily pass. A tube four feet six inches 
 long, and two inches in diameter, being exhausted, did not show on its ex¬ 
 posed end any appearance of crystallization. Near the camphor the deposit 
 was pretty copious, but in advancing from it the crystals were more sparsely 
 scattered, until towards the upper extremity none could be seen. Now 
 the maximum quantity of vapour that can exist in a void, or among other 
 gases, provided the mixture be in aequilibrio, depends on the lowness of the 
 temperature of any one part of the vessel, and hence a long tube one of 
 whose extremities is kept cold, does not exhibit these configurations readily, 
 because the quantity of vapour in it is small, owing to the coldness of one 
 part of the void space. It is not necessary, therefore, that the sun should 
 shine on the camphor, the effect of the rays taking place entirely on the 
 vapour filling the void. 
 
27 
 
 95. I now come to develop a singular action which certain bodies exert 
 over this process. Take a receiver, able to maintain a vacuum for some time, 
 and having cut out a ring a, Fig. 12, Plate II., of tin foil, an inch and a half or 
 thereabouts in internal diameter, and half an inch wide, paste it upon the 
 receiver as at a, Fig. 16, moreover, accommodate the receiver with its cam¬ 
 phor as usual, and having exhausted, expose it to the direct ravs of light, 
 so that the ring a shall be on the perihelion side. In the course of a short 
 time that surface will be found studded in various directions with crystals, 
 as is to be expected; but it will be found that none of these crystals tres¬ 
 pass within a certain distance of the ring, and that not one is to be seen 
 within the circle circumscribed by it. The ring, therefore, exerts a kind 
 of protecting action on the glass, forbidding the deposition of crystals with¬ 
 in certain limits; such a result is depicted in Fig. 13. 
 
 96. This action of a ring, formed of good conducting materials, might be 
 supposed to arise either from its adding something to the surface of the 
 glass, or taking something away from the glass with which it is in contact. Or, 
 on the other hand, it might be imputed to some change impressed on the ray 
 of light. Take therefore a ring a, Fig. 14, and place it before the receiver b , 
 at a distance of half an inch, the ring being of the same dimensions as in 
 the last experiment, it will be discovered that although the ring does not 
 touch the glass, it still protects it, no crystals coming within a certain dis¬ 
 tance of the regions overshadowed by the metal. Nay, even at a distance 
 from the line of shadow, not a crystal is to be seen, nor are any visible in 
 the illuminated centre. 
 
 97. Even after crystals have been formed on the surface of the jar, if it 
 be placed in the sunshine with a ring before it, as in the foregoing experi¬ 
 ments, the ring will be found not only to exert a protection on the glass, 
 hindering any further deposit, but will even remove the crystals that are 
 there. 
 
 98. This is indeed a remarkable circumstance; a part of the perihelion 
 surface is shaded from the sun, and thereby rendered cooler, yet the crys¬ 
 tals deposit themselves on the hottest surface, and avoid that where it is 
 cold. I know of none of the commonly received doctrines, that will give 
 the shadow of an explanation of the matter. We see, however, how it 
 happens that in the experiment of admitting a column of light through a 
 hole in a screen, no crystalline deposite was effected, the protecting agency 
 of the metal, whatever its power might be due to, seemed to hinder it. 
 
 99. To give the particulars of one of these experiments. On the 11th 
 of July, I prepared an arrangement, such as the foregoing, the thermome¬ 
 ters in the shade were at 76° Fall., and in the sun at 99° Fah., distance of 
 the ring from the jar half an inch, its internal diameter .75, width half an 
 inch. After proper exposure, the jar was examined, there were no cry¬ 
 stals on that part opposite the central opening of the ring, and the nearest 
 crystal to the natural border was ^ inch distant from where the shadow 
 was projected on the glass. 
 
 100. Vapour of water exhibits similar phenomena, a thin lamina of tin 
 foil in the form of a cross, a ring, or any other shape, effectually prevents 
 the deposit of water near it. 
 
 101 . Instead of placing the ring outside of the glass, now let it be placed 
 on the inside, as at a, Fig. 15, so that it may be within one-eighth of an inch of 
 the surface. When the crystals have fully formed, it will be discovered, 
 that the ring has exerted the same kind of protecting agency that it did 
 when on the outside of the glass. 
 
28 
 
 102. Hitherto, a class of bodies has been tried, a3 protectors, which 
 are without exception good conductors of electricity, such as the metals. 
 Certain indications led me to make trial of resinous matters, which are non¬ 
 conductors of electricity. Having made the region about a fig. 16, of the 
 air-pump jar, very warm, over a spirit lamp, a ring of rosin was spread on 
 it, about the same size as the ring of tin foil, which had been formerly 
 there. This ring of rosin was transparent, admitting the light to pass it 
 readily, and at a certain distance appeared of a fair amber colour. Having 
 arranged the jar as usual and exposed it to the sun, after a certain length 
 of time well marked crystals were deposited on the perihelion side, on 
 which the rosin was; these crystals not only came up to the verge of the 
 rosin and filled also the inner circle, but were found on the rosin itself. 
 
 103. Metallic plates of various shapes, and under various circumstances 
 were exposed with a view of causing condensation upon them ; it was not 
 found possible however either to cause the formation of aqueous dew, or 
 crystalline deposit, except when their temperature was below that of the 
 medium in which they were exposed. 
 
 104. At this stage of the inquiry, it becomes important to know, whether 
 along with the rays of light, of heat, and of chemical action, there are not 
 also rays of radiant electricity, emitted by the sun. Almost all operations 
 which disturb the equilibria of light and heat, disturb too that of electricity, 
 and it is well known that, upon this fact, Dr. Hare founds the explanation 
 of the action of certain voltaic arrangements, especially the calorimoter; 
 an explanation, the correctness of which, later researches make more proba¬ 
 ble. If light, heat and electricity are set in motion by the force of chemical 
 action, and are often found co-existing, there is nothing improbable in meet¬ 
 ing them together in the case before us. It is very true, that as yet we 
 have not met with any example of electricity, under what we understand 
 as a radiant form, but that it consists of undulations of an elastic medium, 
 like the undulations of light and heat, is not to be doubted. The experi¬ 
 ments of Nobili give proof of an interference, analogous to the interference 
 of the rays of light, which has served so well to refer the motions of that 
 fluid to the undulations of an elastic medium; the analogies of light and 
 heat are every where kept up, and we look with confidence that they will 
 be extended hereafter to electricity. 
 
 105. “ Quelle imposante decouverte ne serait-ce pas, si l’on parvenait a 
 deduire de la lumiere rayonnante, les proprietes par lesquelles les electri- 
 cites neutralizees se signalent.” (Berzelius T. de Ch. T. 1, p. 45.) The 
 tendency of the experiments here communicated, is to show that certain 
 substances, conductors of electricity, have the faculty of depriving glass of 
 that power by which it causes the condensation of vapours upon it when 
 exposed to the sun; that deposition will not take place on metallic surfaces, 
 but that certain vitreous and resinous bodies, interfere in no manner with 
 the process. The inference appears inevitable, that electricity brought 
 into play in some unusual manner, is the cause of the phenomenon. 
 
 106. By the action of the solar ray, electricity of high tension can be 
 developed. A copper electrical condenser was taken, the plates of which 
 were about one-fortieth of an inch apart, and six inches in diameter; there 
 was nothing more in their construction than is met with in the usual ar¬ 
 rangement. Another condenser was also provided, which was connected 
 with a gold leaf electrometer, the plates being one inch in diameter, and 
 separated from each other by a very thin coat of gum lac varnish. Trials 
 were repeatedly made to discover whether the apparatus was trustworthy. 
 
29 
 
 It is a common complaint against instruments intended to indicate low 
 charges of electricity, that they furnish evidence of an accumulation when 
 none has been communicated; it is necessary therefore to examine each in¬ 
 strument by strict tests, to be certain that this charge cannot be preferred 
 against it. Having obtained this preliminary evidence in a satisfactory man¬ 
 ner, and having decided the effectual goodness of the instruments in other 
 particulars,the following trial was made. The six inch condenser was ex¬ 
 posed to the sun-beam for one hour, on a clear bright day; the charged 
 plate was then parted, and applied to the one inch condenser; the plates of 
 this being parted, a small but perfectly distinct electric action was obtained. 
 This experiment is not however devoid of sources of error, as from the 
 friction occasioned by touching the plate of one condenser with the plate of 
 the other, or the heating action of the ray, which might cause currents of 
 air to brush over it, but it was found, by purposely rubbing one plate of the 
 condenser on the other, that no charge of electricity could be produced, 
 even if the friction were continued during some time; and on maintaining 
 the temperature of the condenser at the same point to which it was brought 
 by the sunbeam, in order to produce like currents of air, no divergence 
 whatever of the gold leaves was produced. 
 
 107. When the tension of electricity is high, one of the most delicate 
 methods of detecting its presence, is by the light it emits in vacuo; the ex¬ 
 citation caused by the tremulous motion of a column of mercury in a 
 barometer tube, is rendered visible by the bright light it gives out, when no 
 other method could discover it. On this principle, attempts were made to 
 detect electrical action in the sunbeam, by exposing metallic plates oflarge 
 dimensions to the ray, and causing any electricity they might gather, to 
 give out light in a vacuum; these trials did not prove satisfactory. 
 
 108. It has been stated in another part of these papers, that the cloud 
 which rises from phosphorus when slowly oxydating. is endowed with great 
 mobility; for certain purposes it makes a very good electroscope. When a 
 piece of this substance is shielded from the air by a bell jar, and not exposed 
 to disturbing action of any kind, a fine sheet of vapour rises vertically up¬ 
 wards. If at a distance of several feet, an excited stick of wax be presented, 
 the vapour curls from its path, and leans over to the side of the glass ad¬ 
 jacent to the cause of the disturbance. If such a jar be exposed to the sun, 
 a like disturbance is exhibited; as soon as the rays fall on it, it seems as 
 though they caused each particle to repel its fellows, the straight column 
 which before passed to the top of the jar, separates into confused masses 
 which pass forward to the perihelion side. 
 
 109. No direct proof existing that rays of electricity are emitted by the 
 sun, and as it does not fall within my limit todiscuss their hypothetical action, 
 it may be sufficient to give the proof, that if the surface be admitted to be 
 electrified, these deposits should take place. If a receiver be taken clean, 
 dry, and exhausted, and on any part of its interior surface a glass rod 
 be made to pass, the line which it describes will be stellated with camphor 
 crystals, if any of that odoriferous substance be present. This curious tact 
 was first observed in the case of an exhausted vessel, which had a small 
 syphon gauge shut up in it, the extremity of which rested against the glass; 
 by accident the gauge was moved half round the glass, and in a short time 
 alter a line of crystals was observed coinciding with the line of motion; it 
 was found possible afterwards to repeat this result at pleasure; the appear¬ 
 ances were such as are represented in Fig. 17., Plate II. 
 
 110. Upon the hypothesis here assumed, the deposit of crystals becomes 
 
30 
 
 a phenomenon analogous to the curious configurations described by Lich- 
 tenburg, when powders are dusted on the surface of an electrified plate; so 
 close is the resemblance, that one who sees crystallization produced by the 
 sun for the first time, would be led almost involuntarily to refer them to the 
 same cause; suppose it granted, that when light falls on any surface that 
 surface is electrified, it will exert an attraction on any particle within its 
 vicinity; but, if a conducting substance be placed in contact with the sur¬ 
 face, not only will it hinder deposit on the place which it occupies, but also 
 it will rob the glass around it for some distance; here we find an explana¬ 
 tion of the action of a tin foil ring. Again, if that conducting substance be 
 so placed as to cast its shadow on the glass, no deposit should take place on 
 that shadow, nor for a certain distance around it, because the electricity of 
 the adjacent parts would pass towards the unelectrified spaces, thus con¬ 
 ferring by a surface conduction, a low charge to all the shaded parts. 
 
 111. YVe meet, however, if we pass beyond these simple explanations, 
 with so many difficulties, that we are not encouraged to seek further con¬ 
 firmation of this hypothesis; there are some facts which prove almost de¬ 
 monstratively, that electricity is not the agent in question. If, instead of a 
 ring of rosin we make use of a ring of sealing wax, or a ring of pitch, these, 
 though they are non-conductors, do not fail to protect; the action of a me¬ 
 tallic ring when placed inside of a jar, cannot, so far as I know, receive any 
 explanation, especially if we are to admit the non-conducting power of a 
 spacefilled with camphor vapour only. It is plain and obvious, that trans¬ 
 parency and opacity have nothing to do with it; glass and rosin, it is true, 
 do not protect; but oil, which is equally transparent, protects as powerfully 
 as a metal. 
 
 112. Are we to refer this singular action, to the rays of light, to the rays 
 of heat, or to the chemical rays? By the action of absorbent media, at¬ 
 tempts have been made to satisfy this question. A barometer tube/r? e, 
 Fig. 18, had a conical tube fixed on its outside, so that the interstice could 
 contain liquids at c cl without leaking. Into this torricellian vacuum, I 
 passed a piece of camphor, and exposed the arrangement to the sun; having 
 tilled the interstice with water, it was found to have crystals on the aphelion 
 side, there being a ring of them as at e e, Fig. 19, all round the tube. This 
 fact being observed, the water was poured out and a solution of sulphate of 
 copper and ammonia introduced; on examination it was found, that on the 
 side nearest the sun no crystals were to be seen, but on the other side there 
 was a dense bed of them, extending exactly half way round the tube, and 
 very much resembling the shape of Fig. 20. A yellow liquid, the bichro¬ 
 mate of potassa was next introduced, a result to all appearance exactly like 
 the former was again produced, but having observed that the thickness of 
 the media had a very sensible effect, apparently due to their becoming 
 warm, and not casting off their caloric with sufficient rapidity by radiation, 
 I made an alteration in the arrangement, by interposing between the torri- 
 celian vacuum and the light, a trough capable of containing the different 
 solutions. This trough being filled with solution of bichromate of potassa, 
 and the ray tested that it could not blacken chloride of silver; in about one 
 hour the tube presented the following appearance:—there were some pretty 
 large crystals which extended round the tube, as at a Fig. 21, which, on 
 the aphelion side, suddenly mounted up, forming a kind of hyperbola, on 
 the anterior semi circumference not a solitary one was to be seen. The 
 trough being now filled with sulphate of copper and ammonia, the arrange¬ 
 ment of the crystals was found to be in every respect like the former. 
 
31 
 
 113. Supposing that this result might in some measure depend on the ray 
 having been subjected to reflexion, before passing through the trough, I 
 repeated the trials, when the sun’s altitude was small enough to permit the 
 rays to pass without requiring reflexion, yet still the same results were uni¬ 
 formly obtained; so that whether the chemical or the calorific rays were 
 stopped, crystallization took place on the aphelion side of the tube. 
 
 114. May it not therefore be, that this attractive force originates when¬ 
 ever the colorific ray impinges on a surface; it does not necessarily follow 
 from the phenomena, that any peculiar class of rays are emitted by the 
 sun, which bring about this action, but if there are such, it is a question of 
 interest to find what is the reason that good conductors of electricity, render 
 their action nugatory. 
 
 115. Botanical authors have long been aware of the important effects 
 which solar radiations exercise over the colour of vegetables. A plant, 
 w’hich grows in the dark, is of a pale whitish colour, and of a transparent 
 aspect, possessing none of that greenness and vigour which is so character¬ 
 istically developed on exposure to the sun; its consistency is watery, and al¬ 
 though its growth may not be stunted, its appearance is very sickly, its 
 secretory actions are not duly performed, and all its vital operations are 
 carried on in a state of force. There is no longer any evolution of nitrogen 
 from the leaves, and consequently no apparent production of oxygen gas. 
 Light, which seems to act merely as a stimulus on the green organs of 
 vegetables, indirectly bringing about the decomposition of carbonic acid, 
 though accessory is not however essential to the growth of plants. Sub- 
 teranean cavities, and places far removed from the direct solar ray, have a 
 color of their own ; and in the abysses of the ocean, at depths to which no 
 solar beam can penetrate, and where there is a perpetual night, green plants 
 are found flourishing. 
 
 116. The green colour of leaves, is presumed to be an immediate conse¬ 
 quence of the act of decomposing carbonic acid. (Decandolle phy. des 
 plantes) It appears to me, that there is some obscurity, if not an actual er¬ 
 ror, in the view which botanists take of this matter. They suppose, that by 
 the stimulus of light, some portion of the green organ is enabled to decom¬ 
 pose that gas, completely, or to accomplish its actual resolution into an 
 equivalent volume of oxygen, with the entire deposition of the carbon in the 
 solid form; that it is moreover this carbon, so deposited, that gives origin to 
 the green colour, seeing it forms the cliromule verte itself. Much useless 
 ingenuity has been thrown away by some chemists in explaining, how car¬ 
 bon, the colour of which is black, or a deep Prussian blue, can produce a 
 lively green, and even if their supposing that the modifying action of a 
 yellow tissue spread over it were correct, of which there is much doubt, 
 considering the thinness of that tissue, and the lightness of its tint, yet cer¬ 
 tainly we have no necessity to resort to any such explanation. The deposit 
 is not carbon chemically, it contains both oxygen and hydrogen in unknown 
 proportions. Of all the physical characteristics of a body, colour is the 
 most inefficient, it is even proverbial, that after uniting in a new mode, com¬ 
 pounds never bear the colours of their constituents ; nay more, carbon it¬ 
 self is not essentially of a black colour, as the diamond proves. 
 
 117. To a deposit of some compound, in which carbon enters as an ingre¬ 
 dient, we are to refer the green colour of leaves, but not to carbon itself! 
 On this point, vegetable physiology has been thrown into error by incor¬ 
 rect information, as respects the chemical part of the phenomenon. The 
 earlier chemists, who did not possess those extremely delicate methods of 
 
32 
 
 gas analysis, which are now available, gave wrong evidence in this matter. 
 They stated that on exposing a plant to the sunshine, in contact with car¬ 
 bonic acid, the carbon was separated in a concrete state, the oxygen being 
 left—but such is not the fact; by no known laws can such a change be 
 brought about, and hence any reasoning based upon it, as to the colour of 
 plants, is irrelevant. For when a plant exposed to the sun decomposes car¬ 
 bonic acid, a certain volume of oxygen disappears at the same time; in lieu 
 of this, and in obedience to the laws which guide the transit of gases through 
 tissues, (Jour. Frank. Inst. Vol. XVIII., p. 27) an equivalent volume of nitro¬ 
 gen is surrendered by the plant in return. Sometimes it is carbonic oxide 
 which is absorbed, sometimes oxalic acid, or other compound of carbon with 
 less proportion of oxygen. I do not here indicate from whence that nitro¬ 
 gen is derived, since botanists assert, that some plants contain no nitrogen 
 at all; it may however exist in their juices, as gas exists in spring water, or 
 may be retained in a compressed state on their surfaces, it is however a re¬ 
 markable fact, that nitrogen is present, and perhaps not less remarkable, 
 that its presence has hitherto been entirely overlooked. 
 
 118. The carbon thus taken from the acid, does not pass through the 
 tissue of the leaf in a concrete form, or give rise to a concrete deposit; 
 it bears with it a certain part of the oxygen with which it was formerly 
 united, the rest being set free; the carbon and oxygen so conveyed into the 
 plant, entering into combination with hydrogen, gives rise to the chromule 
 verte; hence we see, that the green colour depends indirectly on the de¬ 
 composing action, that when this goes on without interruption, that is fully 
 developed. 
 
 119. I took five pea plants out of the garden, as nearly resembling each 
 other in size, and other particulars as might be: they had just appeared 
 above the surface of the earth, and were beginning to put out leaves. 
 These plants I designate by the numerals 1, 2, 3, 4, 5. Each one was 
 planted in a small glass vessel, with a hole in the bottom for the purpose 
 of supplying it with water, after the manner of a common flower pot. 
 Number 1 was placed in a box, into which light passed which had traversed 
 a solution of sulphate of copper and ammonia. No. 2, in a similar box into 
 which light was admitted after having undergone the action of chromate of 
 potassa. No. 3 was placed in the open air. No. 4 in a box, into which 
 light passed which had been transmitted through sulphocyanate of iron. 
 No. 5 was shut up in a dark closet. This arrangement was completed on 
 the second day of May. With a pair of compasses the height of each plant 
 was ascertained, and of that, and of the number of leaves, a memorandum 
 was taken. In three days time an examination was made. 
 
 No. 1, had attained three times its former height, and doubled its num¬ 
 ber of leaves. • 
 
 No. 2, not quite twice its former height, no new leaves, in appearance 
 not so plump and transparent as formerly. 
 
 No. 3, twice its former size, with no fresh leaves. 
 
 No. 4, four and a half times its former size, and double its number of 
 leaves. 
 
 No. 5, three and a half times its former size, the leaves looked yellowish. 
 
 120. It is here proper to remark, that the increase of size is not to be 
 taken as an index of any action of the absorbing medium. Some years ago, 
 I had occasion to notice, that rapidity of growth was greatly influenced 
 by the quantity of aqueous gas in the atmosphere. Whether the observa¬ 
 tion possesses any novelty, I am not prepared to say, but if any one causes 
 
33 
 
 plants to grow in glass vessels, containing the maximum quantity of vapour 
 which their atmosphere can hold, at the temperatures under trial, their un¬ 
 usual increase of dimensions, will present a strikingly remarkable pheno¬ 
 menon. 
 
 121. In fourteen days, from the commencement of this experiment, an¬ 
 other examination was held. 
 
 No. 1, all its leaves of a grass green. 
 
 No. 2, of a darker green. 
 
 No. 3, green, but of a bluish tint when compared with a plant taken from 
 the garden. 
 
 No. 4, of a bright green. 
 
 No. 5, pale whitish yellow, with no fresh leaves, but grown to thirteen 
 times its former height, and apparently in a vigorous condition. 
 
 N. B.—With respect to No. 4, the plant under sulphocyanate of iron, I 
 was not aware at the time of making this trial, of the singular properties of 
 that substance in relation to light; in the course of a fortnight, which had 
 elapsed, the solution from being of a deep blood red, had become perfectly 
 colourless. No reliance is therefore to be placed on this result. 
 
 122. Among a number of experiments which were instituted with an 
 intention of illustrating the same point, and which gave analogous results, it 
 may be mentioned that the seeds of common garden cress, were caused to 
 germinate and grow in the boxes mentioned above. And no matter what 
 was the substance through which the light passed, the young plants after 
 reaching a certain size, were always green,—but those which grew in the 
 dark had yellow leaves and white stalks. 
 
 123. The general result of these trials goes to prove, that it is not this 
 or that species of ray, which gives rise to the colour of leaves, the absence 
 of the chemical ray, or of the calorific ray does not appear to affect it, nor 
 have we any direct proof that the colorific ray exercises any influence. 
 Humboldt has stated, that in the mines of Germany, plants as the poa an¬ 
 nua, et compressa , plantago lanceolata, &c., grow in recesses where the 
 sun’s light never comes, and provided hydrogen gas be present, their colour 
 is green. In the Atlantic ocean he saw a marine plant fucus vitifolius, 
 brought up from a depth of 190 French feet, where according to the calcu¬ 
 lations of Bouguer, the light was only equal to that emitted from a candle 
 at 203 feet distance, and yet its colour was green. Decandolle mentions 
 that artificial light, as that of lamps gives the same result; a proof that it is 
 certainly not the chemical and perhaps not the calorific rays which cause 
 the phenomenon. 
 
 124. Perhaps light in this case acts only as a kind of stimulus; it would 
 be desirable to make trial of some plants whose leaves are naturally white; of 
 this class there are several individuals; would they or would they not cause 
 the decomposition of carbonic acid? From many indications it is not impro¬ 
 bable that there is a variety of chemical rays, each of which brings about 
 changes of a character appropriate to itself. As yet, we have not learned 
 to distinguish these from each other, and are not provided with the means 
 of effecting their separation. A remarkable observation which appears to 
 me to be very much in point, was made many years ago, by Prof. Silliman; 
 it has not obtained that attention which it deserves; he states, that on expo¬ 
 sure of a mixture of chlorine and hydrogen to the light of a fire, an explo¬ 
 sion was produced. I quote the fact, however, only from memory, and 
 have endeavoured to substantiate it under a variety of circumstances, but 
 with a want of success probably due to the absorbing action of the glass 
 
 E 
 
34 
 
 jars used, or to the nature of the light. It is desirable that this experiment 
 should be once more repeated; it would settle an important point, that 
 chemical rays of different characters exist. I have referred to this before 
 in speaking of the perehilion motion of matter; for it is more than probable, 
 that there are chemical rays not absorbable by the chromates of potassa. 
 
 Note. —In the foregoing papers the reader is requested to make the following correc¬ 
 tions of typographical errors. 
 
 Errata 
 
 Sec. line 
 
 instead of 
 
 read. 
 
 Sec. 
 
 line 
 
 instead of 
 
 read 
 
 2 
 
 2 
 
 coherent 
 
 inherent 
 
 56 
 
 7 
 
 least 
 
 last 
 
 3 
 
 8 
 
 same light 
 
 sun’s light 
 
 53 
 
 3 
 
 temperature 
 
 temperatures 
 
 9 
 
 8 
 
 prism a 
 
 prism d 
 
 58 
 
 3 
 
 moveable 
 
 invisible 
 
 19 
 
 25 
 
 in the practical 
 
 in practical 
 
 58 
 
 4 
 
 moveable 
 
 invisible 
 
 40 
 
 8 
 
 glasses with 
 
 glasses fig. 19 with 
 
 59 
 
 20 
 
 laternal 
 
 lateral 
 
 40 
 
 10 
 
 various apertures 
 
 narrow apertures 
 
 68 
 
 11 
 
 Sulphocyrate 
 
 sulphocyanate 
 
 40 
 
 16 
 
 chemist 
 
 chemists 
 
 63 
 
 15 
 
 Sulphocyrate 
 
 sulphocyanate 
 
 41 
 
 1 
 
 pressing 
 
 falling 
 
 68 
 
 23 
 
 platiniurn 
 
 platinum 
 
 41 
 
 12 
 
 multipule 
 
 multiple 
 
 69 
 
 24 
 
 an analysis 
 
 on analysis 
 
 43 
 
 8 
 
 40°Fah. 
 
 110 Fah. 
 
 71 
 
 5 
 
 hydrodide 
 
 hydriodide 
 
 45 
 
 5 
 
 isometric 
 
 isomeric 
 
 72 
 
 2 
 
 cloride 
 
 chloride 
 
 48 
 
 3 
 
 c + 20 
 
 C + 20 
 
 94 
 
 15 
 
 aequilibris 
 
 tequilibrio 
 
 48 
 
 6 
 
 c + 6 
 
 cio 
 
 99 
 
 4 
 
 natural 
 
 internal 
 
 50 
 
 10 
 
 26+ N 
 
 20 + N 
 
 114 
 
 1 
 
 wherever 
 
 whenever