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 A .: 
 
11. — On Dew, By Mr John Aitken. 
 
 (Read 21st December 1885.) 
 
 The immense amount that has been written on the subject of clew renders it 
 extremely difficult for one to state anything regarding it which has not been 
 previously expressed in some form. It has been examined over and over by 
 minds of every type, and from every point of view; so that every possible 
 explanation of the different phenomena seems to have been given, and so many 
 passing thoughts recorded, that from the literary point of view the whole 
 subject seems exhausted. As a necessary result, these different treatises 
 are in many respects contradictory; and it would be quite impossible to 
 construct anything like a consistent explanation and account of our subject^ 
 from the very voluminous writings of those who have treated it from the purely 
 literary point of view, and whose ideas have been evolved from their inner 
 consciousness, according to what seemed to them the fitness of things, and 
 without questioning nature as to the truth of their conclusions. On the 
 scientific side of the subject, however, the writings are not so voluminous, and 
 additions to it are still required to enable us to determine which of the many 
 conflicting opinions are correct. 
 
 In ancient times it was thought that the moon and stars had an important 
 influence on dew, probably because there is most dew on those nights when 
 these orbs shine brightly on the earth; thus confusing two things which have a 
 common cause, and making one the effect of the other. Apjstotle placed the 
 knowledge of this subject far in advance of his time. He defines dew to be 
 humidity detached in minute particles from the clear chill atmosphere. The 
 Romans, led by the Avritings of Pliny, returned again to the primitive idea that 
 dew fell from the heavens. This idea retained its position during the course of 
 the Middle Ages. Then began an endless variety of theories, such as, that the air 
 is condensed into water by the cold, that the moon's rays caused it, and so on. 
 
 In the beginning of the eighteenth century clearer ideas began to be formed, 
 and a reformation took place, in which, as in most reformations, the swing of 
 the pendulum went to the extreme on the opposite side. Dew was no longer 
 believed to descend from the heavens, for Gersten advanced the idea that it 
 rose from the earth ; and in this opinion he was followed by M. Du Fay and 
 Professor Musschenbroek, the latter, however, afterwards made some observa- 
 tions which caused him to change his opinion. Gersten Avas led to think that 
 dew rose from the ground, because he often found grass and low shrubs moist 
 
 VOL. XXXIII. PART I. B 
 
10 MR JOHN AITKEN ON DEW. 
 
 with it, while trees were dry. M. Du Fay followed up these observations with 
 experiments, made by placing sheets of glass at different heights from the ground. 
 He found that dew formed on the lowest pane first, and only appeared on the 
 highest at a later hour; he also found that the lowest pane collected most 
 moisture. Other observers gave somewhat different explanations of the pheno- 
 mena connected with dew; but owing to a want of clearness, the subject did not 
 advance much till the masterly Essay on Dew by Dr Wells made its appearance. 
 
 Dr Wells' experiments were so simple, and his interpretation of the different 
 phenomena connected with dew so clear, that he has been justly considered the 
 gxeat master of this subject. In his Essay he struck a medium between the 
 two previous theories as to the source of the moisture that forms dew. He 
 did not think with the ancients that it fell from heaven, nor Avith Geksten that 
 it rose from the earth, but that it was simply condensed out of the air in 
 contact with the surfaces of bodies cooled by radiation below the dew-point of 
 the air at the place. This opinion has, so far as I am aware, been generally 
 received up to the present time. 
 
 Some experiments I have recently made on this subject have caused me to 
 differ entirely from Dr Wells as to the source of the vapour that forms dew. 
 As everything wTitten by Dr Wells is, so to speak, stereotyped and final, there 
 seems to be the greater reason that any of his conclusions that seem doubtful 
 should be carefully criticised and fully investigated; I shall therefore give an 
 account of the experiments that have caused me to differ from so great an 
 authority. 
 
 Dr Wells thought that almost all the moisture deposited as dew at night 
 was taken up by the air during the heat of the day ; so that, according to his 
 idea, vapour ascended from the earth during the day, and again descended and 
 became condensed as dew on the surface of the earth at night. My observa- 
 tions have led me to the very opposite conclusion. All my experiments indicate 
 that dew, on bodies near the surface of the earth, is almost entirely formed 
 from the vapour rising at the time from the ground; at least this would appear to 
 be the case generally in this climate, to which my experiments have been confined. 
 
 After Gersten gave his reasons for supposing that dew ruse from the 
 ground, and Du Fay extended the subject, Dr Wells combated their conclu- 
 sions, and successfully showed that their experiments did not prove that 
 vapour rose from the ground, and that all the phenomena adduced in favour of 
 their theory could be equally well explained according to his own. With 
 regard to Du. Fay's reason for thinking that dew rises from the ground — 
 namely, that it appears on bodies near the earth earlier than on those at a 
 greater height — he says :* " But this fact readily admits of an explanation on 
 other grounds, that have already been mentioned. 1. The lower air, on a 
 
 *An Essay on Dew, by William Charles Wells, p. 109. 
 
MR JOHN AITKEN ON DEW. 11 
 
 clear and calm evening, is colder than the upper, and will, therefore, be sooner 
 in a condition to deposit a part of its moisture. 2. It is less liable to agitation 
 than the upper. 3. It contains more moisture than the upper, from receiving 
 the last "which has risen from the earth, in addition to what it had previously 
 possessed in common with other parts of the atmosphere." Then he goes on to 
 give reasons why vapour cannot be rising out of the ground, but adds, that some 
 of it must be from this source, as bodies near the surface of the ground get dewed 
 sooner than those higher up, though equally cold with them, but says, " the 
 quantity from this cause can never be great," and proceeds to give his reasons, 
 which are not altogether satisfactory, and need not be quoted here. He then 
 sums up as folloAvs : — " These considerations ..... warrant me to conclude 
 that on nights favourable to the production of dew, only a very small part of 
 what occurs is owing to vapour rising from the earth ; though I am acquainted 
 with no means of determining the proportion of this part to the whole." 
 
 I shall now proceed to detail the observations which have caused me to 
 differ from the conclusion so distinctly set forth by Dr Wells in the above 
 quotations. I need not say that all my experiments only confirm the con- 
 clusions of that observer as to the formation of dew — that is, as to the 
 conditions most favourable for the deposition of moisture on the surfaces of 
 bodies during dewy nights, while the earth is radiating heat into space. The 
 point on which we differ is as to the source of the vapour that condenses 
 on the radiating surfaces^a point which Dr Wells admits there were no 
 facts to determine, his own opinion being formed by experiments that did not 
 bear directly on the subject. 
 
 When I began to doubt the truth of the generally received opinion as to the 
 source of the vapour, I found a difficulty in beginning my investigation, as it 
 was not easy to arrange experiments to give a direct answer to the question. 
 My intention at first was to test, by means of a delicate hygrometer, the 
 humidity of the air at different heights from the ground and under different 
 conditions. This plan had, however, soon to be abandoned, owing to the 
 impossibility of making anything like accurate observations with any 
 instruments at present in use. 
 
 For some time I have had in my possession a hair hygrometer constructed 
 by Chevallieb of Paris. This form of instrument is perhaps one of the best 
 for the purpose ; yet on making a few test experiments with it, for the special 
 purpose under consideration, its indications were found to be nearly valueless. 
 For instance, if the instrument was removed from saturated to drier air, and 
 again replaced in the saturated, it was impossible to get the pointer back 
 again to the same position on the scale ; and as the amount of dryness it would 
 be required to measure was a very small degree removed from saturation, the 
 error in the indications might be greater than the actual amount of dryness. 
 
12 MR JOHN AITKEN OF DEW. 
 
 Then again, all such hygrometers, as well as wet and dry bulb thermometers, 
 will have their indications affected by radiation; they will surround themselves 
 with an envelope of cooled air, as there is but little wind during the time the 
 observations require to be made. Their indications would therefore be of little 
 value, and investigation by means of them had to be abandoned. 
 
 What first caused me to doubt the present theory, and led me to suppose 
 that dew is formed from vapour rising from the ground, was the result of some 
 observations made in summer on the temperature of the soil at a small depth 
 under the surface, and of the air over it, after sunset and at night. On all 
 occasions in which these temperatures were taken, the ground a little below 
 the surface was found to be warmer than the air over it. It is evident that, so 
 long as these conditions exist, and provided the supply of heat is sufficient to 
 keep the surface of the ground above the dew-point, there will be a tendency 
 for vapour to rise and pass from the ground into the air, the moist air so 
 formed will mingle with the air above it, and its moisture will be condensed, 
 forming dew wherever it comes in contact with a surface cooled below its 
 dew-point. 
 
 These considerations suggested another method of experimenting than by 
 the use of hygrometers. If vapour is really I'ising from the ground during 
 night, it seemed possible that it might be trapped on its passage to the air, 
 and that this might be accomplished by placing over the soil something 
 that would check the passage of the vapour, while it allowed the heat to 
 escape. To carry out this idea, I placed over the soil shallow boxes or trays, 
 made of tinplate and painted. These trays were 3 inches (76 mm.) deep, 
 and more than a foot (305 mm.) square in area; they were placed in an 
 inverted position over the soil to be tested. 
 
 The action of these trays will be somewhat as follows : — Supposing the 
 
 roof of the small enclosure formed by the covering tray is not by the passing 
 
 air or by radiation cooled below the temperature of the ground. Then 
 
 evaporation will cease when the air between the tray and the ground is 
 
 saturated, and no dew will collect on the inside of the enclosure. But if the 
 
 tray is cooled below the temperature of the ground, vapour will condense on the 
 
 inside, and more vapour will rise from the ground to supply its place, and this 
 
 will go on so long as the ground is the warmer of the two. The effect of these 
 
 trays will be very much the same as if there was no enclosure, and the air 
 
 over the grass was nearly saturated, motionless, and of a lower temperature 
 
 than the soil. But it is evident the trays will check the evaporation on most 
 
 nights, on account of the slow circulation inside, and also on account of the 
 
 air inside being always nearly saturated, which is not the case outside the 
 
 enclosure, so that under most conditions it seems Hkely there will be less 
 
 evaporation under the trays than outside them. This will be particularly the 
 
MR JOHN AITKEN ON DEW. 13 
 
 case on those nights when thei^e is wind, and the air is not saturated, a con- 
 dition which seems to be very frequent in our climate at ordinary elevations. 
 We must remember that the air may not be saturated when dew is forming; and 
 the dew-collecting surface requires to be cooled below the temperature of the 
 air before it collects moisture. 
 
 In experimenting with these trays different kinds of ground were selected, 
 and the trays placed over them after sunset, that is, after the earth had ceased 
 to receive heat, and the heat-tide had begun to ebb. They were generally 
 examined between 10 and 11 p.m., and again in the morning. 
 
 Dew on Grass. 
 
 Confining our attention to the trays placed over grass, the result of the 
 experiments was that, on all occasions yet observed, there was — 1. Always 
 more moisture on the grass inside the trays than outside. 2. There was always 
 a deposit of dew inside the trays. 3. There was often a deposit outside the 
 trays, but the deposit on the outside was always less than on the inside, and 
 sometimes there was no deposit outside when there Avas one inside. 
 
 Now I think these facts prove that far more vapour rises out of the ground 
 during the night than condenses as dew on the grass. This excess is evidenced 
 by the greater amount of moisture on the grass inside the trays than outside, 
 and by the amount of dew condensed inside the box. Under the ordinary con- 
 ditions found in nature, this excess is carried away by the wind and mixed up 
 with the air, while some of it is deposited on bodies further away from the ground. 
 It should be noticed that the inside of the tray was more heavily dewed than 
 the outside. This shows there was a higher vapour tension inside than outside 
 the enclosure, which proved that the vapour rising from the ground outside 
 the tray had got mixed up with drier air, as it did not form so heavy a coating 
 of dew as the inside air, even though it had the advantage of a slightly lower 
 temperature than the inside, on account of it being the side of the metal 
 from which the heat was radiating. 
 
 It may be as well to notice here some objections that may be made to this 
 way of testing the point. It may be said, that though so much vapour does 
 rise under these trays, yet if they were removed and the grass freely exposed, 
 the vapour would not rise, and that the vapour rises because the tray keeps 
 the ground under it warm. Observation certainly shows that the ground under 
 the trays is kept slightly Avarmer than outside them. At night a ther- 
 mometer is higher on the grass under the tray than on that outside, and next 
 morning the ground at 3 inches below the surface is from 1 to 2 degrees Avarm^er 
 under the tray than outside its influence. This objection to the protecting 
 influence of the trays has an appearance of reason about it; but if we examine 
 the facts, I think it will be admitted that instead of being an argument 
 
14 MR JOHN AITKEN ON" DEW. 
 
 against this method of exj)erimenting it is rather a reason for it. We must 
 remember the tray does not heat the ground ; it does not add anything to its 
 store of heat, and enable it to evaporate more moisture ; it simply prevents so 
 much of its store of heat escaping. Now heat escapes from the ground at 
 night in tAvo ways — fii-st, by radiation, and second, by absorption — to supply the 
 latent heat of evaporation. From the area covered by these trays radiation 
 goes on much as at other places ; the painted metal will radiate as much heat 
 as the grass, but evaporation is checked, as there is but little circulation under 
 the trays ; and further, there is the heat recovered by the condensation 
 inside the box. It would thus appear that the reduced evaporation and heat 
 of condensation will be the principal causes of the higher temperature inside 
 than outside ; so that the trays, instead of increasing the evaporation, would 
 rather seem to decrease it ; and that the lower temperature outside is due to 
 the greater evaporation there taking place, as both surfaces are exposed to the 
 same loss by radiation. 
 
 There is an objection that might be made to the whole theory that dew is 
 formed from vapour rising from the ground. It might be urged that it is 
 impossible for the vapour to rise from the ground, and that these trays interfere 
 with the conditions existing in nature. On a cold clear night, for instance, when 
 the grass gets cooled before the dew-point, it might be said that it is quite 
 impossible for the vapour to rise up through it, as it would be all trapped on 
 its passage to the surface by contact with the cold blades, and that the trays 
 placed over the grass prevent this condensation by stopping the radiation from 
 the grass, and thus they allow the vapour to come up. 
 
 A little explanation will, however, show this objection to be groundless. 
 On a dewy night no doubt the top of the grass is at a temperature below the 
 dew point, and if Ave may take the temperature of a thermometer placed on the 
 grass to be the same as that of the grass, wliich we may do without sensible 
 error, if we then remove the thermometer and place it among the stems of the 
 grass, the thermometer will rise; and if we place the bulb among the stems close 
 to, but not in the ground, we shall find it to be very much warmer than at the 
 surface. On dewy nights I have frequently found it as much as 10 to 12 
 degrees warmer. From this we see that the warm air dift'using upwards with 
 its burden of vapour only meets with a very small amount of surface cooled 
 below the dew-point, so that the greater part of the vapour is free to escape 
 into the air. 
 
 Fairly considered, I think these trays more nearly represent natural 
 conditions than might at first sight appear. Indeed, precisely similar results 
 have been observed with natural conditions. If we examine plants with large 
 blades, we shall often find, on dewy nights, that those leaves which are close to 
 the groimd have thek under surfaces heavily dewed, while their upper surfaces 
 
MR JOHN AITKEN ON DEW. 15 
 
 are dry. The effect of the trays is very much the same as that of these large 
 leaves on a perfectly cahB night. The only difference is, the trays will lose 
 more heat on account of their better conducting power, and more vapour will 
 be condensed under them than under the bad conductor, while the temperature 
 of the soil will be more nearly reduced to what it would have been if no large 
 close surface prevented the free evaporation. 
 
 The experiments described were made in August and September, when the 
 ground was very dry, owing to the unusually small rainfall during the previous 
 months. On all occasions the inside of the tray was dewed, however dry the 
 soil, and the inside was always. more moist than the outside. 
 
 After these experiments were made, another method of testing the point 
 under investigation suggested itself, and though, unfortunately, rather far on 
 in the season for satisfactory work of this kind, I at once proceeded to carry it 
 out, as it afforded a means of checking my previous experiments with the 
 trays ; but by this time October had arrived, and the conditions had very much 
 changed. The temperature had fallen considerably, and the rainfall had greatly 
 increased the humidity of the soil. 
 
 It is very evident that if vapovir continues to rise from the ground during 
 dewy nights, as well as during the day, the ground giving off' vapour must 
 lose weight. If this could be shown to be the case, it would prove in a more 
 satisfactory manner than the previous experiments that vapour does rise from 
 the ground during night, and that, therefore, dew on bodies near the surface 
 of the ground is really formed from the vapour rising at the time, and not from 
 the vapour that rose during the day. 
 
 In the first week of October experiments were begun to test this point, by 
 weighing a small area of the surface of the ground, before and after dew had 
 formed, to see whether the ground continued to give off vapour or not while 
 dew was forming. For this purpose a number of shallow pans 6 inches 
 (152 mm.) square and ^ inch (6 '3 mm.) deep were prepared. One of these 
 pans was selected, and a piece of turf slightly smaller was cut from the lawn 
 and placed in it. The pan with its turf was then carefully weighed with a 
 balance sensitive enough to turn with ^ grain; but in experiments of this kind, 
 which must be done quickly, accuracy of only one grain was aimed at, lest the 
 time required for more accurate weighing might cause loss of weight by evapora- 
 tion. To prevent loss from this cause, the weighing was done in an open shed. 
 
 The turf was cut at sundown, and Avhen dew began to form. The earth 
 was removed from it till it weighed exactly 3500 grains (226 '79 grammes). 
 The pan with its turf was then rapidly restored to the lawn, and put in its 
 place, where the turf had been cut out, and in as good contact with the ground 
 as possible. The pan and turf were then brought back, the under side of the pan 
 carefully cleaned and dried, and all weighed again to make sure nothing was 
 
16 ; ME JOHN AITKEN ON DEW. 
 
 lost in the manipulations ; after which it was again restored to its place in 
 the lawn, and left exposed while dew was forming. A few experiments were 
 made in this way, in all of which the ground was found to lose weight. For 
 instance, on the 7th October, the small turf freely exposed to the sky at 
 5.15 P.M., Avhen weighed again at 6.30 p.m., was found to have lost 5^ grains 
 (0-356 grammes), and by 10.15 p.m. it had lost 24 grains (1"555 grammes). 
 FviUer particulars of these experiments will be given further on. 
 
 In making these experiments, the first thing done was to sink two thermo- 
 meters in the ground, one to a depth of 3 inches (76 mm.), the other to a depth 
 of 1 foot (305 mm.), and to place a third thermometer on the surface of the grass. 
 Eeadings were taken v/hen the experiment began, and again when the pans 
 were removed for weighing. During the time the turfs were exposed, generally 
 about 5 hours, the soil at 3 inches below the surface lost from 2 to 5 degrees, 
 while at 12 inches the loss was small. No doubt part of the heat was lost by 
 radiation, but in grass-land, where the surface of the soil is protected by a fairly 
 good non-conductor, much of the heat will be spent in evaporating the moisture. 
 
 These experiments prove clearly that under the conditions then existing, 
 the soil loses weight, and that vapour really rises from the ground even while 
 dew is forming ; therefore the dew then found on the grass must have 
 been formed out of the vapour rising from the groimd at the time. The dew 
 on the, grass was, in fact, so much of the rising vapour trapped by the cold 
 grass. The blades of grass acted as a kind of condenser, and held back some 
 of the vapour which would have escaped into the air. 
 
 It must not be supposed that these experiments in any way contradict the 
 well-known observations of Wells and others who have worked at the subject. 
 It has long been the custom to expose different substances to radiation during 
 the night, and to estimate the amount of dew on different nights by the increase 
 of weight due to the moisture collected on them. It must be noticed that the 
 conditions of the two sets of experiments are quite different. In those for 
 estimating or measuring the amount of dew, the collecting body must not be 
 in heat communication with the earth, an essential condition being that it shall 
 receive no heat by conduction from surrounding bodies ; whereas, in the experi- 
 ments with the' turf, the essential condition is that the body experimented on 
 shall be in as good contact with the ground as possible. The result of these 
 two conditions is, that in the former, the exposed surface loses heat by radiation 
 into space, and soon gets cooled below the temperature of the air, and when 
 cooled below the dew-point, dew collects upon it ; while in the latter case the 
 exposed surface is in good heat communication with the ground, and tends to 
 keep hotter than the other surface ; then being always moist it tends to give off 
 vapour, which "diffuses away from the hot ground and escapes into the air 
 above, but in part is trapped by coming into contact with the cold grass. 
 
MR JOHN AITKEN ON DEW. 17 
 
 The experiments were generally stopped at night. It would be of no use to 
 let them go on till morning, unless one were in attendance at sunrise; for the 
 early morning heat radiated from the sun and sky would cause an increased 
 evaporation, and make the loss appear too great. On one occasion, however, 
 Avhen the morning was dull, weighings were made, and the soil was then found 
 to have lost weight during the late night and morning. 
 
 The following simple observation is sufficient to convince us that, under the 
 ordinary conditions of our climate, vapour is almost constantly escaping night 
 and day from soil under grass. Go out any night, but it is best when terrestrial 
 radiation is strong, place one thermometer on the grass, and push another under 
 its surface, among the stems, but it need not be into the soil, and note the differ- 
 ance in temperature. As an example, I found, at 10.45 p.m. on the 10th October, 
 this difference to be as much as 18'5 degrees. The thermometer on the surface 
 of the grass was 24°, while the other, only about 1^ inches underneath it, and 
 not in the soil, was as high as 42° '5, the temperature of the air at the time 
 being 32° '5. Of course, this difference varies, and is not always so great as on 
 this occasion, when the sky was clear and the air still. An experiment of this 
 kind causes us to doubt the value of the radiation observations made by com- 
 paring the readings of a thermometer placed on the grass with the temperature 
 of the air in the screen ; because the temperature of the thermometer on the 
 grass varies greatly according to its position. If its bulb is supported near the 
 tips of the stems, the temperature is much lower than when it is allowed to 
 press the grass close to the ground, because in the latter position it receives a 
 good deal of heat from the earth. 
 
 It might be objected that these experiments having been made late in the 
 year, and when the soil was damp, they do not prove that evaporation would 
 take place in summer when the soil was dry. Other considerations, however, 
 lead us to suppose that this nightly evaporation does go on even after a con- 
 tinuance of dry weather, though I have no- direct experiments to prove it, other 
 than those made with the inverted trays. But I find that soil, after it has been 
 kept for some time in a house, and when it looks dry and incapable of support- 
 ing vegetation, still gives off vapour, and saturates the air over it. This was 
 shown by placing over some dry-looking soil a glass receiver, in which was 
 hung the hair hygrometer. The instrument soon showed an incease of humidity 
 inside the receiver, and after a time indicated saturation. To check the read- 
 ing of the hygrometer, it was quickly removed and placed in saturated air, when 
 it was not found to change its reading. 
 
 Now as soil, even when it appears dry, tends to give off vapour, and saturate 
 the air in contact with it, it is evident that under most conditions of our 
 climate the vapour tension at the surface of the ground, amongst the stems of 
 the grass, must, owing to the higher temperature, be very much greater than 
 
 VOL. XXXIII. PART I. C 
 
18 MR JOHN AITKEN ON DEW. 
 
 at the tops of the blades ; and as the air and vapour are warmer, they tend to 
 rise and diffuse themselves, and so come into contact with the colder blades at 
 the surface, where the moisture gets deposited as dew. 
 
 Having proved that, under the conditions existing during the experiments, 
 the ground was giving off vapour during the night, I then proceeded to 
 test the value of the observations previously described, and which were made 
 by placing shallow trays over the grass, in order to see if those experiments 
 were of any value. A small tray, similar to those used in the earlier 
 experiments, was prepared. It was made to fit tightly into one of the shallow 
 pans, in which, as before, was placed a small turf cut from the lawn. After the 
 turf and its pan was weighed, the tray was placed over it, and the whole 
 removed, and put in its place in the lawn. This was done at the same time 
 as the other experiment previously described, in which the tui-f and pan, 
 after being weighed, was freely exposed to radiation and evaporation. The 
 result was that the tray was found to check the evaporation. The inside 
 of the covering tray was dewed very much like another one placed over un- 
 disturbed grass. The turf covered by the tray lost only 6 grains (0-388 
 grammes) during the five hours, or about ^ of the amount lost by the one freely 
 exposed to the air. This shows that the trays check the evaporation ; we may 
 therefore conclude that the amount collected by them is less than would be 
 given off" by the exposed parts of the grass. 
 
 There seems to be reason for supposing that the amount lost per unit of 
 area in these experiments, with the freely exposed turfs, is too low an estimate 
 for the loss of the lawn at the parts where it was undisturbed, because the 
 under sides of the pans were not in good contact with the ground beneath them. 
 The experimental turf would not therefore be so warm as the rest of the 
 ground, and its evaporation would therefore be less. Most of the heat was 
 conveyed upwards towards the experimental turfs by the rising vapour, which 
 condensed on the under sides of the pans on which the turfs rested, as they 
 were always found to be dripping wet underneath when removed from the soil. 
 
 The question now comes to be, Does this evaporation take place from grass- 
 land on all nights and in all weathers ? So far as my observations at present 
 go, evaporation is constantly going on, however strong the radiation. On all 
 nights on which the inverted trays have been exposed, dew has collected 
 on their inner surfaces. There is, however, an indirect way of testing this 
 point which may be noticed here, as it is specially applicable to observations on 
 grass land. As soil capable of supporting vegetation tends to saturate the air 
 in contact with it, it will be admitted that so long as the soil is hotter than the 
 air in contact with the grass, vapour will tend to difiuse upwards. Now I find 
 by placing a minimum registering thermometer on the grass, and another on the 
 top of the soil among the stems of the grass, that there is always a difi'erence 
 
MR JOHIS^ AITKEN ON DEW. 19 
 
 between the minimum on and under the grass, often amounting to a considerable 
 number of degrees, this difference being greatest on nights when radiation is 
 strongest, and least when windy and cloudy. It is only as the day advances 
 that the temperature on the grass approaches that under it; this is caused by 
 the upper thermometer being heated by solar radiation sooner than the lower ; 
 but as the air is by this time drier, there is no tendency for it to lose moisture 
 by contact with the colder soil, though some of the dew condensed on the grass 
 will, after it evaporates, diffuse downwards, and condense on the soil. It may 
 therefore be safely concluded that, on almost all nights in this climate, vapour 
 does rise from grass-covered land, and it is this vapour that we see as dew 
 on the exposed surfaces of the grass. 
 
 Dew on Soil. 
 
 While the experiments previously described were being made on ground 
 covered with grass, parallel ones were made on bare soil. The inverted 
 trays placed over soil always showed a greater amount of condensed vapour 
 inside them than those over grass. Sometimes there was a heavy deposit 
 of dew inside, while there was none outside. This would be owing to the soil 
 radiating directly to the trays, and to the amount of heat brought up and con- 
 veyed to the trays by the vapour. The temperature of the trays was thus in 
 some cases kept above the dew-point of the air outside. 
 
 Experiments were also made by weighing a small area of the surface soil, 
 to see if it also lost weight like the grass-land during dewy nights. One of the 
 small pans was covered with a thin layer taken from the top of the 
 soil. The pan and its soil was then weighed and put on the surface of the 
 bare ground at the place where the soil had been taken out. It was left 
 exposed the same time as the other trays with the turfs. On weighing, the 
 soil was found to have lost 23 grains (1-490 grammes) in five hours, or nearly 
 the same as the turf Alongside this pan was placed another one of the 
 same area, and with the same weight of soil, but covered with a small tray, 
 to see whether the covering trays decreased the evaporation from soil as well as 
 from grass. The result was the same as was found with the turf — a decrease 
 in the evaporation. The protected soil lost only 8 grains (0-518 grammes). 
 
 The following are the details of a few of the experiments on grass-land and 
 on bare soil made on different evenings, and show the temperatures and the 
 loss of moisture per 0-25 square foot, or 0-023225 square metre, during the 
 experiments :* — 
 
 * Tkrougliout this investigation I have adhered to the Fahrenheit scale, as it is the one generally- 
 used for meteorological purposes in this country, and hecause it possesses what appears to me practical 
 advantages over the Centigrade scale. The degrees are of a more suitable size, and combine ease in 
 reading with accuracy. This scale also avoids a fruitful source of error, experienced by mauy, when 
 taking readings above and below zero. 
 
20 MR JOHN AITKEN ON DEW. 
 
 October 7, 1885. 5.30 p.m. 
 
 Grass. Soil. 
 
 Temperature of soil, 3 inches below surface, 47°'5 46° 
 
 12 „ M 47°-5 44° 
 
 
 At 6.30 P.M. 
 
 
 
 
 Grass exposed, lost 
 
 
 5i grains, 
 
 or 
 
 0-356 grammes. 
 
 II under tray, " 
 
 
 3 
 
 
 0-194 
 
 II 
 
 Bare soil exposed, n 
 
 
 5i II 
 
 
 0-356 
 
 II 
 
 11 under tray, n 
 
 At 
 
 2f 1, , 
 10.30 P.M. 
 
 
 0-178 
 
 Grass. 
 
 II 
 
 Soil. 
 
 Temperature on surface. 
 
 
 
 
 36°-5 
 
 40° 
 
 11 of soil, 3 inches below surface. 
 
 44°-5 
 
 42°-l 
 
 12 
 
 \ II 
 
 11 
 
 
 47° 
 
 44° 
 
 Orass exposed, lost 
 
 
 24 grains, 
 
 or 
 
 1-555 grammes. 
 
 11 under tray, n 
 
 
 6 M 
 
 
 0-388 
 
 M 
 
 Soil exposed, « 
 
 
 23 11 
 
 
 1-4&0 
 
 II 
 
 ri under tray, n 
 
 
 8 11 
 
 
 0-518 
 
 ri 
 
 October 12, 1885. 5.80 p.m. 
 
 Grass. Soil. 
 
 (CO 
 
 Temperature of soil, 3 inches below surface, 44° 45° 
 
 12 1, 1, 44°-5 43-5 
 
 At 10.15 P.M. 
 
 Grass. Soil. 
 
 Temperature on surface, 31°-5 35°-5 
 
 11 of soil, 3 inches below surface, 42° 40°-2 
 
 12 „ 11 44° 43°-5 
 
 Grass exposed, lost 30 grains, or 1-944 grammes. 
 
 II under tray n '6 n 0-388 
 
 Soil exposed, n 22 i, 1-425 
 
 II under tray, n 6J m 0-421 
 
 There was a little wind on this occasion, and very little dew formed. 
 During the night the min. on the grass was 28°-5, under it 41°; and it was not 
 till 10 o'clock next morning that the thermometer on the grass was as high as 
 the one under it. 
 
 The following reading were taken about the 20th October, the exact 
 date unfortunately is omitted in note-book : — 
 
 5.15 P.M. 
 
 Temperature of air — Dry bulb, 42-5 ; Wet bulb, 40. 
 
 Grass. Soil. 
 
 Temperature on surface, 39° 42° 
 
 II of soil, 3 inches below surface, 46° 48° 
 
 13 11 1, 46°-3 46°-l 
 
MR JOHN AITKEN ON DEW. 
 
 At 10.40 P.M. 
 
 21 
 
 Temperature of air, 
 
 Dry bulb, 38° 
 SSI 
 
 Wet bulb, 36° 
 33° 
 
 Grass. 
 Temperature on surface, 31" 
 
 rr of soil, 3 inches below surface, 44° -2 
 
 12 " „ 46° 
 
 at 4' 0" ■ 
 near ground. 
 
 Soil. 
 34° 
 43° 
 46° 
 
 Grass exposed, 
 
 11 under tray, 
 Soil exposed, 
 II under tray. 
 
 lost 
 
 9 grains, or 0-583 grammes. 
 
 16^ 1, 
 9 11 
 
 Next morning at 9 a.m. 
 
 0-518 
 1-069 
 0-583 
 
 Temperature on surface, 
 M of soil. 
 
 Grass exposed. 
 
 It under tray, 
 Soil exposed, 
 rr under tray, 
 
 12 
 
 lost 
 
 Grass. 
 
 39°-5 
 
 3 inches under surface, 42°-5 
 
 45°-5 
 
 SoU. 
 39°-5 
 40°-5 
 45°-o 
 
 19 grains, 
 13 „ 
 
 30 rr 
 
 18 11 
 
 or 
 
 1-231 grammes. 
 0-842 
 1-944 
 1-166 
 
 These figures canuot be supposed to represent anything definite, they 
 only indicate a condition of matters which has not been previously observed. 
 They show that evaporation in our cKmate is going on night as well as day 
 during dry weather, but the extent to which it takes place cannot be gathered 
 from these observations, as they are far too few for the purpose — too few alike 
 with regard to seasons, humidities, and exposures ; nor can the proportionate 
 amount of evaporation from bare soil and from grass-land be arrived at from 
 the weights given. These readings can only be considered true for the place 
 and moisture at the time of the year when the experiments were made. For 
 instance, the inverted trays over soil in my early experiments always indicated 
 a larger evaporation from soil than from grass, while the later ones did not. 
 But the early experiments were made over soil freely exposed to sunshine 
 during the whole day, while the later ones were made at a place less freely 
 exposed, on account of the situation where the first experiments were made 
 being too far from the place of weighing. It is evident the amount of sunshine 
 will be an important factor in this nightly evaporation, as it will greatly 
 determine the amount of heat stored up during the day, and available for 
 evaporation during the night. 
 
 I extremely regret the season was so far advanced before these experiments 
 were begun, as most of the weather suitable for the purpose was past. I have, 
 however, endeavoured to check my results as well as possible. Still I feel that 
 what has been done is only preliminary. Similar experiments would require to 
 
22 MR JOHN AITKEN ON DEW. 
 
 be made during the whole year, to determine whether this evaporation is 
 constantly going on or not in fair weather, and to determine its amount 
 under different conditions. The varieties of soils, of humidities, and exposures 
 are so great that an enormous number of experiments would require to be 
 made to determine with any degree of accuracy the amount of evaporation 
 that takes place from any large tract of laud. 
 
 The temperatures of the soil and of the air during these experiments were 
 not high, but we must remember they were taken in October. In summer we 
 have to deal with much higher temperatures and greater vapour tensions, and 
 therefore the possibilities of heavier dews. On the 18th August I find the 
 temperature ^ inch under the surface of the soil at 4 p.m. was 82°, at 3 inches 
 underneath it was 72°, the temperature of the air being 66°. At 9 p.m., at 3 
 inches deep, the temperature was 60° under grass and under bare soil. The 
 temperature on the grass was 45°, while a thermometer placed on bare soil 
 was 52°. Next morning the temperature at 3 inches under grass was 56°, 
 and at the same depth under soil 52°. The soil at 3 inches down had thus lost 
 20 degrees during the night, and that nearer the surface would have lost a 
 good deal more. Much of this loss would be spent in evaporating moisture. 
 On this occasion it will be noticed that at night the difference between the 
 temperature on the surface of the grass and on the bare soil was as much as 
 7 degrees, and this easily explains why the ground kept dry while the grass 
 got wet. 
 
 So far as my limited observations go, evaporation is constantly going on 
 from soil under grass, but on a few occasions it was doubtful whether the reverse 
 process had not taken place, and vapour got condensed on the surface of bare 
 soil. On one or two occasions in autumn, I observed soil which had been dry 
 the previous day to be damp in the morning. The soil had evidently received 
 an increase of moisture. But the question still remains, Whence this 
 moisture ? Came it from the air, or from the soil underneath ? The latter 
 seems the more probable source, as the higher temperature below would 
 determine a movement of the moisture upwards by the vapour diffusing ; and 
 the surface soil being cold, the vapour would be trapped by it before it escaped 
 into the air, in the same way as it is trapped by grass on grass land. 
 
 During summer it is difficult to trace the vapour condensed on the surface 
 of the soil to its source, and to say definitely whether it came from the air or 
 from the ground underneath. But on the morning of the 12th October I had 
 an interesting opportunity of studying this question. During the night the 
 radiation had been very powerful, the surface of the soil was greatly cooled, 
 and a thin crust of frozen earth formed. After the sun had thawed the sur- 
 face it was very wet. An examination of the soil before the sun had acted on 
 it, showed that the vapour condensed near its surface had come from under- 
 
MR JOHN AITKEN ON DEW. 23 
 
 neath. On lifting the small clods on the surface, it was observed that their 
 under surfaces and sides, when close to each other, were all thickly covered 
 with hoar-frost so thickly as to be nearly white, while the upper surfaces 
 exposed to the passing air had but little deposited on them, — the interpreta- 
 tion of which seems to be, that the vapour rising from the hot soil underneath 
 had got trapped in its passage through the cold clods. Its presence under- 
 neath and on the sides of the clods was an evidence that the moisture Avas 
 on its passage from the ground, when it met with the cold surface which im- 
 . prisoned it. 
 
 This hoar-frost on the sides and under the clods could not be due to tapour 
 condensed from the passing air, because the upper surfaces of the clods had 
 scarcely any deposited on them, and that in spite of the fact that the upper 
 surfaces would be the colder, as they were those from which the radiation 
 was taking place. It seems probable that even the vapour condensed on the 
 upper surfaces of the clods was part of the vapour escaping from the soil, and 
 was not taken from the passing air. 
 
 The occasions when the earth is most likely to receive vapour condensed 
 upon it from the passing air, are not on clear nights when the radiation is 
 strong, but rather when after strong radiation and cooling of the surface 
 the weather changes, becomes cloudy, and a warm moist wind bloM^s over the 
 land. Occasions of this kind are seen most frequently after frosts, and 
 undoubtedly much moisture is then condensed on the soil, but the moisture 
 so condensed is not what we call dew. 
 
 Dew on Eoads. 
 
 There is considerable difference among works on dew as to the absence 
 of dew on roads, but almost all agree in stating that it is never formed on 
 roads ; and the presence of dew on grass, while none is visible on roads, is 
 generally attributed to the greater radiating power of vegetation over that of 
 the material of which our roads are composed. Now I find that this state- 
 ment as to facts is wrong, and the explanation is also inaccurate. Dew really 
 does form on roads in great abundance on dewy nights, and the material of 
 the road is practically as good a radiator as the grass. 
 
 The reason why it is generally said that dew is not seen on roads is 
 owing, not to the less radiating power of the stones, but to the fact that dew 
 has not been looked for at the proper place. The blades of grass are practi- 
 cally non-conductors of heat, while stoues conduct fairly well. The result of 
 this is that we are not entitled to look for dew on the upper surfaces of stones, 
 as on grass, but it must be sought for on their under sides, because the stones 
 are good conductors, and the vapour tension under them is much higher than 
 at their upper surfaces, owing to the higher temperature of the air laden with 
 
24 MR JOHN AITKEN ON DEW. 
 
 moisture rising from tlie ground. If we examine a gravel walk on a dewy 
 evening, we shall find the under sides of the stones, especially those near the 
 solid ground, to be dripping wet; and we may occasionally see isolated patches 
 of stones wet on the upper sui'face, probably due to an openness in the ground 
 at the place permitting a free escape of vapour. 
 
 Another reason why the upper surface of the gravel does not get wet, is 
 that it is in good heat communication with the ground; the stones are thus 
 kept warm; and as a good deal of the vapour rising from the ground is trapped 
 by the under surfaces of the stones, the vapour which escapes these surfaces is 
 not enough to saturate the air at the temperature of the exposed surfaces of 
 the gravel. The following temperature, taken at 10 p.m. on the 25th September, 
 will give an idea of the difference in temperature on the surface of grass and 
 on gravel, and show why no dew is formed on the top of the stones while it 
 collects on the grass. A thermometer placed on the surface of the gravel was 
 34°, while one placed near it, but on grass, was 30°, or 4° lower. At the 
 surface of the soil vmder the grass the temperature was 40°, and it was almost 
 exactly the same temperature at the bottom of the gravel which was 1^ inches 
 deep. 
 
 We see from the above that hot vapour, rising from the ground under 
 grass, ascends till it comes into contact with the cold blades, and is condensed 
 on their exposed surfaces; whereas on the gravel road the under sides of the 
 stones are nearly as cold as their exposed surfaces, and much of the warm 
 vapour gets condensed under them, while the vapour which escapes to the 
 surfaces has its dew-point lowered by mixing with the surrounding air, and 
 the upper surfaces of the stones being in good heat communication with the 
 ground, are not cool enough to condense this vapour and form dew. 
 
 A simple manner of studying the formation of dew on roads is to take, say, 
 two slates, and place one of them on the gravel and one on a hard part of the 
 road. If these slates are examined on a dewy night, their under sides will be 
 found to be dripping wet, though their upper surfaces and the road all round 
 them are quite dry. This experiment also shows us that under most conditions 
 of our climate vaj)our does rise from hard dry-looking roads on dewy nights. 
 
 In studying questions of this kind, and for showing the importance of the 
 heat communicated by the earth to the radiating body, the following experiment 
 may be useful. Place on the grass, soil, or road, a slate and a piece of iron, 
 say an ordinary 7 lb. weight. Alongside of these place another slate and 
 weight; but instead of the latter resting on the ground, elevate them a few 
 inches on small wooden pegs driven into the earth. If we examine the 
 surfaces of these bodies on dewy nights, the following will be the general 
 result. While the grass all round is wet with dew, we shall find that the 
 upper surfaces of the slate and the weight resting on the ground keep dry, and 
 
MR JOHN AITKEN" ON DEW. ' 25 
 
 those of tlie elevated ones get wet like the grass, the reason for this is that the 
 bodies on the ground as well as the elevated ones are constantly losing more heat 
 by radiation than they receive by absorption; but those in contact with the 
 ground have heat communicated to them by conduction and by the condensa- 
 tion of vapour on their under surfaces; their temperature is thus prevented 
 from falling as low as that of the elevated bodies, which only receive heat 
 from the passing air ; the latter are thus cooled more by radiation than those 
 on the ground. Bodies out of heat communication with the ground thus tend 
 to cool more than those in contact with it; and while the former get cooled 
 beloAV the dew-point and collect dew, the latter keep Avarmer than the dew- 
 point, and thus tend to keep dry, or if wetted to become dry again. 
 
 These considerations suggest a simple method of testing whether the surface 
 of any particular part of the ground is giving off vapour or not. It is very 
 evident that so long as the temperature of the surface of the soil is above the 
 dew-point of the air, vapour will lise from the ground, and that if the 
 surface is cooled by radiation below the dew-point, evaporation will cease, 
 and vapour will condense upon it. In order to test this, all that is necessary is 
 to place on the ground, and in good heat communication with it, some substance 
 that is a good conductor, and shows dewing easily. A piece of metal 
 covered with black varnish does well. It is painted black, not in order to radiate 
 copiously, but because black shows any deposit of dew most quickly and easily. 
 So long as this test surface keeps dry while in contact with the ground, the 
 soil round it must be giving off vapour, because the temperature of its surface 
 is higher than the dew-point. But if the temperature of the ground falls 
 below the dew-point it will collect moisture, and this test surface will 
 collect dew also, and will thus tell us that the surrounding soil is receiving 
 moisture. In experiments such as these we are simply converting a small 
 area of the earth's surface into a condescending hygroscope, and our test 
 surface tells us whether the earth's surface at the place is cooled by radiation 
 below the dew-point or not. So long as no dew forms on the test surface 
 vapour is being given off. 
 
 These test surfaces must not be large, at most only two or three centimetres, 
 because if large they would check the free passage of the vapour to the air, 
 and so prevent the soil under them from cooling to the same amount as the 
 surrounding ground ; and further, it is difficult to get good contact with large 
 surfaces, without which only a part of the test surface keeps clear, while 
 the part not in contact gets dewed, even though the temperature of the surface 
 of the ground is above the dew-point. This was confirmed by observations 
 made on a frosty night. On lifting each test plate, it was observed that the 
 soil was frozen to it under the clear parts, and no soil adhered under the parts 
 that were dewed. In my experiments I have used small copper discs covered 
 
 VOL. XXXIir. PART I. D 
 
26 MR JOHN AITKEN ON DEW. 
 
 with black varnisli, ordinary glass mirrors, and also small black mirrors, in 
 order to get rid of the objection to ordinary silvered mirrors, namely, that they 
 might not be good radiators. On no occasion up to the beginning of November 
 have I yet seen dew on any of these at night, but it is difficult to say whether 
 dew had not formed on them on some mornings, as the. air was thick and 
 misty, and the deposit then observed might have fallen as fine rain. 
 
 The changes in temperature of the surface of the soil due to radiation, give 
 rise to a downward movement of heat during the day, and to an upward move- 
 ment of it during the night. These heat changes will be accompanied by 
 corresponding movements of the moisture in the soil. During day, after the 
 surface is heated, the vapour tension being higher above than below, a 
 downward movement of moisture will take place ; and at night this process 
 will be reversed, the tension of the vapour at a depth being greater than near 
 the surface, the vapour rises and condenses in the colder soil. Part of the 
 latent heat so liberated by the rising vapour is spent in radiation from the 
 surface, part in evaporating moisture, and a little in heating the air cooled by 
 contact with cold grass, &c. 
 
 We may conclude that, owing to the heat received during the day, and 
 probably also to the internal heat of the earth, vapour continues to rise from 
 the ground long after the sun has set, and in many conditions the vapour 
 continues to rise the whole night; but under certain others it seems probable 
 that the reverse will occasionally take place, and vapour condense on the 
 ground. This is most likely to take place soonest on bare soil, especially on 
 those parts of it that are in bad heat communication with the ground under- 
 neath. But over grass-land in most conditions of our climate, when dew is 
 forming, the evaporation seldom seems to stop, but goes on night and day, on 
 account of the surface of the soil being protected by the grass from losing its 
 heat so quickly as the bare soil. The escaping vapour rises till it meets with 
 some surface not in good heat communication with the ground, and which has 
 been cooled by radiation, in the manner set forth by Wells and others. 
 These remarks refer to weather when dew is most abundant, as in spring, 
 summer, and autumn, and do not apply to those conditions in which a warm 
 vapour-laden air is brought over a cold ground. 
 
 Dew and Wind. 
 
 It is well known that during windy nights no dew is formed. We pre- 
 viously knew that wind acts in two ways to prevent the formation of dew; 
 to these two ways we must now add a third. Wind prevents the formation of 
 dew — (1) by mixing the hot air above the surface of the ground with the air 
 cooled near its surface, this tends to prevent the air being cooled to the dew- 
 
MR JOHN AITKEN ON DEW. 27 
 
 point; (2) the wind by its passage over the surface of radiating bodies 
 prevents these surfaces being cooled much below the temperature of the air; 
 the wind thus tends to prevent the air in contact with these surfaces being 
 cooled below the dew-point ; and (3) wind blowing over the surface of the 
 ground rapidly carries away the vapour rising from the soil, and mixes it up 
 .with a large quantity of drier air. The wind thus tends to prevent an 
 accumulation of damp air near the ground. 
 
 To illustrate this third effect of wind, let us use the observations made on 
 the evening of October 12. The sky was cleai% and there was a considerable 
 amount of radiation, but a slight wind was blowing. The bare soil in the test- 
 pan lost 22 grains and the corresponding turf lost 30 grains in about five hours. 
 Almost no dew was formed on the grass, but trays placed over the bare soil 
 and over grass had their inside surfaces covered with moisture, though not so 
 heavily as was generally observed on dewy nights. The reason why so little 
 dew formed on this occasion was, partly, that the wind prevented the tempera- 
 ture of the air near the ground falling as much as it would have done if it had 
 been calm. In the screen the temperature only fell to 40°. On the grass, 
 however, it fell to Sf-S, and on the soil to 35°'5; but a good deal depended on 
 the exposure of the thermometer to the wind. From the above we see that, 
 though wind was blowing, the thermometer on the grass fell a good deal below 
 the temperature of the air, and showed a considerable amount of radiation. The 
 wind apparently prevented the formation of dew on this occasion, principally by 
 preventing an accumulation of moist air near the surface of the ground. The 
 inverted trays showed that if the wind had fallen dew would have formed, 
 because it formed in the still air under the trays. The deposit was not so heavy 
 inside the trays on this occasion as was often seen in dewy nights, because the 
 wind prevented the radiation cooling the top of the trays to the same extent as 
 when it was calm. 
 
 Dew and Vegetation. 
 
 When I began to make observations on dew, one of the first things I did 
 was to make a tour of the garden on a dewy night, and to examine the 
 appearance of the plants. A very short survey was sufiicient to show that 
 something else was at work than radiation and condensation to produce the 
 effects then seen. Let me briefly describe what I saw, and what at once 
 struck me could not be explained by the ordinary laws of radiation and con- 
 densation. Certain kinds of plants were found to be covered with moisture, 
 while others were dry. Many plants of the Brassica family were heavily 
 covered with glistening drops; while beans, peas, &c„ growing alongside them, 
 were quite dry. Again, in clusters of plants of the same kind some were wet, 
 while others were not; and not only so, but some branches were wet, while 
 
28 MR JOHN AITKEN ON DEW. 
 
 others on the same plant were dry. These differences Avere noticed to be quite 
 irrespective either of their exposure to the sky, or to the probable humidity of 
 the air surrounding them. 
 
 In illustration of this latter point, small clusters of dwarf French poppies 
 may be mentioned. Most of the plants were quite diy, whilst others growing 
 amongst them were dripping with moisture; and while some branches were 
 dry, others on the same plant were studded with drops, aiid the general 
 surface of the leaves in some cases wet. On examination of these plants 
 next day, it was observed that those that were wet at night were all plants 
 in vigorous growth, and the shoots that were dewed were those in which the 
 vegetation seemed most active. It was also observed that it was always the 
 same plants and branches that were dewed night after night during the 
 short time the observations were made. 
 
 A closer examination of the leaves of broccoli plants showed better than 
 any others that the moisture collected on them was not deposited in the 
 manner we should expect if it had been deposited as an effect and according 
 to the laws of radiation ; nor was it deposited in accordance with the laws of 
 condensation; indeed, every appearance was at variance with these laws. 
 Examination showed that the moisture was collected in little drops placed at 
 short distances apart, along the very edge of the leaf, while the rest of the leaf 
 was often dry. Now, if the moisture had been condensed by cold produced by 
 radiation, then it would have been most abundant on the upper surface of the 
 leaf; but there would have been none on its windward edge. This is well 
 seen when we expose a small glass plate on a dewy night; the windward edge 
 is always dry, and the deposit is spread evenly over the rest of the plate up to 
 the opposite margin, because the temperature of the air when it first strikes 
 the plate is higher than the dew-point, and it has to travel over more or less 
 of the surface of the glass before it is cooled enough to deposit its moisture. 
 Again, if these drops on the edge of the leaf had been deposited according to 
 the laws of condensation, then the moisture would have been deposited on the 
 surface more in accordance with the distribution of temperature at the different 
 points; the moisture would therefore have been more equally distributed, and 
 not been in large isolated drops. 
 
 On further examining these plants, I placed the lantern behind the blade, and 
 then observed that the position of the beautiful sparkling diamond-like drops 
 that fringed its edge had a definite relation to the structure of the leaf; they 
 were all placed at the points where the nearly colourless and semi-transparent 
 veins of the leaf came to the outer edge, at once suggesting that these veins 
 were the channels from which the drops had been expelled. 
 
 These isolated drops on the edges of the leaves were therefore evidently 
 not dew, but an effect of the vitality of the plants. An examination of grass 
 
ME JOHN AITKElSr ON DEW. 29 
 
 blades showed that they also tend to have large drops attached to them, 
 while the rest of the blade is dry, and these drops were always found to be 
 situated at certain definite points; they were always near the tips of the blades. 
 These large drops seen on plants at night are therefore not dew at all, but are 
 watery juices exuded by the plants. 
 
 Now this excretion of water by the leaves of growing plants is not a new 
 discovery — it has been long well known. But what seems extremely curious is, 
 that its relation to dew has never been recognised, at least so far as I am 
 aware, and it must be admitted that it is one of considerable importance. 
 
 It is well known that plants transpire from their leaves an immense amount 
 of moisture, which passes off in an invisible form. Prof. J. Boussingault found 
 that mint transpired 82 grammes of water per square metre in sunshine, and 
 36 grammes in shade; but if the roots of the plants were removed, they only 
 transpired 16 and 15 grammes respectively. This simple experiment proves 
 that the root sends into the stem of the plant a supply of water, that it acts as 
 a kind of force-pump, and keeps up a pressure inside the tissues of the plant. 
 This supply sent in by the root is in most conditions removed by means of 
 transpiration from the surface of the leaves. 
 
 Now what will be the result if transpiration is checked, while the root con- 
 tinues to send forward supplies'? It will evidently depend on two things — 
 first, the pressure the root is capable of exerting before its action is stopped; 
 and second, the freeness with which the water can escape from the leaves. If 
 the root pressure is small, it will cease with the transpiration ; but if it is great, 
 the sap will be forced into the plant, and if nature has provided any outlets it 
 will escape at these openings. 
 
 Dr J. W. MooL* has given great attention to the subject, and has experi- 
 mented on a number of plants. The method he employed in his researches 
 was to place the leaves under the most favourable conditions for the excretion 
 of drops, by diminishing the transpiration as far as possible, and by supplying 
 them with water. He substituted for root pressure, a pressure produced by a 
 column of mercur}^ Out of 60 plants experimented on by Dr Mool, he found 
 that the leaves of 29 excreted drops without being injected, 13 leaves became 
 injected and excreted drops, and 18 became injected, and did not excrete at 
 all. He says that the excretion takes place by water-pores, and by ordinary 
 stomata, while in some cases it occurs at surfaces possessing neither of 
 these organs. 
 
 I have recently made a few experiments on this subject in its relation to 
 dew. As, however, the season was far advanced before the experiments were 
 begun, but little could be accomplished, for the activity of the plants was nearly 
 over, and grass was almost the only plant possessing sufficient vitality for 
 
 * Nature, vol. xxii. p. 403. 
 
30 MR JOHN AITKEN ON DEW. 
 
 experimenting. I however removed a branch of the poppy, which, during 
 summer, had shown snch a tendency to exude moisture, and connected it by 
 means of an india-rubber tube with a head of water of abovit one metre. 
 After placing a glass receiver over it, so as to check evaporation, it was left for 
 two or three hours, when it was found to have excreted water freely — some 
 parts of the leaves being quite wet, while di'ops had collected at other places. 
 
 The broccoli plants which had excited my interest in summer were also 
 experimented with. A full-grown leaf was fitted into the apparatus, and the 
 pressure applied. In a little over an hour it also exuded water, and soon got 
 fringed with drops along its edge in exactly the same way that was observed on 
 it in summer. Another leaf from the same plant, but much younger, being 
 about one quarter grown, on being tested in the same way did not excrete at all, 
 after the pressure had been applied for twenty-four hours. Here we have the 
 same result as that noticed in summer — one leaf exudes, while another on 
 the same plant does not. 
 
 If the water pressed into the leaf is coloured with aniline blue, the drops 
 when they first appear are colourless, but before they grow to any size, the 
 blue appears, showing that little water was held in the veins, but the whole 
 leaf got coloured of a fine deep blue-green, like that seen when vegetation is 
 very rank, showing that the injected liquid had penetrated through the 
 whole leaf. 
 
 Most of my experiments on this subject were made with grass. I find 
 that even in the middle of October, after having been severely frosted two or 
 three times, which had probably reduced its vitality, it still exuded so abun- 
 dantly that drops collected in air which was not saturated. A turf placed in 
 a cellar, dry enough to keep glass quite free from dewy deposit, soon collected 
 drops. These drops always appear near the tips of the blades ; they are not 
 exuded from every blade, and sometimes from only one on each stalk, but 
 generally from more ; and it is always from the blades that seem to have the 
 greatest vitality, and are nearly, but not quite full grown. Sometimes it is the 
 youngest blade that exudes, but if it is very small, it is the second youngest. 
 As the blades grow old they cease to exude ; but this seems to be due to 
 some change in the blade at the point where it exuded, and not to a diminution 
 of root pressure, as it exudes freely when the tip is cut off. 
 
 The question might be here raised, Are these drops really exuded by the 
 plant 1 Are they not due to some condensing power possessed by the leaves, 
 by the presence at these points of some substance possessing an affinity for 
 water vapour, or some process by which they may extract moisture from the 
 air ? To get an answer to this question, I selected a small turf, placed 
 over it a glass receiver, and left it till drops were excreted. Removing 
 the receiver, a blade having a drop attached to it was selected. After being 
 
MR JOHN AITKEN ON DEW. 31 
 
 carefully dried, the tip of the blade was placed in a small glass receiver, so 
 as to isolate it from the damp air of the larger receiver. This small 
 covering glass measured about 10 mm. in diameter by about 15 mm. in 
 height. Its open end was closed by means of a very thin plate of metal 
 cemented to it. In the centre of this plate was pierced a small opening, of 
 the same size and shape as the selected blade of grass. The tip of the blade 
 was entered about 5 mm. into this small receiver, and to prevent moisture 
 entering and coming in contact with the tip of the blade, an air-tight joint between 
 the blade and the metal was made with india-rubber solution. The tip of 
 the blade was thus isolated inside the small receiver in which the air was 
 dry. The large glass receiver was then placed over the turf to prevent 
 evaporation from the lower part of the blade, or the experiment was made in a 
 room where the air was not very dry. After a time, generally some hours, the 
 turf was examined. A drop was always found to have formed on the tip of 
 the blade inside the small receiver, and this drop was, as nearly as could be 
 judged, always as large as the drops formed in the moist air under the large 
 receiver. It would thus appear that these drops are really exuded by the 
 plant, and not extracted from the air. 
 
 These exuded drops seem to be almost entirely the result of root pressure, 
 because if we cut off the roots, and place the stems in water, putting over all 
 a glass receiver standing in water so as to saturate the air, and as a test 
 that the conditions are favourable, placing a small turf alongside the 
 cut grass under the receiver, we shall find that scarcely any drops make 
 their appearance on the rootless stems, while those with roots have drops 
 attached to them. Again, if we take one of these rootless stems, and attach it 
 by means of the india-rubber tube to a head of water, it is found to exude 
 drops at the tips of its blades in moist air in the same way as when it was 
 attached to its roots. 
 
 These excreted drops are formed on grass on other than dewy nights. 
 After rain, if there has been no wind, and the air near the ground becomes 
 saturated, a reai-rangement of the drops takes place. Some time after the rain 
 has ceased, most of the blades will be found to be tipped with a drop at the 
 same point as the exuded drop appeared at night — a position which no falling 
 rain drop could keep. This tendency of plants to exude moisture explains 
 why the grass is almost always wet during autumn. At that season evapora- 
 tion is slow, and as the plants are constantly pouring in supplies to the drops, 
 it takes a long time for the slow evaporation to overcome the wetting effect 
 and dry up the grass. 
 
 The question as to what degree of humidity in the air is necessary before 
 plants will exude drops, would seem to be greatly determined by the rate at 
 which the supply is sent into the leaf If the supply is greater than the 
 
32 ME JOHN AITKEN ON DEW. 
 
 evaporation from the whole surface of the leaf, the drop grows ; but if the 
 supply is less, it does not form, or if formed, it decreases in size. The rate of 
 the supply will evidently depend on the kind of plant and the amount of its 
 vital activity at the time. The formation of drops on plants that exude 
 moisture will therefore depend on the rate of supply, the humidity of the air, 
 and the velocity of the wind. It is not easy to get a satisfactory experimental 
 answer to this question, on account of the soil near the grass tending to 
 moisten the air over it. A small turf placed in an elevated position in the 
 centre of a room has been observed to have drops on it, when there was a 
 difference of more than one degree between the wet and the dry bulb, thermo- 
 meter hung alongside. As the drops are exuded at the tips of the blades, 
 it is probable the air in contact with them was not much moistened by the 
 small area of soil underneath. 
 
 These observations entirely do away with the explanation usually given of 
 the tendency of grass to get wet early and heavily on dewy nights. It has 
 generally been explained by saying that grass is a better radiator than most 
 substances, and therefore cools more, and sooner, than other bodies. We 
 now see that those drops that first make their appearance on grass are not 
 drops of dew at all, and their appearance depends, not on the laws of dew, but 
 on those of vegetation. Hence the varied distribution of moisture on plants 
 and shrubs on dewy nights. 
 
 We have seen that much of the moisture that collects on plants at night 
 does not form like dew on dead matter. Dead matter gets equally wet where 
 equally exposed, and the moisture does not collect on it in isolated drops, as 
 it does on plants. Those drops which appear on grass on clear nights are not 
 dew, and they make their appearance on surfaces that are not cooled to the 
 dew-point. If the radiation effect continues after these drops have been 
 forming for some time, true dew makes its appearance, and now the 
 plants get wet all over their exposed surfaces in the same manner as dead 
 matter. This latter form of wetting or true clew is of rarer occurrence than 
 we might at first imagine. On many nights on which grass gets wet, no true 
 dew is deposited on it; and on all nights, when vegetation is active, the exuded 
 drops always make their appearance before the true dew ; so that when we 
 walk in early evening over the wet lawn, it is not dew that we brush off the 
 grass with our feet, but the sap exuded by the plant itself The difference 
 between these exuded drops and true dew can be detected at a glance. The 
 moisture exuded by grass is always excreted at a point situated near the tip 
 of the blade, and forms a drop of some size, which may form while the rest 
 of the blade is dry, but true dew collects evenly all over the blade. The 
 exuded liquid forms a large glistening diamond-like drop, whereas dew coats 
 the blade with a fine pearly lustre. 
 
ME, JOHN AITKEN ON DEW. 33 
 
 I feel that the dissecting hand of science has here done an injury to our 
 poetic feehngs. Every poet who has sung of the beauties of nature has added 
 his tribute to the sparkUng dew-drop, and Ballantine in his widely-known 
 song has taught a comforting lesson from the thought that " ilka blade o' grass 
 keps its ain drap o' dew." No doubt the drop of dew to which the poets refer 
 is the large sparkling diamond-like gem that tips the blades of grass, and which 
 we now know is not dew at all. While, however, our interpretation of nature 
 has changed, the teaching of the poet remains, and the sparkling dew-drop may 
 still teach the same comforting lesson. We must, however, change our views 
 regarding the source of the refreshing influence. We may no longer look upon 
 it as showered down from without, but as welling up from within — no longer as 
 taken by the chill hand of night and given to refresh and invigorate exhausted 
 nature ; we must rather look upon it as suggesting that we are provided with 
 an internal vitality more than sufficient to restore our exhausted powers, after 
 the heat and toil of the day are past. 
 
 Radiation. 
 
 I have said in a previous part of this paper that the surface of bare soil and 
 of roads will radiate at night as much heat as grass. It may be thought I have 
 said this simply because we do not now require that grass should be the more 
 powerful radiator to enable us to explain its greater wetness on dewy nights. 
 Though it is not now necessary to suppose that grass is a powerful radiator, yet 
 there is nothing in the above experiments to prove it either a good or a bad one. 
 It therefore seemed desirable that some definite experiments be made on this 
 point, and also to determine the radiating powers of different substances at 
 night, as this is always an interesting and important j^oint in questions con- 
 nected with the deposition of dew ; and the radiating power of grass, though 
 not the principal cause of its wetness at night, might be still considered to play 
 a subordinate part. 
 
 We have already a great number of experiments on the radiating powers of 
 diff'erent substances. Unfortunately most of the accurate measurements of this 
 kind are from laboratory experiments, and do not appear to bear very directly 
 on our subject. Feanklin's early experiments, made with diff'erent coloured 
 cloths placed on snow, seem to have given our ideas an unfortunate bias on 
 this subject. From observing the diff'erent depths to which cloths of dif- 
 ferent colours sunk in snow, when exposed to solar radiation, he came to 
 the conclusion that the dark colours absorb most heat, and this conclu- 
 sion seems for long to have influenced our ideas. If the heat radiated and 
 absorbed by a surface was composed entirely of visible rays, then no doubt 
 the colour of a body would be an index of its radiating and absorbing powers. 
 
 VOL, XXXIII. PAKT I. E 
 
34 MR JOHN AITKEN ON" DEW. 
 
 But as the eye gives us uo iuformation about the greater proportion of the 
 radiant energy, its indications are of no value in determining the radiating and 
 absorbing powers of different surfaces. 
 
 Experiment shows that different surfaces have different absorbing powers 
 for different rays. Melloni, for instance, found that white lead absorbed only 
 about half as much heat from a Locatelli lamp as lamp black did, while it 
 absorbed as much as lamp black when the source of heat was copper at 100° C. 
 It is evident from this, that we cannot take the result of experiments made in 
 the laboratory, and apply them to surfaces exposed to the temperature of 
 the sky on a clear night. It may be possible that the radiating and 
 absorbing powers of different surfaces may bear the same proportion to each 
 other when the temperature is 0°, and they radiate into space, as when their 
 temperature is 100°, and they are exposed to surfaces at the ordinary tem- 
 perature of the laboratory. This may be so, but till it is proved we can- 
 not apply these laboratory experiments to the cooling effect of radiation at 
 night. 
 
 Some experiments on the radiating power of different substances exposed 
 to a clear sky were made by Daniell. He used for his purpose two similar 
 parabolic reflectors. In the focus of each was placed the bulb of a ther- 
 mometer. In expei'imenting he turned the reflectors to the sky, and coated 
 the bulbs of the thermometers with the substances to be tested. Comparing 
 garden mould with black wool, his measurements show, from the average of 
 three readings given by him, that while the black wool fell 9° below the 
 temperature of the air, the mould fell only 6°. The difference between the 
 radiating powers of chalk and black wool, as given by him, was not quite so 
 great. There seems to be an objection to this method of experimenting. 
 The different surfaces here lose more heat by radiation into space than they 
 receive. To supply this loss, they receive more heat by radiation from the 
 reflector than they give, and they also receive heat from the surrounding air, 
 conveyed to them by connection currents. Now in the experiment as arranged 
 by Daniell, the two surfaces will not receive the same amount of heat from 
 the latter source. The wool surface will not have such a free circulation of air 
 over it as the other one ; it will therefore not receive so much heat, and its 
 temperature will thus tend to fall lower. 
 
 It appeared that something more might be done in this direction, and on con- 
 sideration it was thought that the radiation thermometers, described by me in 
 a previous paper, might be suitable for the purpose. It may be remembered 
 that the principle on which these radiation thermometers is constructed is, 
 that a large surface is more highly heated than a small one by radiation during 
 the day, on account of the absorbed heat being more slowly taken away by 
 the passing air from the former than from the latter ; and for a similar reason 
 
MR JOHN AITKEN ON DEW. 35 
 
 a large surface is colder at night than a small one, as the small surface receives 
 more heat, per unit of area, from the air than the large one. The absorbing 
 and radiating surface of these instruments is a large flat area, painted black, 
 and its temperature is taken by means of a thermometer, with its bulb placed 
 under the centre of the radiating surface.'"" 
 
 The construction of these radiation instruments has been altered, and those 
 used in this investigation were made of metal in place of wood, as described in 
 the previous paper, the radiating surface being a thin plate of metal, 14 inches 
 (355 mm.) square. A thin metal tube is fixed close to and parallel with the 
 under surface of the plate. One end of the tube terminates at the centre 
 of the plate, and the other at the edge. The thermometer is placed in 
 this tube with its bulb under the centre of the plate, and to prevent heat 
 escaping or being absorbed at the back, a considerable thickness of cotton wool 
 is placed under it. The instrument is practically a shallow box, 14 inches 
 square by 2 inches (51 mm.) deep, packed with cotton wool. One of the 
 flat areas of the box is exposed to radiation, and its temperature is taken by 
 means of a thermometer placed under its surface. In the following I shall 
 refer to this instrument simply as the thermometer box. 
 
 One of the advantages of this form of instrument for solar radiation 
 experiments is, that the readings given by different instruments agree with 
 each other, at least this is the case so far as my experience goes; and it is well 
 known that the vacuum radiation thermometers are unsatisfactory in this 
 respect, no two almost ever reading alike. For instance, the vacuum 
 radiation thermometers used at the Indian Stations, when compared with 
 another of the same pattern as standard, were in some cases found to 
 diff"er as much as 15°, though they were exact copies of each other, and 
 similarly exposed.! I find that when the different instruments of the kind 
 used by me are compared they agree very well when of the same size. It is of 
 course necessary that they be of the same size — this results from the principle 
 of their construction. It seems possible that we might make boxes of different 
 sizes, and from them determine the law of variation for size; so that, knowing 
 the size of the surface used in any particular set of observations, we could 
 determine what temperature its readings corresponded to in another instrument 
 of a different size, or all readings might be reduced to a standard size, say the 
 temperature of a very large surface. 
 
 I may mention that the tempei'ature given by an instrument of the size 
 here described when placed in sunshine is a good deal above that indicated 
 by a vacuum thermometer, which had been carefully prepared for me by 
 Casella of London. Generally the readings were about 12 per cent, higher. 
 
 * Thermometer Screens, Proceedings of the Royal Society, Edinlurgli, No. 117, 1883-84-. 
 + Eeport of the Meteorology of India, 1879, by H. F. Blanford, P.E.S. 
 
36 MR JOHN AITKEN ON DEW. 
 
 One objection to these large-surface radiation thermometers is that they are 
 more affected by wind than the vacuum ones. If it is a question of solar 
 energy we are considering, this certainly is an objection, but if it is one of 
 climate it will scarcely be so. I need not say that for questions of terrestrial 
 radiation at night the vacuum thermometer is of no use. 
 
 In using these thermometer boxes for determining the radiating powers of 
 different surfaces at night the following method was employed :— Two precisely 
 similar boxes were prepared, and their upper surfaces painted black. They 
 were placed in an elevated position in the open air, commanding a clear 
 view of the sky all round. They were first exposed without anything on their 
 surfaces, to see if their readings were exactly alike. In constructing them care 
 was taken to put the same amount of cotton wool in each, in order that their 
 non-conducting powers and heat capacities might be the same, so that both 
 misht take the same amount of heat to warm them, and both lose the same 
 amount of heat at the back. Ou trial both instruments were found to read 
 alike when similarly exposed. 
 
 As the sky radiation is a rather variable quantity, it would not do, on most 
 nidits, to leave one of these test surfaces bare, and use it as a standard with 
 which to compare the other, over which we have put the substance to be 
 tested, because the uncovered surface will follow the changes in the radiation 
 more easily than the other, and will change more, and sooner, than the 
 one covered with the substance to be tested, particularly if the substance is 
 a bad conductor. The method generally adopted was to place both surfaces as 
 nearly as possible under the same conditions. For instance, the first substances 
 tested were black and white cloths of different materials; of each kind a black 
 and a white was selected, each pair being as imich alike as possible, of the 
 same material, of the same weight, and of the same texture. A black one 
 was placed over one thermometer box, and the corresponding white one 
 over the other. After a time the readings were taken, and the position of 
 the cloths reversed, the black being placed over the box where the white 
 was, and mce versa, and readings again taken. Then if radiation remained 
 constant one of the cloths was removed, and the other compared with the 
 black surface. 
 
 The following table shows the results of some experiments made on the 
 radiating power of black and white cloths tried in this way. The readings 
 were taken on the evening of the 14th November. The sky on the occasion 
 was quite cloudless. The air was very dry, and had scarcely any movement — 
 an unusually favourable condition for conducting experiments of this kind. 
 The radiating surfaces were placed at a height of about one metre from the 
 ground, and a protected thermometer for taking the temperature of the air was 
 placed alongside at the same height. 
 
MR JOHN AITKEN ON DEW. 
 
 37 
 
 Air. 
 
 Substance. 
 
 Eadiation. 
 
 Substance. 
 
 Kadiation. 
 
 35° 
 
 No. 1, black 
 
 28° 
 
 No. 1, white 
 
 28° 
 
 35° 
 
 „ white 
 
 28°'5 
 
 „ black 
 
 28°-5 
 
 35°-5 
 
 jj )j 
 
 28°-5 
 
 JJ 3J 
 
 28°-5 
 
 35°-5 
 
 Paint black 
 
 28°-5 
 
 >J >3 
 
 28°-5 
 
 36° 
 
 No. 1, black 
 
 28°-5 
 
 Paint black 
 
 28°'5 
 
 36° 
 
 No. 2, „ 
 
 29° 
 
 No. 2, white 
 
 29° 
 
 35° 
 
 )y J) 
 
 29°-2 
 
 )> i> 
 
 29°-3 
 
 35° 
 
 „ white 
 
 29° 
 
 „ black 
 
 29° 
 
 35°-5 
 
 )j )> 
 
 28°-7 
 
 3J J) 
 
 28°-8 
 
 35°-5 
 
 >i J) 
 
 28° 
 
 Paint „ 
 
 28° 
 
 35° 
 
 No. 3, white 
 
 27° 
 
 No. 3, black 
 
 27° 
 
 35° 
 
 „ black 
 
 27° 
 
 „ white 
 
 27° 
 
 34°-5 
 
 Paint „ 
 
 26°-5 
 
 )> JJ 
 
 26°-5 
 
 34°-5 
 
 )> 3f 
 
 26°-5 
 
 J> » 
 
 26°-5 
 
 In the above table, the first column shows the temperature of the air at the 
 height of the radiating surfaces. In the second and fourth columns are the 
 substances whose radiating powers are compared, No. 1 being black and 
 white cotton cloths, No. 2 merino cloths, and No. 3 thick woollen cloths. 
 In the third and fifth columns are the temperatures of the radiating surfaces. 
 The following was the manner of conducting the experiments : — Take the first 
 on the table. A black cotton cloth was spread over one thermometer box, 
 and a white cotton one over the other; after a time, when the readings were 
 taken, the temperature of the air was 35°, the black cloth 28°, and the white 
 one 28°. The black cotton was now removed from its box, the white one 
 put in its place, and the black one where the white one previously was. This 
 Avas done to check any error from difference of exposure to wind or difference 
 in thermometer boxes. After a time the readings were taken, and found to 
 be — air 36°, white cotton 28-5, and black cotton 28-5. The radiation of the cloth 
 was then compared with the radiation from the black paint on the surface of 
 the radiation box. This was possible on this night, as there was no wind, and 
 radiation was fairly constant. 
 
 In my first experiments with black and white cloths, they were found to be 
 cooled to an unequal amount ; but as the cloths used on this occasion were 
 what first came to hand, and happened to be of unequal thickness and texture, 
 and as there was wind blowing at the time, the heating effect of the passing 
 air acted unequally on the different cloths, and prevented them from being cooled 
 to the same amount ; hence the necessity of using cloths of equal texture in 
 experiments of the kind, especially when wind is blowing. 
 
 It will be observed that these experiments do not show any difference in 
 the radiating powers of white and black cloths ; nor do they show any differ- 
 ence in the radiating powers of cotton, wool, and paint. All radiate equally well, 
 
38 
 
 MR JOHN AITKEN ON DEW. 
 
 and have their surfaces cooled to the same amount when exposed to the same 
 radiation. It will be noticed that the temperature of the radiating surfaces 
 varied during the experiments, and was from 6 to 8 degrees below the 
 temperature of the air. 
 
 These experiments make no claim to any great degree of accuracy ; the 
 conditions under which they are made make it difficult to get correct results, 
 as the readings have to be taken with the aid of a lantern in the open air on 
 cold nights, and as special thermometers had not been prepared for the radia- 
 tion boxes, the thermometers used had to be partly withdrawn from the boxes 
 before reading; there may therefore be a slight inaccuracy in the temperatures 
 given. The error from this cause is not likely to be more than a quarter of a 
 degree, and if there had been any great difference in the radiating powers 
 of the surfaces, it would have shown on a scale of 6 to 8 degrees. 
 
 The following table gives the result of a comparison made between the 
 radiating powers of grass and garden soil, on a calm evening when the air was 
 dry. One of the thermometer boxes was sprinkled over with the soil, and over 
 the other was put a layer of cut grass just sufficient to conceal all the black 
 surface, and pressed down so as to make as flat a surface as possible : — 
 
 Temperature of air. 
 
 Temperature of grass. 
 
 Temperature of soil. 
 
 34' 
 
 25°-5 
 
 25° 
 
 35° 
 
 27° 
 
 26°-5 
 
 35° 
 
 27° 
 
 26° 
 
 35° 
 
 . 27° 
 
 26°-5 
 
 35° 
 
 27° 
 
 26°-5 
 
 From the above it will be seen that the garden soil was colder than the 
 grass on this evening. "When the grass was removed from the box and the soil 
 compared with the black paint on the other box, the soil was found to 
 be a little colder than the black paint, but not so much as it was colder than 
 the grass. The reason for the soil being colder than the black paint would 
 appear to be due to the evaporation taking place from its surface; the dew- 
 point at the time was very low, and the top of the soil showed signs of 
 drying. Compared with grass, this was not the reason for the difference, as 
 the grass was slightly damp. The higher temperature of the grass would rather 
 appear to be due to the nature of its surface. The passing air would communi- 
 cate more heat to its irregular surface than it would to the more even one of 
 the soil. Grass and soil were compared on other evenings on which the air was 
 not so dry, and the exposed surfaces had vapour condensed on them; on 
 these occasions the two surfaces radiated almost equally well. 
 
 This comparison of the radiating powers of grass and soil gives no support 
 
MR JOHN AITKEN ON" DEW. 
 
 39 
 
 to the idea that the greater wetness of grass on dewy nights is owing to its 
 greater radiating power. The radiating powers of the two surfaces seem to be 
 practically the same; and if neither grass nor soil received heat from the ground, 
 the soil would cool lowest, because the grass in its natural condition would get 
 more heat from the passing air, on account of its surface being irregular and in 
 small pieces, as we know that small surfaces receive from this cause much more 
 heat, per unit of area, than larger ones, this being particularly the case when 
 there is wind. From which we see that the smallness of the blades in grass is 
 really an advantage, and prevents their surfaces being cooled by radiation so 
 much as they would be if they were larger. 
 
 The number of substances tested for their radiating powers at night is not 
 so great as was hoped for, on account of the rare occurrence of evenings on 
 which work of this kind can be done in this climate; for not only must the sky 
 be free from passing clouds, in order that the amount of radiation may be as 
 constant as possible, but the air must also be very dry, in order that the dew- 
 point may be lower than the temperature of the cold radiating surfaces. 
 If vapour gets condensed on the radiating surfaces, the radiation from the film 
 of ice, or water, will interfere with the results. In making the experiments, 
 a large sheet of glass was generally exposed alongside of the radiation boxes, to 
 show if vapour was being deposited on the radiating surfaces. But even with 
 this precaution we cannot be sure we are testing the radiating powers of some 
 substances experimented on, because some kinds of matter have an aflRnity for 
 water, and condense vapour on their surfaces from unsaturated air. As an 
 example of uncertain results, I may mention a comparison made between salt and 
 sugar. These two substances have been found by other observers to radiate 
 very unequally at 100° C. — sugar radiating twice as much as salt. When tested 
 at night, they were found to radiate equally well; bat as both substances have 
 an affinity for water, their surfaces would have a film of moisture over them, 
 which would increase the radiating power of the salt, and thus make the test 
 of no value. 
 
 Among the few substances that have been found to radiate less heat at 
 night than a black surface is sulphur. On the night of the 7th December, when 
 the air was very dry and the glass plate kept undewed, the following read- 
 ings were taken : — 
 
 Temperature of air. 
 
 Temperature of black surface. 
 
 Sulphur. 
 
 27° 
 
 26° 
 
 21° 
 19° 
 
 23° 
 21°'25 
 
 The sulphur was sifted over the one thermometer box and the other left bare. 
 
40 
 
 MR JOHN AITKEN ON DEW. 
 
 It will be observed that the black surface radiated one half more than the 
 sulphur. This experiment suggests that a sprinkling of sulphur might be used 
 as a protection to delicate plants on frosty nights, but whether it would pay or 
 not experience alone can determine. 
 
 Polished tin was also tested, a sheet of tin being placed over one box, and 
 another sheet painted black put over the other, so as to make the conditions of 
 both boxes similar. The amount radiated by the tin was small; when the 
 temperature of the black surface fell 7°, the tin only fell about 1°, more or less, 
 according to the perfection of the pohsh of its surface. 
 
 For meteorological purposes the following observations made on the radiat- 
 ing power of snow will be useful. I regret that owing to the absence of snow 
 so far this winter, I have only had one opportunity of making observations on 
 this substance. In the following table will be found the readings given by the 
 thermometer boxes, one of which was left bare, and gave the radiation of black 
 paint, while over the other was put a thin layer of snow. This was done on 
 the forenoon of the 10th December, and readings were begun shortly after 
 mid-day, and taken from time to time till evening: — 
 
 Hour. 
 
 Air. 
 
 Black. 
 
 Snow. 
 
 Difference. 
 
 12'30 P.M. 
 
 28° 
 
 24° 
 
 21° 
 
 -3° 
 
 1 
 
 28° 
 
 24°-5 
 
 22° 
 
 -2°'5 
 
 2 
 
 26°-8 
 
 22° 
 
 20° 
 
 -2° 
 
 5 
 
 23° 
 
 15°-5 
 
 16° 
 
 -|-0°-5 
 
 5-30 „ 
 
 21° 
 
 15° 
 
 15°-5 
 
 + 0°-5 
 
 6-30 „ 
 
 21° 
 
 1.3° 
 
 13°-5 
 
 -|-0°-5 
 
 8 
 
 19° 
 
 9° 
 
 10° 
 
 +r 
 
 In the above table, the first column gives the hour at which the temperatures 
 were taken. In the second cohimn are the temperatures of the air; in the 
 third are the temperatures of the black radiating surface ; in the 'fourth are 
 the temperatures of snow surface; and in the fifth column are the difi^erences 
 between the temperature of the snow and the black surface at the hour the 
 readings were taken. The day on which this comparison was made was fine, 
 clear, calm, and frosty, with the sun shining brightly. The radiating surfaces 
 had a clear view of the sky, but were protected from the direct rays of the 
 sun. 
 
 It will be observed that while the sun was high the snow surface was very 
 much colder than the other; while the black surface only fell 4° below the 
 temperature of the air, the snow fell 7°. As the day advanced, and the sun 
 sunk towards the horizon, this difference decreased to 2° -5 at 1 o'clock and to 
 2° at 2 o'clock; and at 5 o'clock, by which time the sun had set, the snow 
 
 i 
 
ME. JOHN AITKEN 01^ DEW. 41 
 
 was a little warmer than the black surface, a condition in which it remained 
 during the evening. 
 
 The reason for the snow being colder than the black surface during the day 
 would seem to be, that both surfaces radiate and absorb " dark heat " about 
 equally well, both surfaces therefore throw off about the same amount of heat; 
 but while this is the case, their absorbing powers for the heat of the sun are 
 very different, and though the sun was not shining directly on the surfaces, 
 yet there is a considerable amount of its heat reflected to the surface of the 
 earth from the atmosphere overhead. Now a black surface absorbs most of 
 this reflected heat that falls uiDon it, while the snow absorbs very little. Hence, 
 while both surfaces are radiating about the same amount of heat, the black 
 surface is absorbing far more than the snow, and thus keeps warmer. As 
 the sun sinks, the amount of its heat reflected by our atmosphere gets less and 
 less, and the difierence in the temperature on the two surfaces diminishes; and 
 when at last the sun is quite under the horizon the temperature of the two 
 surfaces becomes nearly equal. It will be, however, observed, that they never 
 become quite the same, the snow being generally about half a degree warmer 
 than the other. The whole of this difference is not, however, owing to differ- 
 ence in radiating powers ; the snow Avill tend to give a slightly higher reading 
 on account of its surface being rougher than that of the paint, thus causing it 
 to receive more heat from the passing air than the black surface. And, further, 
 from the conditions of the experiments, the readings being taken during a 
 falling temperature, and the snow not being a good conductor of heat, the 
 thermometer under it will take longer to fall than the one under the blackened 
 metal. From these conditions it seems probable that there is not much differ- 
 ence between the radiating and absorbing powers of snow and black paint at 
 night, while the difference is very considerable during the day. 
 
 It has been suggested to me that this difference in the radiating and 
 ■ absorbing powers of snow and black surface, such as soil, &c., will enable us to 
 explain a difficulty long felt, regarding the hour at which the diurnal variation 
 of temperature begins in countries covered with snow. Over those parts of the 
 surface of our globe, where there is no snow, the temperature of the air begins 
 to rise before sunrise, whereas in snow- clad regions this change does not take 
 place till the sun is above the horizon — the explanation would appear to be 
 that where the surface of the ground is dark it absorbs the heat of the sun, 
 and warms the air whenever the rays begin to shine into the air overhead; 
 but where covered with snow it is but little warmed by these early reflected 
 rays, and it is not till the sun gets higher and shines on the surface of the 
 earth that its effects begin to be felt. 
 
 VOL. XXXIII. PART J. 
 
42 MR JOHN AITKEN ON DEW. 
 
 General Remarks. 
 
 We see as a result of these experiments, that in our climate at least, water 
 vapour is almost constantly rising from the ground, and this takes place from 
 fallow land, fi'om grass land, and from roads, even on nights on which there is 
 heavy dew. There seems to be but little doubt that the tide of vapour almost 
 always flows outwards from the earth, and ebbs but rarely, save after it has 
 been condensed to cloud and rain. The question as to whether any surface is 
 in a condition to lose or gain moisture on a dewy night depends on its more 
 or less perfect heat communication with the earth. Those surfaces, such as 
 soil, rock, stones &c., which are in good heat communication with the earth, 
 tend to keep warm, and to lose moisture ; while those surfaces not in good 
 communication with the earth, such as leaves of plants, roofs of sheds, &c., 
 tend to lose their heat, and gain moisture. This is the reason why grass 
 tends to collect true dew, while stones on the ground remain dry. Grass is a 
 bad conductor, and forms a non-conducting layer over the ground, preventing the 
 earth from losing its heat. The inside of this covering is hot by contact with 
 the earth, while its outside is cooled by radiation; and as the grass is a bad 
 conductor, its exposed surface gets cooled by radiation to a lower temperature 
 than the better conducting soil and stones ; hence the appearance of dew on it, 
 while the earth is dry. 
 
 Since vapour is constantly rising from the earth on dewy nights, it follows 
 that any measurements of dew we may make ought not to be added to the 
 rainfall, as the water so collected is in no sense a measure of the moisture 
 returned to the earth at night, nor is it even a proof that any water is then 
 returned. The amount of dew measured is simply a somewhat rough indication 
 of the amount of moisture received by plants and other bodies not in heat com- 
 munication with the ground ; while the ground itself does not receive any, but 
 is rather giving off vapour. 
 
 Dew is most copious during clear weather, and these experiments show us 
 that this condition of weather has a threefold action in the production of dew 
 — ^first, cloudless skies are necessary at night, in order that radiation may be 
 strong, and the surfaces of bodies cooled low enough to condense the vapour; 
 second, clear skies are necessary in order that a copious evaporation may take 
 place under a hot sun during the day ; and third, the same conditions are 
 necessary that the ground may be highly heated by the sun, and a large amount 
 of heat stored up during the day to be spent in evaporating an abundant 
 supply of vapour during the night. 
 
 What are known as radiation fogs are generally supposed to be due to cold 
 air flowing down, at evening, from higher levels to lower and warmer ones 
 and the mixing of the airs resulting in a foggy condensation. The more 
 
ME JOHN AITKEN ON DEW. 43 
 
 probable explanation now seems to me to be, that they are caused by the 
 uprising of the hot air and moisture from the ground, mixing with the colder 
 air above the grass, much in the same way as a fog is produced over a river in 
 sunny weather when the water is warm. So far as my observations go, these 
 fogs generally form over flat damp fields, after hot sunny days, and they have 
 been seen where there was no high ground from which cold air could flow in. 
 
 There almost seems reason for supposing that much of the moisture 
 collected on grass, and which looks like true dew, may under many conditions 
 be fine rain from fog formed in the manner above described. Because the hot 
 air and vapour rising through the grass will tend to form fog, where it mixes 
 with the cold air near the upper part of the grass, and if there is little wind 
 this fog will settle on the blades. Under most conditions this fog will not 
 form above the grass, and will not therefore be visible. It will often not form 
 above the blades, because the hot moist air may there meet with too much dry 
 air to supersaturate it, but there seems reason for supposing that it will be 
 often formed amongst the stems of the grass. 
 
 During frosts we have excellent opportunities for studying the condensation 
 of the vapour of our atmosphere, because it remains in the position where it is 
 condensed and is easily seen, being neither absorbed by the ground nor dropped 
 from the plants, &c., on which it may be deposited. I took the opportunity 
 afforded by two nights of this kind for observing two opposite conditions of 
 the air, and I shall here describe the' eff'ects of the radiation on the nights of 
 the 14th and 15th November. During the afternoon the canopy of clouds that 
 had hung over the earth for some days was gradually drawn aside, and moved 
 away southwards. By 5 p.m. on the 14th the sky was cloudless. There was 
 only a very slight movement of the air from the north, the radiation was strong, 
 and the air dry. These conditions continued all night, and the minimum 
 thermometer in the screen fell to 25°. 
 
 Next morning the ground and the grass were frozen. It was what is called 
 a black frost. There was no hoar-frost on the trees, and what little there was 
 on the grass was irregularly distributed. All the little hollows, of about a foot 
 square in area and under, had a deposit of hoar-frost, while the higher parts of 
 the grass had none. As there was no wind, and only a slow movement of the air, 
 this peculiar distribution would not be caused by the heating eff'ect of the passing- 
 air on the higher and more exposed blades, but was probably owing to its 
 dryness. The small hollows being less freely exposed to the circulation, the 
 air in them became more moistened from the vapour lising out of the ground 
 than the air a little higher up, where it got mixed with a larger amount of the 
 dry air. The test surface on the ground was quite dry at 9 p.m., and also next 
 morning, showing that the ground had been giving oflf vapour all night. 
 
 In contrast with this, let us now look at the condition of matters on the 
 
44 MR JOHN AITKEN ON DEW. 
 
 following morning. During the whole of the 15th the air remained calm and 
 frosty, and the cold intensified during the night. On the morning of the 16th 
 the minimum thermometer indicated a temperature of 19°. On this occasion 
 we had a hoar or white frost. Grass, fences, shrubs &c., were all white, and 
 the trees even to their top branches. The air on this occasion had evidently got 
 cooled to near its dew-point, and moisture had condensed on almost every ex- 
 posed surface, causing nature to present a remarkable contrast to its appearance 
 on the previous morning, though both mornings were frosty. 
 
 We shall now refer in more detail to some of the points most worth 
 noticing on these mornings. It was observed that the distribution of the hoar- 
 frost on the grass on the morning of the 16th was the reverse of what it was 
 the previous morning, the high blades on this occasion having rather a thicker 
 coating of hoar-frost than those lower down. The reason for this was that 
 the higher blades were exposed to the passing air, which on this occasion was 
 saturated; whereas those on the hollows had to depend for their supply on 
 what rose from the ground, which could not be much under the conditions, as 
 the bottom of the grass and the top of the soil were cooled below the freezing- 
 point, and most of the rising vapour would be trapped before it reached the 
 surfaca We may also note here that the test surfaces on the soil were quite 
 dry, and that the slate and iron weight resting on the grass were free from 
 deposit ; while the elevated slate and weight, the grass, and almost everything 
 else had a coat of hoar-frost, showing that the ground kept hot enough to give 
 off moisture. An examination of the bare soil also showed that at most parts 
 of its surface vapour was being given off. Wherever the contact with the 
 ground underneath was good, no hoar-frost formed, and it was dejposited only 
 on the small clods that were lying on the surface; of course, there was plenty 
 of hoar-frost on the under sides of the large clods, but none on their upper 
 surfaces. 
 
 The slates and weights resting on the grass were frequently examined while 
 the frost lasted, which it did till the morning of the 19th. During aU this time 
 the radiation was great, and the temperature very low. The minimum on 
 different nights was as low as 12°"5, l^'S, and 19°. During all that time, though 
 the ground received no heat direct from the sun, and but little in any way 
 from above, yet the supply from beneath was sufficient to keep the tempera- 
 ture of the surface above the dew-point, and the slate and weight in contact 
 with the ground remained black amidst the surrounding whiteness. 
 
 Another peculiarity of mornings such as this — -which are of frequent occur- 
 rence during winter — is the deposition of moisture on trees. During my observa- 
 tions in summer I never saw shrubs dewed to a height of more than a few feet 
 from the ground, while in winter tall trees frequently have vapour deposited on 
 them to the top branches. But as my observations in warm weather are very 
 
MR JOHN AITKEN ON DEW. 
 
 45 
 
 limited on this point, and owing to a simple wetting not being so conspicuous 
 as hoar-frost, it is possible trees may occasionally get wet in summer with dew 
 without its being observed. It however seems probable that it will be of much 
 more frequent occurrence in winter than in summer, owing to the much longer 
 absence of the sun during winter nights. 
 
 Radiation from Snow."- 
 
 In a previous part of this paper reference has been made to the I'adiating 
 and absorbing powers of snow. In the experiment detailed, comparative 
 readings are given of the temperature of snow and of a black surface exposed 
 in shade on a bright day. The temperatures taken under the conditions then 
 existing showed the snow to be much colder than the black surface. This 
 conclusion has since been confirmed by a number of readings with the radia- 
 tion thermometer under different conditions of climate. Of these observations 
 it will only be necessary to give those taken on two days. The following 
 temperatures were taken on the 19th Januaiy. In these tables the contents 
 of the columns are arranged as in previous one. 
 
 Hour. 
 
 Air. 
 
 Black. 
 
 Snow. 
 
 Difference. 
 
 10.0 A.M. 
 
 20°-2 
 
 16°-2 
 
 12°-0 
 
 -4°-2 
 
 2.0 P.M. 
 
 30°-0 
 
 30°-0 
 
 26°-0 
 
 -4°-0 
 
 2.30 „ 
 
 30°-l 
 
 30°-2 
 
 26°-4 
 
 -3°-8 
 
 3.30 „ 
 
 30°-2 
 
 30°-2 
 
 28°-0 
 
 — 2°-2 
 
 In the morning the sky was clear, but by 2 p.m. it became overcast, with a 
 thin uniform covering of clouds ; and at 3.30, it was ^beginning to snow. The 
 next readings were taken on the 5th February. 
 
 Hour. 
 
 Air. 
 
 Black. 
 
 Snow. 
 
 Difference. 
 
 10.0 A.M. 
 
 23°-0 
 
 28°-8 
 
 25°-0 
 
 -3°-8 
 
 12.0 P.M. 
 
 25°-6 
 
 3r-i 
 
 26°-l 
 
 -5°-0 
 
 2.0 „ 
 
 30°'0 
 
 34°-8 
 
 30°-0 
 
 -4°-8 
 
 3.30 „ 
 
 31°-0 
 
 34° -5 
 
 30°-5 
 
 -4°-0 
 
 5.15 „ 
 
 31°'5 
 
 29°-0 
 
 29°'0 
 
 -o°-o 
 
 On this occasion also the sky was overcast. In these two tables exactly 
 the same result is recorded, the black on both occasions being again much 
 warmer than the snow. These two tables are given, as they were taken under 
 quite different conditions. When the temperatures given in the first table 
 
 *Eead March 1, 1886. 
 
46 ME. JOHN AITKEN ON DEW. 
 
 were taken, the air was warmer than the radiating surfaces, while in the 
 second the air was colder than the exposed surfaces, except towards evening. 
 A number of other readings were taken under different conditions, but with the 
 same results ; the snow was always colder than the blacker surface during day, 
 while other observations made at night show their radiating powers are then very 
 similar. 
 
 Part of the cooling produced by the snow surface shown in the above tables 
 would be due to evaporation from the snow. The amount due to this cause 
 was not great, as the difference between the wet and dry bulb thermometers 
 was small at the time. I regret, however, that the readings of these instruments 
 have been lost, so I write from memory. It will, however, be observed that the 
 difference in the radiating powers of the two surfaces continued during the 
 whole time the readings were taken on the 19th January, though the weather 
 changed to snow, and the air at that time would be nearly saturated. The 
 readings given in the tables therefore give the total cooling effect of the snow, 
 which is produced by two causes — radiation and evaporation. 
 
 This small absorbing power of 'snow for heat, reflected and radiated from 
 the sky during the day, must have a most important effect on the atm-osphere, 
 causing its temperature to be much lower when the ground is covered with snow 
 than when free from it. So that when a country becomes covered with snow 
 — other things being equal — it will be accompanied by a depression of the 
 mean temperature of the air ; and, further, as cold tends to produce a stable 
 condition of the atmosphere, not creating the current disturbances of heating, 
 it would appear that once a country has become covered with snow there will 
 be a tendency towards glacial conditions. 
 
 But this poor absorbing power of snow is not the only way in which it 
 tends to produce a glacial climate. Snow, in addition to being a bad absorber 
 of the heat of the sky, is also a very poor conductor of heat. In illustration of 
 this, let me mention a few temperature observations made while the ground 
 was covered with snow during January last. On the 18th of that month 
 there was about 5^ inches (140 mm.) of snow on the ground; the night was 
 clear, and radiation strong. At 8 p.m. the temperature of the surface of the 
 snow was 3°, and a minimum thermometer also on the snow showed that it had 
 . been at 0° at an earlier hour. Taking a maximum thermometer, the index 
 was brought down below the freezing-point, and the bulb plunged through the 
 snow down to the grass. On examining it a short time afterwards the index 
 was at 32°. In confirmation of this reading, it may be mentioned that on 
 removing the snow the top of the soil was found to be unfrozen. These 
 observations showed that there was a difference of about 30° between the 
 temperature of the top and the bottom of the snow — that is, a distance of 
 5J inches. 
 
MR JOHN AITKEN ON DEW. 47 
 
 During the night the temperature of the air fell 5 degrees lower, so that the 
 surface of the snow would be kept aloout zero for most of the night, yet next 
 morning at 9.45 the bottom of the snow was still at 32°. The temperature of the 
 surface of the snow had risen at this hour only to 8°"5. These observations 
 were repeated on other occasions when the coating of snow was thinner ; there 
 was then less reduction in the temperature of the surface, but on all occasions 
 when the snow was a few inches deep, the surface of the soil remained at 
 32°. As the ground was frozen when the snow fell, it would appear that the 
 earth's heat slowly thawed it under the protection of the snow, and the 
 temperature of 32°, which Avas below the surface w^hen the soil was frozen, . 
 gradually rose to it, where it of course stopped, and the rising heat was spent 
 in melting the snow. 
 
 The protection afforded by the bad conducting power of snow is evidenced 
 in our climate, by the amount of vegetation that takes place underneath it, on 
 those occasions when we have had snow on the ground for a length of time. 
 After the snow is gone, many plants are found to have grown, and some 
 advanced nearly to bloom under its protection. This same influence may, 
 however, be seen at work in a more marked manner in spring on the slopes of 
 the Alps and other lands covered with snow all winter. As the snow recedes, 
 and the surface of the earth is gradually laid bare, the vegetation is found to be 
 .in an advanced state — many of the flowers, if not in bloom, are just ready to 
 open. 
 
 This bad conducting power of snow, compared with soil and rock, pvill 
 evidently tend to lower the temperature of the air over snow-clad lands, as a 
 few inches of snow in our climate conserves the earth's heat, and prevents its 
 surface being cooled below 32°. The surface of the snow thus gets very much 
 colder than the bare surface of the earth would have been if no snow had been 
 on it, and the air is thus cooled much more over a snow surface than over one 
 of bare earth. 
 
 To get evidence of the statement that the surface of a country covered 
 with snow is much colder than it would have been if free from snow, we have 
 only to examine the surface of the ground under the two conditions. The 
 surface of snow covering the ground receives so little heat from the earth 
 that it gets cooled by radiation to such an amount that it is almost always 
 during frost cooled below the dew-point, and is covered with a heavy deposit 
 of hoar-frost. During the late snow I have frequently noticed a thickness of 
 more than 12 mm. of beautiful ice crystals of this kind deposited on its surface. 
 But while the snow is cooled so much, and has this deposit on it, the surface 
 of the soil where it has been laid bare keeps quite free from this deposit, as it 
 receives sufficient heat from below to keep its temperature above the dew-poiut. 
 The air over snow-clad lands is thus always in contact with a highly-cooled 
 
48 MB, JOHN AITKEN ON DEW. 
 
 surface ; whereas when there is no snow, the air rests on a warmer 
 one. 
 
 Taking then these two things together, the bad absorbing power of snow 
 and its small conducting power for heat, we see that when snow has once got 
 possession of a land, the tendency to glacial conditions is greatly increased ; 
 and if it were not for the disturbing effects of heat in warmer climates, 
 it is hard to say how far glacial conditions might spread towards the 
 equator. 
 
 Up to the time when the previous part of this paper was given in* I had 
 been unableto find records of any meteorolgical station at which the condition 
 of the ground with regard to snow had been recorded in addition to the usual 
 temperature observations ; even yet I have not succeeded in getting suitable 
 records for our climate, and one cannot help regretting that observations of 
 this kind are not more frequently kept in this country. 
 
 My conclusions, however, with regard to the effect of snow on climate, I 
 now find are confirmed by the observations of Dr Woeikof, an abstract of 
 whose work in this direction has just appeared in Nature of 18th February 1886 
 (vol. xxxiii. p. 379). Dr Woeikof has approached the subject from the observa- 
 tional side, and as his results bear directly on ours, I shall quote the following 
 paragraph from the abstract referred to : — 
 
 " The year 1877 was a striking instance of how the absence of snow was 
 accompanied by a far less notable lowering of temperature during the preval- 
 ence of anticyclones, than would have been the case had the soil been covered 
 with snow. In 1877 there was no snow in Eastern Eussia until Christmas, and 
 in November and December the anticyclones occurred, accompanied by no 
 wind, or only by feeble breezes. Quite bright weather lasted in December for 
 more than ten days ; and still, in the region which remained uncovered with 
 snow, no great cold was experienced, as visually happens in such circum- 
 stances; the minima were 8° to 9° above their average values. The same 
 conditions were noticed during the winters of 1879-80 and 1881-82 in West 
 Europe, as shown by Dr Billwiller in the Zeitsclirift filr Meteorologie for 
 1882." 
 
 In the next paragraph, the abstract states that, in the opinion of Dr 
 Woeikof, the higher temperature of November, as compared with March in 
 South-East Russia, is due to the ground not being usually covered with snow 
 in November. 
 
 These observations of Dr Woeikof confirm the conclusions arrived at in this 
 paper from a consideration of the properties of snow. In the opinion of Dr 
 Woeikof, the low temperature accompanying the snow is to be attributed 
 to its bad conducting power. While we quite agree with the observer 
 
 * Note added 26th February 1886. 
 
ME, JOHN AITKEN ON" DEW. 49 
 
 that the bad conducting power of snow will be a cause of the lower tempera- 
 ture, yet we think that the observations recorded in this paper show that 
 radiation plays a most important part in producing this result. It has been 
 long well known that snow receives less heat from the direct rays of the sun 
 than the surface of a dark body like soil or rock, and we now also know that, 
 under any condition of sky yet tested, it absorbs but little of the sky radiation. 
 From this we see that as the surface of the snow, under all conditions of 
 sky, receives less radiant heat from without than the surface of the ground, 
 the air in contact with it will be more cooled than the air over bare soil or 
 rock. 
 
 In the radiation temperatures above recorded, the snow was generally 
 about 4 degrees colder than the black surface during the day. We must not 
 from this suppose that these 4 degrees are the greatest difference that could 
 exist in the radiation temperatures of the two surfaces. These 4 degrees 
 simply represent the difference that was maintained under the conditions when 
 the tests were made. If the air circulation had been greater, the difference 
 would have been less, and both would have been nearer the temperature of the 
 air ; and if the circulation had been less, the difference would have been 
 greater. But while there may be a variation in the difference of the tempera- 
 tures of the two surfaces, there will be but little difference in their respective 
 cooling effects, as with a quick circulation a large amount of air will be cooled 
 a little, while, when the circulation is slow, a little air will be cooled a great 
 deal. Not only so, but on account of the temperature of space being so much 
 colderthan the surface of the snow, the temperature of the snow will tend to sink 
 about the same amount below the temperatvire of the air, even though the air be 
 greatly cooled ; so that, under the same conditions of radiation and atmospheric 
 circulation, however much the air may have been cooled, the two surfaces will 
 tend to maintain the same difference in temperature, and the snow will always 
 tend to cool the air more than a black surface, when the surfaces are colder 
 than the air, or to heat it less when the surfaces are warmer. 
 
 It is therefore evident that this radiation effect will be one of the causes 
 tending to produce the lower temperature of the air while snow is on the 
 ground, and it seems probable that the snow will not only reduce the minima 
 temperatures, but also the maxima. It seems also probable that the bad 
 absorbing power of snow will have a most important influence in retarding the 
 approach of warm weather, as the earth under snow receives far less heat from 
 the sun with the approach of spring than if the ground had been free from 
 snow. This is, of course, altogether apart from the question of the retardation 
 of the approach of warm weather by the sun's heat being spent in melting the 
 snow, instead of warming the air. 
 
 VOL. XXXIII. PART I. G 
 
50 MR JOHN AITKEN ON DEW. 
 
 Summary. 
 
 Our principal conclusions may be summed up as follows : — First, From the 
 experiments made with (a) the inverted trays, (6) by weighing small areas of 
 turf, and (c) by observations of the temperatures on and under the grass 
 during dewy nights, we have concluded that vapour is almost constantly rising 
 from grass land, by night as well as day, in our climate. Second, From experi- 
 ments made with (a) inverted trays, (Jb) by weighings of small areas of soil, and 
 (c) by observations made with small test condensing surfaces, on dewy nights, 
 we have concluded that, under most conditions of our climate, vapour rises 
 from uncultivated areas of soil during night as well as day. It follows from 
 these two conclusions that dew never " falls " on the earth : and for reasons 
 given, it is only deposited on plants, and other bodies, not in good heat com- 
 munication with the ground. Third, That the greater part of the dew condensed 
 on bodies near the ground is formed of the vapour rising at the time from the 
 earth, and very little of it from the vapour that rose during the day. Fourth, 
 That dew forms copiously on roads, but owing to the stones being good con- 
 ductors of heat, the vapour is deposited on the under sides of the stones, and 
 not on the top as on grass. Fifth, Wind hinders the formation of dew by 
 preventing an accumulation of damp air near the ground. Sixth, The " dew- 
 drop " formed on grass and other plants is not dew at all, but is formed of the 
 exuded sap of the plant. Seventh, Almost all substances, such as black and 
 white cloths, garden mould and grass, radiate equally well at night. Among the 
 few exceptions observed are polished metals and sulphur. Eighth, A covering 
 of snow on the ground lowers the mean temperature of the air. 
 
 FURTHEE EEMARKS ON DEW. 
 
 Appendix. 
 
 (Readl9tli July 1886.) 
 
 Since the preceding paper was written, a few opportunities have occurred 
 for continuing the investigation under different conditions of climate, and some 
 additions have also been made to strengthen the exudation theory of the " dew- 
 drop " on grass and other plants, of which I shall here give a short account. 
 
 At the beginning of the paper, evidence is adduced to show that water 
 vapour is almost constantly rising from the ground during night, as well as day, 
 and it is there noted that the experiments were somewhat unsatisfactory on 
 account of the rather damp condition of the soil at the time, the dry season 
 being over before the investigation was made. In order to supplement these 
 observations, a few others were made in the beginning of July of this year, when 
 the soil was in about as dry a condition as it almost ever is in this climate. 
 
MR JOHN ATTKEN ON DEW. 51 
 
 An inverted tray was placed over bare soil whicb was so dry and powdery 
 that it rose in dust when the edge of the tray was pressed into it. The tray 
 was not placed on the ground till after 9 p.m. — that is, after the soil had lost a 
 good deal of heat and moisture — ^yet in two hours the inside was wet, and in 
 the morning it was covered with drops. While this experiment was going on, 
 another tray was placed on the lawn at a place where the grass was burnt quite 
 brown. The inside of this tray was also wetted, and to about the same amount 
 as the one over the bare soil. It may be noted here, that the positions for 
 making these tests were selected on account of their extreme dryness. The 
 bare soil was light and open, while the lawn was extremely dry, having been 
 laid down a few years ago with every precaution to ensure this condition ; the 
 under soil is dry and sandy, it was moreover well drained, the upper soil was 
 removed, and a good depth of ashes put in its place, then a layer of sand, on 
 which was put the turf with its inch or two of soil. A drier position could 
 scarcely have been selected for the trial, and yet the result showed that in our 
 climate bare soil and grass land, even when very dry, continue to give off vapour 
 during dewy nights. 
 
 A few experiments were also made on this subject in March last at Hyeres, 
 in the south of France. In order that the test might be as severe as possible, 
 I selected a spot where the soil seemed driest and most exposed to the sun and 
 the wind; and as the soil was stony and lying on the hill-side, it could 
 have no supply of water from below. An inverted tray placed over this arid 
 ground collected a surprisingly great amount of moisture. The following are 
 the notes of one of the tests made at Hyeres on the 27tli March last : — At 5.15 
 P.M. the sun had ceased to shine on the place selected for the experiment ; at 
 that hour the temperature of the soil was 73° F. at 3 inches below the surface, 
 and 59° at 12 inches. At 5.45 the tray was put on the ground. Examined at 
 7.15 P.M. the outside was dry, but inside was quite wet, and by 10 p.m. the drops 
 were so large they ran down the inside when the tray was placed vertically. 
 This great amount of wetting was probably due, not only to the ground being 
 highly heated during the day, but also to a considerable fall in temperature at 
 night, by the cooling produced under the clear skies of that climate. 
 
 I much regret I have been unable to get any trustworthy information 
 regarding the movement of the vapour near the surface of the ground in barren 
 and desert countries. I have, however, received some information from 
 travellers who have been in Australia and parts of South Africa, where rain 
 does not fall for months at a time, and it goes to prove that even in these dry 
 countries vapour rises from the ground at night, as they often found the under 
 side of their waterproof bedding placed on the ground to be wet after camping 
 out at night. One would scarcely have expected much moisture to be collected 
 in this way on account of the warmth of the sleeper's body keeping up 
 
52 ME JOHN AITKEN ON DEW. 
 
 the temperature of the condensing surface. It therefore seems probable 
 that the moisture will collect under those parts of the waterproof which are 
 beyond the influence of the sleeper's body. 
 
 While the experiments above referred to on bare soil and grass land were 
 being made on ground exceptionally dry in July, those with slates placed on 
 the road were repeated. As in the experiments described in the first part of 
 this paper, the slates placed on the hard dry part of the road and on the 
 gravel got quite wet on their under sides at night, thus showing that, even 
 under the exceptionally dry conditions existing at the time, vapour was still 
 rising at night from the hard and arid roads. 
 
 In connection with the action of stones lying near the surface of the ground, 
 we may here refer to a result observed by agriculturists, on which it seems to 
 throw some light. It has been remarked that the removal of small stones from 
 fields where the soil is light and open, often has a prejudicial effect on the 
 crops. It must be admitted that accurate information on this point is not 
 easily obtained, but the impression in some parts of the country certainly is, 
 that the removal of small stones from fields is not to be recommended. Now 
 the removal of stones may act prejudicially in different ways. In some cases 
 the disintegration of the stones may add to the richness of the soil; their 
 removal may therefore in some cases decrease the natural fertility of the field. 
 ISTo doubt the removal of the stones will permit of a more rapid evaporation from 
 the soil during the day, but it does also seem probable that the peculiar action 
 of stones, in trapping the moisture before it comes to the surface at night, will 
 have a beneficial effect on light and dry soils. Part of the moisture is trapped 
 by them before it can escape from the surface at night ; and when the sun 
 rises, the stones becoming warmer than the soil under them, the moisture 
 leaves the stones and condenses in the soil underneath. In this way stones 
 would seem to have a sort of conserving action on the moisture, and tend to 
 check the prejudicial effects of continued dry weather. 
 
 In confirmation of this conserving action of bodies lying on the surface of 
 the ground and improving its fertility, I would refer to a letter in Nature, 
 vol. xxxiii. p. 583, by Lieut.-Col. A. T. Fraser. As this letter is so 
 interesting, and bears directly on our subject, I may be allowed to quote 
 from it here. At the beginning of his letter he says : — " Having had occasion 
 to lay out a large quantity of iron hoes and picks, without handles, on the hard 
 ground of an open inclosure in one of the driest districts of India (Bellary), 
 where, in fact, these implements had been collected in the face of a scarcity, it 
 was found, after they had lain a couple of months, that a thick, weedy, but 
 luxuriant vegetation had sprung up, enough, though there was no rain, to 
 almost hide the tools." 
 
 Lieut.-Col. Fraser also says he had previously noticed in the tropics a 
 
MR JOHN AITKEX ON DEW. 63 
 
 similar stimulating effect produced on vegetation when tools were left lying on 
 the surface, but was unable to account for it till he read the abstract of the 
 first part of this paper, when he at once recognised the manner in which the 
 tools lying on the surface acted in hot and dry climates by checking the 
 escape of vapour at night. No doubt much of the increased fertility observed 
 would be due to the wetting the plants received by the vapour condensed on 
 the under sides of the tools ; still it will be admitted something must also be 
 owing to the reduced evaporation during the day, produced by the metal tools 
 checking the escape of the vapour. 
 
 This conservation of moisture by stones and other bodies lying on the 
 surface of the ground, and its up and down movement during night and day, 
 may be easily seen in the experiment with the slates. Examined at night the 
 Q'oad under the slates is as dry as the exposed parts, while the under sides of 
 the slates are wet ; but if examined at a certain time in the morning, the under 
 sides of the slates will be found to be dry, ivMle the road under them is wet. 
 But, if the slates are not examined till a later houi% both the slates and the 
 road under them will be found to be dry, the moisture being driven deeper and 
 condensed among the stones lower down. 
 
 Many people, after reading the conclusions arrived at in the first part of this 
 paper, seem to have a difficulty in accepting the theory that vapour is con- 
 stantly rising from the ground during night as well as day, and that the dew 
 on grass is formed of this rising vapour, because it seems to them to contradict 
 the teaching of Dr Wells, and it also appears to be at variance with their 
 experience. A little consideration will show that the results established by 
 Dr Wells are in no way affected by it. That investigator certainly did 
 not think that much vapour rose from the ground at night, yet he was well 
 aware that some might rise ; his investigation was however principally con- 
 fined to the condensation of the vapour after it is in the air, and he gave 
 comparatively little attention to its source ; whereas in this investigation 
 Wells' results are accepted with regard to the condensation, and an attempt is 
 made to extend the subject by investigating some parts not worked out by him. 
 
 Others again have made a difficulty to the acceptance of this theory by 
 extending the conclusions arrived at in this paper to other conditions than those 
 for which they are true. They have assumed that if dew on grass and on bodies 
 near the ground is formed of vapour rising at the time, then the dew found on 
 bodies higher up in the air must also be formed of vapour rising at the time 
 from the ground immediately underneath'; and as this conclusion is opposed to 
 experience, they seem inclined to dismiss the whole theory as unworthy of 
 consideration. When we come to investigate what is taking place in nature 
 we will see that this extension of the conclusion is by no means justified. 
 
 The reason why it is concluded that the dew on the grass, and on bodies 
 
54 MR JOHN AITKEN ON DEW. 
 
 close to the ground, is formed of vapour rising from the ground, is that at 
 night the ground under the grass is always in a condition to give off vapour, 
 and this rising vapour will tend to displace that which rose during the day; 
 the stems and blades of grass will thus become surrounded by vapour that 
 has risen during the night. A further reason is, that the ground under the 
 grass is much warmer than the air over it, and the air in contact with the 
 moist earth is nearly saturated ; the tension of the vapour in the air rising 
 through the stems of grass is thus higher than that in the air over them, and it 
 is therefore in a more favourable condition for condensing and forming dew 
 than the air higher up. This hot rising vapour will often indeed yield dew 
 when the air over the grass can give none. 
 
 When, however, we come to consider what takes place higher up above the 
 grass, or even at the tops of the blades, we meet with a much more compli- 
 cated condition of matters, and we are now able to say very little about the 
 source of the vapour condensed at these higher positions. Whenever we get 
 above the protection of the grass, into the parts of the atmosphere exposed to 
 air-currents, we can say very little as to the source of the vapour existing there, 
 either as to the place where it changed to vapour, or the time when this 
 change took place. No doubt some of the molecules in this upper air will 
 have risen but recently from the ground, but some of them will certainly — if 
 there is the slightest wind — be molecules that have risen during the day, and 
 no doubt some of them will have ascended into the air many days previously ; 
 and while some will have but recently come from the ground immediately under- 
 neath, others will have travelled from lands and oceans far away. But while 
 this may be so, it in no way affects the conclusion that vapour is almost con- 
 stantly — night as well as day — given off by the ground, and that dew on grass, 
 and on bodies close to the ground, is part of this rising vapour trapped by their 
 cold surfaces. 
 
 Curiously enough, history here repeats itself The theory that dew rises 
 from the ground has before now been wrecked by the observation that dew forms 
 on bodies placed high above the ground, and in situations where no vapour 
 could have risen to them from beneath. Professor Musschenbroek rejected 
 Gersten's theory of rising dew after he found dew was deposited on bodies 
 placed on the leaden roof of his observatory. He thought the dew formed 
 under those conditions could not have risen from the ground, but must have 
 fallen from the atmosphere. There must, therefore, evidently be a foundation 
 somewhere for the rejection of the theory on the grounds stated, though it must 
 be admitted it is difficult to find, as the statements are in no way opposed to 
 each other. It will, therefore, be as well for us to consider here the cause of 
 this appearance of opposition. 
 
 One cannot help thinking that a good deal of the difficulty experienced in 
 
MR JOHN AITKEN ON DEW. 55 
 
 reconciling the statement, that the dew formed on bodies near the surface of 
 the earth is formed of vapour rising from the ground, with the fact that dew is 
 found on bodies high up in the air is caused by a want of clearness in our 
 ideas, or perhaps rather to a persistence of primitive ideas. Dew was in olden 
 times often spoken of as falling from the heavens, and even yet we talk of 
 falling dew. This expression is to a certain extent associated with the idea of 
 falling rain — a process in which the moisture passes from place to place through 
 the air, and falls on bodies exposed to it ; and many seem to think that if dew 
 comes out of the ground it should be found only on bodies exposed to the earth. 
 That in fact rising dew is the converse of the old falling dew, whereas dew is 
 only so much moisture taken by a cold surface from the store of vapour in the 
 air. This explanation of the difficulty does seem somewhat absurd, but we all 
 know that old habits of thought have a curious way of asserting themselves, 
 and it seems the only way of explaining a difficulty so many have felt. 
 
 Let us picture an imaginary state of matters, in which all the conditions shall 
 be as simple as possible. Suppose the vapour to be constantly rising from the 
 ground, and that the air is absolutely still ; and further, let us imagine that the 
 vapour flows upwards through the air in a continuous stream, only varying in 
 velocity at different hours. At 6 p.m. we will suppose the molecules of vapour 
 that left the earth at 6 a.m. to have arrived at a height a. Let dew now begin 
 to form, then the moisture condensed at 6 p.m., on bodies at the height a, will 
 be vapour that rose from the ground at 6 A.M., and bodies at intermediate heights 
 will have the vapour on them that rose at the intermediate hours. At 6 a.m. 
 the following morning the vapour that rose at 6 on the previous evening will 
 be at an elevation h, and the dew forming on all bodies lower than & at this hour 
 will be vapour that rose during'the night. So that in these ideal conditions, if 
 we knew the rate of ascent of the vapour, we could tell the hour at which the 
 vapour — condensed at any height — rose from the ground. 
 
 If this imaginary condition of matters was correct, then there would be 
 some reason for expecting that dew would only be deposited on bodies placed 
 over such areas as yield vapour. In nature, however, the conditions are much 
 more complicated. The vapour does not flow upwards in a uniform stream, but 
 is mixed with the air by eddies and wind currents, and carried to bodies far 
 from where it rose ; so that while we can say something about the source of the 
 vapour condensed on bodies near the surface of the earth, yet the molecules in 
 the higher air have no history we can interpret. While the vapour rising from 
 the ground plays an important part in the phenomena of dew, as it is not 
 only the source of that formed on bodies near the ground, but it also increases 
 the amount deposited on bodies high up, yet the rising vapour is not essential 
 to its formation, as dew may be deposited even though the country for many 
 miles all round is dry and incapable of yielding any vapour. In such case 
 
56 MR JOHN AITKEN ON DEW. 
 
 the supply of vapour to form dew would depend on the evaporation of the 
 dew, and on what was brought in by the winds. 
 
 The "Dew-Drop." 
 
 The statement that the " dew-drops " formed on plants at night is not dew 
 at all, but is formed of the exuded sap of the plant, has been rejected by some 
 on account of its being contrary to all accepted ideas on the subject, while some 
 who have accepted it, have given only an indifferent assent. It has therefore 
 seemed desirable that further evidence be advanced in support of the state- 
 ment, and also that some simpler methods be devised for studying the pheno- 
 mena connected with the exudation of moisture by plants, so that those not 
 accustomed to making difficult experiments may be able to demonstrate the 
 point for themselves. 
 
 One of the simplest experiments of this kind is to cut a piece of turf, or, 
 better still, lift a single grass plant with a clod of earth attached to it, which 
 can generally be easily found in any garden. The leaves and stems of a single 
 plant being separate and open, the phenomena are more easily observed on it 
 than in the confused vegetation of a turf. Place the plant on a plate, and 
 invert a tumbler or other vessel over it, so as to enclose the plant and rest it on 
 the plate. This should be done when the soil is not too dry, otherwise water 
 will require to be given to the plant. After it has been kept in moist air 
 for about an hour drops will begin to exude, and the tip of nearly every 
 blade will be found to be studded with a diamond-like drop. 
 
 In the above simple experiment there is nothing to tell us where the 
 moisture came from to form the drops. It might be contended that it was 
 condensed by the plant out of the moist air. It has, however, been shown in 
 the first part of this paper, that when the tip of the blade is isolated from 
 all supply of moist air, the drop at the end grows as quickly as the drops 
 at the ends of the blades exposed to saturated air. This experiment is, 
 however, a somewhat difi&cult one for any one not accustomed to work of 
 this kind. The point may, however, be proved in a much simpler way. Take 
 any exuding plant with a single stem, such as a broccoli or poppy, if it is grow- 
 ing in a pot so much the better, as it is more convenient both for making and 
 seeing the results of the experiment. Prepare a circular disc of metal — -say tin- 
 plate—with a hole in its centre large enough for the stem of the plant to pass 
 through, then cut the disc in two through the centre. Now place the disc on 
 the pot with the stem of the plant passing through the hole, and join the two 
 halves of the disc, either by soldering, or by cementing over the joint a strip of 
 sheet india-rubber. A large glass receiver is now placed over the plant with 
 its edges resting on the metal plate. In this way the plant is isolated in air 
 from which it can extract no moisture, the metal plate preventing vapour 
 
MR JOHN AITKEN ON DEV\^. 57 
 
 rising from the soil underneath. If any drops now appear on the leaves, it is 
 evident the air cannot be the source of the moisture. 
 
 When tested in this way, it will be found that any exuding plant will soon 
 become studded with drops, and present exactly the appearance it would at 
 night. The time the drops take to appear depends on certain conditions ; 
 amongst these are — 1st, the kind of plant experimented on; 2nd, the state of 
 vital activity in the plant at the time; and 3rd, the degree of dryness of the 
 leaves at the time it is put under the receiver. If the leaves are very dry, it 
 will take some time to fill the tissues of the plant with sap before the surplus 
 begins to exude. A broccoli plant in fair health and condition may be expected 
 to show drops in less than an hour. These drops are small at first, and 
 gradually grow so large that they fall off by their own weight. 
 
 An experiment of this kind is so easily made by any one, that the interest and 
 the information gained is ample reward for the little trouble taken in making 
 it. Instead of using a metal plate as above described for isolating the plant 
 from the damp soil, a simpler plan is to use a piece of sheet india-rubber, with 
 a radial slit in it, for slipping it round the plant, the two edges being joined 
 with india-ru.bber solution, the glass receiver being, as before, placed over the 
 plant, and its edges resting on the rubber. 
 
 The evidence advanced in the first part of this paper in support of the 
 statement that the " dew-drop " is exuded by the plant is^lst, that the drop 
 at the tip of a blade of grass grows as quickly when isolated from all supply of 
 moist air as when exposed to saturated air ; 2nd, the blades of those plants 
 which have drops attached to them on dewy nights exhibit no drops when 
 separated from the root, even when supplied with water and placed in 
 saturated air ; and 3rd, when hydrostatic pressure is applied to the stalk of 
 the leaf of any plant that has drops attached to it at night, it causes the leaf 
 to exude at exactly the same points as the drops appeared on dewy nights. 
 Although the above evidence is fairly conclusive, yet there is a point where it 
 might be strengthened by the addition of a link to the chain. While it is 
 shown that hydrostatic pressure will produce the same effects as are seen on 
 plants at nights, yet no evidence is adduced to show that there is any internal 
 pressure in those plants on which " dew-drops " have been observed. This 
 omission has now been corrected, and some experiments made in connection 
 with this point. The plants selected for experiment were of the same kinds 
 as those which were observed to have " dew-drops " on them at night, and 
 which showed exuded drops when subjected to hydrostatic pressure, such as 
 broccoli, cauliflower, poppy, &c. For convenience, some plants were grown in 
 flower pots, and the experiments made while they were still small. 
 
 For measuring the pressure inside the plants, U-shaped tubes half filled 
 with mercury were used, and the pressure measured by the height to which 
 
 VOL. XXXIII. PART I. H 
 
58 MR JOHN AITKEN ON DEW. 
 
 the sap forced the mercury. These gauges did their work well enough, but 
 they were somewhat slow in action, as it takes some time for sufficient liquid 
 to be exuded to displace the mercury and force it up the tube. Where high 
 pressures were required to be measured, the U tubes had to be abandoned, on 
 account of their inconvenient height, as well as the length of time required for 
 making an observation with them, and the pressures were measured by means 
 of air-pressure gauges. These gauges were made of a short length of wide 
 thermometer tube, having a bore of less than 1 mm. diameter, a short 
 column of mercury being put in to form an index. The pressures were 
 calculated from the volumes, and corrections made when necessary for 
 temperature. The gauges were occasionally compared with a column of 
 mercury to see that everything was correct. The pressures given cannot be 
 considered correct to more than 10 mm. of mercury, but for the present purpose 
 this degree of accuracy is sufficient, as the pressures are very indefinite, 
 varying with so many conditions that anything like an exact figure cannot be 
 looked for in experiments of this kind. 
 
 I shall now describe in detail an experiment made on a cauliflower, as 
 it is similar to those made on other plants, of which it will only be necessary 
 here to give the results. The pot containing the plant was j)laced on a sheet 
 of metal, and a glass receiver got ready large enough to cover it entirely. The 
 stalk of one of the blades was selected for making the connection between 
 the pressure gauge and the plant. This stalk, while the leaf was still on it, was 
 prepared for making a water-tight joint by filling up the longitudinal groove in 
 its upper surface with beeswax, laid on with a slightly heated iron. The blade 
 was now cut off", and the gauge attached by means of a short length of soft 
 india-rubber tube ; the stalk having been made round by means of the beeswax, 
 a tight joint was easily made. With the exception of this one leaf cut off", all the 
 others were left untouched. The receiver was now put over the plant to stop 
 evaporation from the leaves, and everything left at rest. After a short time 
 the pressure was seen beginning to rise in the gauge, and drops also began to 
 show themselves all round the edges of the leaves. As time went on the drops 
 increased in size, and the pressure went up to 290 mm. of mercury, at which 
 point it stopped. This pressure can be considered correct only for the par- 
 ticular plant under the particular conditions. It simply meant that when the 
 pressure rose to 290 mm., the whole of the supply of sap sent up by the root 
 could find an exit by exudation. If the supply had been greater, or the 
 exuding pores fewer or smaller, the pressure would no doubt have gone higher, 
 and vice versa. 
 
 The plant in the above experiment was'in the same condition as it would be 
 on a dewy night. All evaporation from the leaves was stopped, and transpira- 
 tion having ceased, the root continued to send up its suppUes of sap, first filling 
 
MR JOHN AITKEN ON DEW, 59 
 
 the tissues of the plant, and then producing an internal pressure, which forced 
 the sap to escape by the exuding pores. But what is the condition of the 
 tissues during day when transpiration is going on 1 An answer to this was easily 
 obtained by removing the receiver from the plant, and allowing evaporation to 
 proceed from its leaves. The result was that the pressure inside the plant fell, 
 and a negative pressure took its place ; the mercury first fell in the U tube, and 
 then rose on the other side. The mercury was drawn np in one case to a height 
 of 140 mm., and in another plant to 180 mm., the height seeming to depend on 
 the rate of evaporation, and the perfection and closeness of the tissues of the 
 plant enabling it to stand a greater or less pressure before air forced its way 
 inwards. We see from this that exuding plants during night, and at times when 
 there is little evaporation, have an internal pressure tending to distend their 
 tissues, and have a negative or external pressure during the day tending to 
 press the tissues inwards. This internal pressure may help to explain the more 
 rigid appearance of the leaves of plants at night ; while the negative pressure 
 or degree of vacuum produced inside the leaves by transpiration explains 
 the manner in which water is taken up by cut flowers and branches of plants 
 when their ends are placed in it, and it also explains something of the peculiar 
 curving of leaves when withering. 
 
 I have said that the pressure above measured insid.e of the cauliflower plant 
 Avould have been much greater if the exuding pores had been less in size or 
 number — that, in fact, the pressure then measured was not the maximum root 
 pressure. To test this point, the plant was now cut across near the bottom of 
 the stem, within two or three centimetres of the root, so removing all the leaves 
 with their exuding pores ; and the pressure gauge was attached to the stem 
 near the root. The gauge now rose very rapidly, and in a short time indicated 
 a maximum pressure of 760 mm., the india-rubber connecting tube requiring to 
 be strongly bandaged to prevent it bulging. It seems strange that the delicate 
 tissues of a young plant should be able to produce and resist so great a pressure. 
 We must however remember that this last registered pressure is one to which 
 the plant is never subjected when under natural conditions, but even the 290 
 mm. measured when the plant was exuding freely does seem a great pressure 
 to exist in plants. 
 
 A poppy tested in the same way showed an internal pressure of 175 mm. 
 with its leaves all on, and exuding freely in saturated air. This lower pressure 
 compared to the cauliflower, would seem to indicate that the exuding pores are 
 larger or more numerous in the poppy than in the cau.liflower, as the former are 
 fully as wet as the latter on dewy nights. When the poppy was cut across, 
 and the gauge attached to . the main stem near the root, the pressure rose 
 to as much as 1040 mm. 
 
 The pressure inside grass has not been easily measured, owing to the diffi- 
 
60 MR JOHN AITKEN ON DEW. 
 
 culty of making a tight joint with the gauge, and so delicate a structure as a 
 grass stem. The highest pressure observed before the joint gave way was 160 
 mm. The measurement was taken with all the blades on, and exuding freely in 
 saturated air. This pressiire is just a little less than was found in the poj)py 
 when under similar conditions. No measurements have been made with all 
 exudation stopped, on account of grass not growing in a form suitable for 
 making a measurement of this kind. 
 
 The following are a few readings of the maximum root pressure given by 
 different plants. These plants were small and still in the seed-bed, but too 
 large for transplanting. 
 
 Cauliflower, .... 875 mm. 
 .... 920 „ 
 
 .... 1065 „ 
 
 Cabbage, .... 1310 „ 
 
 From the above figures it will be seen that the pressures given by the 
 different cauliflower plants varied considerably ; and that the cabbage experi- 
 mented on was capable of exerting a root pressure equal to forcing its sap to 
 a height of about 58 feet, thus showing an extraordinary reserve of energy. 
 
 It was shown- in the first part of this paper that the leaves of these plants 
 exuded when hydrostatic pressure was applied to their stalks, and we have 
 now shown that there is abundance of pressure inside these plants at night to 
 produce the exudation. Nothing like an attempt, however, has been made here 
 to give either the exact pressure inside different kinds of plants or the pressure 
 under diff'erent conditions. It has already been stated that the conditions 
 aff"ecting the pressure are much too varied for the figures to be settled by a 
 few experiments ; all that has been attempted is to show that in exuding plants, 
 there is abundance of pressure to produce the results claimed. 
 
 In connection Avith this subject, it was interesting to notice the variation in 
 the exudation of grass during the late continuance of dry weather. While the 
 soil was damp exudation went on as usual, but when the ground got drier the 
 exudation gradually got less and less, and at last it entirely ceased. Even when 
 the grass was covered with an enclosure, and surrounded with saturated air, 
 no exudation took place, and yet the grass was green and growing ; it took 
 some time after the grass was wetted before the activity was great enough to 
 give rise to exudation. During this dry weather, while the grass had ceased to 
 exude, it got moist at nights with the hot vapour rising from the ground, the 
 lower parts, particularly where exposed to radiation, being wetter than the tops 
 of the blades. 
 
 It is very difficult to get an idea of the number of plants that exude, so 
 much depends on the vitality of the plant at the time, and on the amount of 
 
MR JOHN AITKEJSr ON DEW. 61 
 
 moisture in the ground, so that the same plant may emit drops at one time and 
 not at another. We have seen that even so free an exuding plant as grass may 
 cease to discharge ; others cease with a less degree of dryness, and with a 
 less decrease in vital activity. The number that exudes under favourable 
 conditions is, however, much greater than we might at first imagine. In the 
 south of France, in spring, a very great number of plants were observed to 
 exude on dewy nights ; even roses had their leaves fringed with drops, a 
 condition in which I have never seen these plants in this country ; but the 
 activity of vegetable life in spring is very much greater in the south than with us. 
 This question of root pressure in plants is one of vast interest ; so much 
 still remains to be known about it. How is it that one plant must have the 
 soil in which it grows full of water, while another requires it to be only damp 1 
 Another seems to be able to grow on nearly dry soil, Avhilst another still can by 
 means of its air-roots extract moisture from air that is not saturated. What is 
 the source of energy called into action by this latter class to enable it to 
 condense the vapour in the air 1 Is it a chemical process ? or a purely physical 
 one, like the condensation of vapour by Professor Tait's hygrometer when it 
 is falling 1 or is it some unknown function of vitality 1 These questions, how- 
 ever, open up a field much too wide to be considered here. 
 
 I^th July 1886. — After I had written the above paragraph, and as I supposed 
 had closed the paper, it slowly dawned upon me that the surface of the leaves of 
 all the different kinds of plants that have been obseiwed to exude drops behaved 
 themselves in a particular manner towards water. None of them seemed to be 
 wetted by it. The glistening rain-drop on the grass shows that the blades of 
 that plant are not wetted by water, the glistening being due to the reflection 
 from the inside of the drop, where it rests on the blade, but does not touch it. 
 But do all the other exuding plants repel water in the same manner \ As it 
 was raining while these thoughts passed through my mind, a visit to the garden 
 was at once made, and the broccoli, poppy, and all the other exuding plants were 
 examined. Every one of them was found to behave towards the rain-drops in 
 the same manner as the grass. The rain-drops slipped off their surfaces " like 
 water off a duck's back ;" and where water collected in the hollows of the blades, 
 the reflection from its internal surface showed it was not in contact with them. 
 
 The other plants — cultivated and uncultivated — in the garden were then 
 examined, when most of them were found to be quite wet. The difference 
 in their appearance from the exuding ones was very marked. At first sight 
 the leaves of plants that got wet, like potatoes, beans, &c., looked almost as if 
 they were dry, but in reality the water wetted them so perfectly all over, that 
 it ran off", leaving only a thin and even film on their surfaces ; whereas all 
 the plants that exuded drops had their surfaces dry, save certain small areas 
 
62 MR JOHN AITKEN ON DEW. 
 
 where the natural surface of the blade had been destroyed. On thinking over the 
 matter, it became evident that this property of leaves that exude drops at night 
 ouo-ht to have been foreseen by me. The fact that the emitted moisture remains 
 as a drop, shows that the surface of the leaf rejects water; if the leaf surface 
 got wetted with water, the exuded liquid would have crept outwards from the 
 exuding pore, and have wetted the leaf for some distance all around it. These 
 exuded drops behave very much in the same manner as a drop of water 
 attached to the end of a glass rod that is not very clean; the water does not 
 wet the rod, but draws itself up into a drop. If the drop had been attached to 
 a wooden rod or a piece of thread, or anything that was easily wetted, it would 
 not have remained as a drop, but have spread itself all over the surface of the 
 body. 
 
 When examining the plants in the garden during rain, in addition to those 
 plants which I knew exuded drops at night, I noticed a number of others 
 that rejected the rain drops, and kept their surfaces dry in the same manner as 
 the exuding plants. Amongst these were Nasturtium, some of the Brassicese 
 family not previously observed, and also some weeds. Now, it appeared that if 
 the above reasoning is correct, these other dry-surfaced plants ought to exude 
 drops, I therefore marked them, and on afterwards experimenting found that 
 they also discharged drops like the others. 
 
 It almost looked at first sight as if this property of repelling water was a 
 distinguishing characteristic of the leaves of all exuding plants ; but on further 
 considering the matter, the idea soon suggested itself that the other class of 
 plants, the leaves of which got wetted with rain, might also exude moisture, as 
 it was evident that if they did exude the discharge would be masked, for the 
 moisture would not collect on them in drops, but spread itself over the leaves, 
 and so become undistinguishable from dew. It therefore seemed desirable that 
 other experiments be made on this class of plants, to see if any of them exuded 
 moisture. It was evident that special precautions would be necessary to enable 
 us to see the exuded moisture on leaves easily wetted, as it would not be so 
 easily seen as the sparkling drop on water-repelling leaves. 
 
 For investigating this point, the most convenient plant I could find was a 
 strong growing variety of everlasting flower (Helichrysum). This plant was 
 one of those observed to have its leaves wet while it was raining, and no e:^uded 
 drops were observed on it at night. The first thing determined was to see if 
 there was any root pressure to cause exudation. The plant was cut across 
 at the bottom of the stem, and the pressure gauge attached near the root. The 
 root pressure was found to be 950 mm.; that is, this plant had as great an 
 internal pressure as was found in the drop-exuding plants. In order to see 
 whether it exuded when hydrostatic pressure was applied, the upper part of 
 the plant, which was cut ofi" for taking the root pressure, was removed to the 
 
MR JOHN AITKEK ON DEW. 63 
 
 laboratory, where it was connected by means of an india-rubber tube with a 
 head of water of about 1'5 metres, and surrounded with saturated air. After a 
 time drops appeared at the tips of most of the leaves, and also at some other 
 points on them ; but these drops were quite unlike those on grass, broccoli, and 
 other water-repelling plants ; they spread themselves on the leaves, and adhered 
 to them, no reflection being given from the back of the flattened drop. It 
 could, however, be easily seen, when the experiment was made in this way, that 
 moisture is exuded from the plant, whereas at night no exuded moisture is 
 perceptible. The reason for this is, that under the condition of the experiment, 
 the exuded drop only spreads to a certain extent, and the outline of the wetted 
 surface is defined, because the whole surface of the leaf is not wet ; but at night 
 the surface of the leaf is wet with dew, and the exuded drop spreads and 
 thins away by imperceptible degrees into the dewed surface. This was 
 illustrated in the above experiment by breathing on the leaf, so as to bring it 
 into the same condition it is on dewy nights, the drop was then seen to spread 
 rapidly outwards. 
 
 We see from the above that a plant may be exuding, and yet we may not be 
 able to notice it. This is specially the case while dew is forming, that is under 
 natural conditions ; for dew is very generally forming while plants are exuding, 
 and it is difficult to tell from an examination made at night whether any plant 
 whose leaves have an affinity for water is exuding or not. It is therefore much 
 better to test the plants under artificial conditions, by placing them in saturated 
 air, but where no dew can be formed on their surfaces. This can be done by 
 placing them at night under hand-glasses, and well protected from radiation, or 
 even during the day under metal boxes, and well shaded. In this way a few- 
 plants, whose leaves got wet with rain, were tested, and all were found to exude 
 if the evaporation from the leaves was stopped long enough, and time given for 
 the tissues to get filled with sap. In all cases the exuded moisture adhered to 
 the leaf and formed a wet patch. The plants tested were helichrysum, stocks, 
 asters, mignonette, foxglove, celery, lettuce, turnips. 
 
 The plants were taken at hazard, and while some, such as mignonette and 
 stocks, exuded little, the others discharged a good deal. The root pressure of a 
 stock was measured, and found to be only about one-half that of the more freely 
 exuding Helichrysum. The root pressure will, however, be only one factor in 
 determining the amount exuded, as it is evident the rate of supply sent in by 
 the root will be of as much importance ; but no measurements of quantity have 
 been made by me. It may be as well to note here, that though the few plants, 
 taken at hazard, all showed powers of exuding, yet we must not therefore 
 conclude that all plants have this property. 
 
 It is interesting to note the effects of these two ways in Avhich the surface 
 of leaves behave towards their exuded sap and water. Take the diff"erent kinds 
 
64 ME JOHN AITKEN ON DEW. 
 
 of tvirnips, for instance. The Swedish variety exudes freely, the liquid forming 
 little drops fringing the leaves, while the moisture exuded by the other varieties 
 sjDreads itself over the leaves. One result of this is, that after dewy nights the 
 softer varieties dry sooner than the Swedish, because the exuded moisture, by 
 spreading itself over the surface of the leaves, dries up much more quickly 
 than the drops on the others. This seems to be the explanation of a fact fre- 
 quently observed by sportsmen and others who have occasion to walk through 
 turnip fields on autumn mornings, namely, that the softer varieties generally 
 wet them much less than the swedes. Again, after rain the swedes take longer 
 to dry than the others, because their surfaces do not get wet, but the water col- 
 lects in drops, imperfectly attached to them, and also fills the hollows of their 
 leaves ; whereas the other kinds get wet, and the water runs off them, leaving 
 only a thin film on their surface, which dries up much more quickly than the 
 drops on the others. Further, when we walk through turnips immediately after 
 rain, our feet brush the drops from the swedes in showers, which rapidly wet 
 us, while the water adheres to and does not so easily leave the surfaces of the 
 others. 
 
 This last part of the investigation takes us a step further, and shows us that 
 not 'only is the dew-drop a result of the vitality of those plants on which it 
 forms, but that much of the wetness spread over the leaves of others on dewy 
 nights is produced by moisture exuded by the plants. 
 
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