54 



NA TURE 



{May 15, 1884 



air about them, and produce upward currents or a buoying of 

 the athermaru i teles by dilation of the air in con. 



them. 



I'his afford nplete explanation of clow 



flotation of : I ' particles. For some years I have been in 



the habit of wat< hing clouds, and by the use of the above theory 

 have very often b en able to account for forms, dimensii 

 movements which I could not otherwise explain. Some 

 I explained thi above ideas to the Chief Meteorological Officer of 

 State. Signal Service, and received from him sugges- 

 tions which have since afforded me the means of much pleasure 

 in observing the locations and forms and movements of clouds, 

 and although thesi irregular masses are subject to many com- 

 : , .11 r yet observed anything 



tending to weaken this theory of flotation, but have made many 

 Irnndn d of oh i rvations tending to confirm it. 



I trust that it will be taken for granted that I do not wish to 

 attack the hypothesis of Dr. Lodge and Mr. Clark, that heated 

 bodies "bombard" and drive away approaching particles. My 

 object is simplv to show that, as it seems to me, the theory of 

 particles buoyed up by a locally heated fluid, when consid :red ir 

 connection with well-known principles of radiation, &c, is suf- 

 licieiit to account for the phenomenon of the "dust-free coat" 

 described in the article alluded to. 



Referring to the figures on p. 612, an ascending current is 



shown in the neighbourh I of the pipe or rod in Fig. I. The 



theory which I have sketched would indicate that this current 

 had been set up in great measure by the indirect action of the 

 heated tube or n ling air. 



1 should contend thai the dust-free coat may be explained as 

 illow 5 : — 



A given particle which may be assumed to be directly belou 

 the rod is heated by radiation from the rod. It in turn heats 

 and expands the air in contact with it ; the particle with a coat 

 of adherent air becomes lighter than the surrounding atmo- 

 sphere, and the mote, with its jacket of expanded air, ascends 

 towards the rod. As it reaches the point marked "slow 

 moving" in the figure, it begins to find itself in air which has 

 been heal, | contact with the rod, and distributed 



near it by the -mall " circular" currents which always surround 

 a blunt obstacle in a stream of fluid. At the outer limit of the 

 " dust-free coat " the particle or mote i cause it has 



come to a point where the air is so warm that the mote can no 

 longer heal n ■ jackel enough hotter than its surroundings to 

 causebuoyancy. It is arn ted bei tuse it has reached a point 

 where the surrounding medium is as light as its own air-float, 

 much as cork i, arn ted at a surface of water. 



I'he mote witli 11- warm air jacket could ascend through cool 

 and therefore heavy air. but the air warmed by contact with the 

 pipe is too light to float it. 



The dark " tail " above the rod, or tube, i- the upstreaming 

 dust-free air, warmed by the tube, and too light to carry mote-. 

 or in which motes have no; been carried by any current. 



The report of the lecture contains within it-elf some very 

 striking confirmation, of this theory, for example, 1'; I 

 tells us that at a high temperature the dust-free coal i- thicker 

 than at low- ones. 1 In- i- a- cording to the theory of flotation as 

 above set forth, because an approaching mote would sooner meet 

 the increased body of air warmed by contact with the tube to a 

 point sufficient to destroy the buoyancy of the mote and its 

 jacket. Again, hydrogen is a light gas having a very high 

 specific heat ; hence according to this theory the mote would 

 need more heat and more difference of temperature to float than 

 in air, and consequently should nol be able to float u]' to a- n u 

 the rod. Now, Dr. Lodg< tate -hat "in hydrogen it [the 

 dust-free coat] is thicker' than in air." With a surrounding 

 medium of carbonic acid, less heat and less flotation are required 

 for the mote, as the gas is heavier and of lower specific heat, 

 and, quite in accord with the theory, the dust-free coat "is 

 thinner" than in air or hydrogen. 'Again, Dr. Lodge states 

 that the dust-free coat is set up by a "difference of a degree 

 or two," and it would apparently require a much more com- 

 plicated theory than the simple one here advanced to account 

 for this on the bombardment hypothesis, as the action ha- been 

 shown to be — 



(i) Affected by the medium as to thickness of coat. 



(2) Obtainable at different temperatures in the rod. 



(3) Apparently dependent, not on the actual temperature of 

 the rod, but on the differences in temperature between the rod 

 and its surrounding dust- containing fluid. 



The behaviour of cool rods or plates, as slated, is also in 

 accord with this theory. A mote coming within the influence 

 of the plate or rod is cooled by radiation and loses buoyancy in 

 its air jacket. If above the plate, it therefore falls upon it ; if 

 below, it drops away. Dr. Lodge does not explain how a cool 

 plate "bombards" the motes and drives them away from its 

 If clearly explained, the method of experiment 

 developed and now under study by Dr. Lodge and Mr. Clark, 

 and that of Mr. Aitken on the condensation of water about 

 nuclei, will probably be found productive of results of the very 

 highest importance. 



Questions of climate, rainfall, healthfulness of districts, togs, 

 mist-, humidity, &c, can probably be better studied than in 

 any other way by some form of apparatus based upon results 

 obtained by these experiments, if the theory of flotation above 

 set forth is connected with them, as I trust it may be. 



Edw. \V. Serrell, Jun. 



Chabeuil, Drome, France, April 27 



Mr. Serrell is no doubt perfectly correct in his view that the 

 average specific gravity of a warmed and vapour-filled cloud 

 may be often less than that of air. The ascent of the so-called 

 "steam" from a kettle proves this, and he will find the view 

 clearly -tated ill Maxwell's "Heat." p. 280. I did not enter 

 into details in the Dublin lecture, but I was fully convinced of 

 the truth of this statement. 



I lis suppi isition that the dusty air near a hot body gets warmed 

 not by gaseous conduction from the hot body but by intercep- 

 tion of its radiation by the suspended particles. i» not an un- 

 natural one. but it is practically untrue. It is disproved by the 

 fact that the concentrated radiation from the electric light is 

 much less effective in warming dusty (or any other) air, than is 

 the neighbourhood of a warm solid only a few degrees above the 

 atmospheric temperature, 



Mr. Senell's criticism, that we do not clearly explain the 

 down-streaming dark plane from a cool body observed by Lord 

 Rayleigh, is quite legitimate. So far a- I entered into the 

 matter at all, I intended to indicate provisionally a distinction 

 between a cool body and a very cold one — the boundary coming 

 somewhere, say. between ten and thirty degrees below the air, or 

 possibly depending upon actual temperature as well as on dif- 

 ference. I am not prepared to assert that the bombardment of 

 particles towards a cool body begins the instant it is colder than 

 I think it possible that there may be a neutral 

 point below which it begins. 



But Mr. Clark is worldng out this among many other points, 



and I am not sure that his view at present agrees with my hypos 



thesis. He will doubtless make a complete statement when he 



111 account of the quantitative research he is now 



in. 'fill then I prefer to leave the account of cold 



bodies a little vague. 0. J. Lodge 



The Supposed Volcanic Dust Phenomena 

 •fin; reddish circle round the sun, which I suppose must be 

 considered as a kind of very large corona, alluded to by E. 

 Divers of Tokio (Natuke, vol. xxix. p. 283), G. F. Burder 

 (p. 525), and other observers, was invariably visible here, when 

 circumstances favoured, from November 18S3 up to April 3. 

 In the middle of that day, and of the 4th, though circumstances 

 seemed favourable for seeing it, no tinge of red was perceptible ; 

 but it was visible late in the afternoon of the 4th. Since then it 

 ha bei ome more visible again, and from April 21 has been very 

 plain, though not so conspicuous as it was originally. It is red 

 in the middle of the day, and brown towards sunset, the bright 

 space between it and the sun being blue or greenish. 



The semicircle opposite the sun is now far fainter than it 

 was originally, indeed I do not think I should notice it now 

 without looking for it. It is now plainest when the ,1111 is a 

 little above the horizon, which was not formerly the ease, and I 

 have not seen it after sunset lately. This may perhaps be owing 

 to a 1 hange in the height of the volcanic dust, or whatever it is. 



'I'he amount of sediment in the rain strikes me as being very- 

 large. I have at different times in the last few months collected 

 it upon glass and examined it with the microscope : there appear 

 in it a considerable variety of crystals and other transparent 

 Some of the crystals are like those drawn by Mr. 

 Beyerinck (vol. xxix. p. 309). I have usually found a number 

 of irregular transparent pieces, but I cannot say that they have 



