Jan. 31, 1884] 



NATURE 



out^ide these zones was another, caused by the evaporation of 

 the writer from the particles. 



'l"heconcIu>ions arrived at from these experiments are ihat the 

 downward dark plane is produced by thi-i separating action of 

 gravitation, in the space under the cnkl body, and that the 

 upward dark plane is produced (i) by the separating action of 

 gravitation, (2) by the repulsion due to heat, (3) by evaporation, 

 and (4^ by disintegration. 



The effect of centrifugal force is considered. It is pointed out 

 that as the air, in its passage over a body .such as a tube, curves 

 as much in one direction as it does in another, therefore any 

 centrifugal elTect produced in the one part will be reversed in 

 the other, .^n experiment is described in which an air current 

 is cau'ed to curve through i8&° in its passage round the 

 edge of a thin plate, and without any curving in the opposite 

 direction, but no decided centrifugal action could be detected. 



The motiins of the dust particles produced by the repuMon of 

 the hot surface su.gested that electricity might play some part in 

 these phenomena. Experiments were made to test this. The 

 hot body was insulated and connected with an electroscope ; but 

 no electrical disturliance was observed, nor could any electrifica- 

 tion be got from the dust and hot air streaming up from the hot 

 wires. The effects of electrification «ere studied by in-ulating 

 and charging the hot surface. The effect was found to be the 

 apposite of the heat effect. If the potenlial is slight, and the 

 temperature high, the heat is able to keep the dust off the sur- 

 face of the b 'dy and the dark plane district, but if the temperature 

 falls, or the potential is increased, a point is reached when the 

 electrical attraction overcomes the heal effect, and the dust 

 particles break in upon and destroy the dark space. 



It was observed that after the dust particles were electrified 

 they tended to deposit themselves on any surface near them, and 

 experiments were made to determine the best conditions for 

 purifying air in this manner. It was found to be best done by 

 cauing as rapid a discharge of electricity as possible, by means 

 (if points, surfaces being placed near them to increase the electri- 

 fication of the dost, and to augment the rate of the currents of 

 air which were driven from the points. The e surfaces became 

 places on which the du-t deposited itself before lo ing its charge. 

 A large flask was found to be rapidly cleared of a cloud of dust 

 liy means of a point — the dust being almost entirely deposited 

 on the inside surface of the flask. If the end of the conductor 

 in ihe flask terminated in a sphere, but little effect was produced. 

 Electricity has also been found capable of depositing the very 

 fine du-t of the atmosphere. The air in a large flask was puri- 

 fied much more quickly by means of the electric discharge than 

 it could have been by means of an air-pump and cotton-wool 

 filter. 



It is shown that a wet and hot surface repels dust more than 

 twice as strongly as a hot dry one. From this it is concluded 

 that the heat and moisture in our lungs exert a protecting influ- 

 ence on the surface of the bronchial tubes and tend to keep the 

 dust in the air which is ebbing and flowing through them from 

 coming into contact with their surfaces. This was illustrated by 

 placing a hot and wet surface in a current of dense smoke, where 

 il remained some time without receiving a speck of soot, while 

 a similar surface, but cold, was blackentd with the smoke. It 

 is pointed out that on account of the irregularities on the surface 

 nf the tubes, and of the more violent movements of the air in 

 the lungs, and on account of curves and | rojecling edges, the 

 jiroteclion in the lungs is not perfect. .Still it is thought that 

 this repelling aclion at these surfaces must have some influence, 

 and it .'■eems possible it may explain some clinatic effecis, as it 

 is evident that the lungs will be much belter protected in such 

 places as Davos Platz, where the air is cold and dry, and the 

 repelling forces at a maximum, than at places like M.ideira, 

 where the air is w-arm and moist and these forces are at a mini- 

 mum. This point can, however, only be determined satisfactorily 

 by anatomical examinations of lungs which have lived under 

 the different conditions. 



In the experiments it was observed that dust not only tended 

 to move away from hot sui faces, but also that it was attracted by 

 cold ones, .and attached itself to them. To study this effect 

 gla^s plates were put in different positions near the hot platinum 

 11 ire. Very beautilul impressions of the dark plane can be 

 obtained by placing a piece of glass vertically and transversely 

 over the hot wire. The hot air in flowing over ihe glass, deposits 

 its dust on the surface of the plate leaving a clear line in the 

 middle, indicating where the dustless air of the dark plane had 

 passed. In this w.ay the dust is trapped on the glass to which it 



adheres with seme firmness, and not only the impressions fut 

 the dark planes themselves may thus be preserved.^ 



Other experiments to study the repulsion and attraction of hot 

 and cold surfaces were made by placing glass plates on both 

 sides of the hot vvire. An interesting result was obtained when 

 the plates were about I mm. apart. U.^ing magnesia powder, 

 the particles could be seen ri-ing in the current, and approaching 

 the hot wire ; they were then observed 10 be violently repelled 

 towards the cold sin face, to which they adhered. If there was 

 sufficient difference of lemperalure, not a single particle of dust 

 was carried by the current past the hot wire. 



A thermic filter is then described. In this filter the air is 

 passed through the space formed between two concentric tubes. 

 One tube is kept cold by a stream of water, and the other heated 

 by means of steam or a flame. This instrument was shown in 

 aclion ; one end of the filter wa5 connected with a glass flask, 

 in which the condition of the air was tested. So long as the 

 difference of temperature was kept up, and the current not too 

 rapid, the air passing through it .-hovved no signs of producing 

 cloudy condensation on the pres-ure being reduced, ^howing 

 that the filter had trapped all, even the invisible dust particles. 



Some experiments on the effect of diffusion on the distribution 

 of dust at the surface of a diaphragm are described. When 

 carbonic acid diffuses into a space, the du-t comes clo>e to the 

 diffusing surface, but if hydrogen is the diffusing gas, a clear 

 space is formed in front of the diaphragm. 



An explanation is then offered of the repulsion of dust by 

 hot surfaces and its attraction by cold one-. It seems possible, 

 Ihat the dust might be I'epelled in Ihe same way as the vanes of 

 a Crookes' radiometer, by a radiation effect. That this was not 

 the true explanation was, however, proved by placing in the 

 dust-box a polished silver flat test-surface, one half of which 

 was coated w ith lamp-black, when it was found that the dark 

 space in front of the lamp-black was not any thicker than that 

 in front of the polished metal. It is thought that the repul-ion 

 i- due to the diffusion of the hot and cold air molecules. The hot 

 .«urface repels becau-e the outward diffusing molecules are hot, 

 and have greater kinetic energy than the inward moving ones ; 

 and as the side of the dust particle next the hot surface is 

 homljarded by a larger number of hot molecules than the other 

 side, it is driven away from the hot surface. The attraction of 

 a cold surface is explained by the less kinetic energy of the 

 outward ihan of the inward diffusing molecules. Some experi- 

 ments are referred to, to show that Ihe rate at which gas mole- 

 cules diffuse indicate that this diftusion el'lect is sufficient to 

 account for the repulsion and allraclion of the dust. 



If the explanation here given is correct, then the dust is 

 repelled in the same w ay as a vane of a radiometer w hen placed 

 in front of a surface fixed inside the radiometer bulb, and hotter 

 than the residual gas, the principal part of the energy pro- 

 ducing Ihe motion being transferred from the hot surface to the 

 repelled surface by the kinetic energy of the molecules, and not 

 by radiation. 



In illutration of the tendency of dust to move from hot and 

 to deposit itself on cold surfaces, the following experiments 

 were made. Two mirrors, ore hot and the other cold, fixed 

 face to face and close to each other, were placed in a vessel 

 filled with a dense cloud of magneia, made by turning mag- 

 nesium wire. After a short time the mirrors were taken out and 

 examined. The hot one was quite clean, while Ihe cold one 

 was white with magne.^ia dust. In another experiment a cold 

 metal rod was dipped into fome hot magnesia powder ; when 

 taken out it had a club-shaped mass of magnesia adhering to its 

 end, while a hot rod attracted none. 



This tendency of dust to leave hot surfaces and attach itself 

 to cold ones explains a number of familiar things, among others 

 it tells us why Ihe w alls and furniture of a stove-heated room 

 are always dirtier than those of a fire-wanned one. In the one 

 case the air is warmer than Ihe surfaces, and in the other the 

 surfaces are warmer than the air. This effect of temperature is 

 even necessary to explain why so much soot collects in a chimney. 

 It explains something of Ihe peculiar liquid-like movements 

 of hot powders, and perhaps something of the spheroidal 

 condition. 



For practical applications, it is suggested that this effect ot 

 temperature might be made available in many chemical works 

 for the condensation of fumes, and that it might also be used 



^ Specimens of these trapped dark planes we 

 some of them made of white powder deposited 

 of charcoal deposited on opal glass. 



shown at ih^ meetings, 

 blackened glass, ethers 



