FORMATION OF SMALL CLEAR SPACES IN DUSTY AIR. 243 



may be placed on a match and introduced into the box after being lighted. 

 When the dust is thick enough, and uniformly distributed through the box, 

 bring the light to a focus on the tube. For the present the tube must be 

 neither heated nor cooled. Using the magnifying glass ; it will in all proba- 

 bility be found that there is a clear space all round the tube or on some part 

 of it, and that the air currents are carrying off the clear space in an irregular 

 manner, or there may be seen rising over the tube a regular dark plane, depend- 

 ing on the relative temperatures of the air and the tube. 



Now remove the beam of light from the dust-box and leave it for some time. 

 If left long enough, and the box kept free from changes of temperature, it 

 will be found that all air currents have ceased, and a close examination of 

 the experimental tube will show that the dust is now in contact with it at the 

 sides and on the top. But if we look on the under side of the tube we shall 

 there see a clear space, like that shown in fig. 1, which represents the tube seen 

 endways." It will be observed that this does not agree with Dr Lodge's 

 observations ; but I think I have taken every precaution, and the conclusion 

 which I have come to is, that bodies have not sharply defined dust-free coats, 

 and that when the bodies and the air have the same temperature, the dust 

 comes into contact with the sides and top of the bodies. t Now what is the 

 cause of this clear space under the tube ? Clearly 



Gravitation, 



which brings me to the first of the causes of the dark plane. When the air 

 comes to rest, the temperature of the air and the tube being the same, there is 

 nothing to keep the dust from coming into contact with the tube. But gravita- 

 tion is at work on the particles, and while the air is still the particles are all 

 falling, and as the upper surface of the tube stops those falling on it, there are 

 no particles to supply the place of those falling from the space under the 

 tube, and the result is that a dustless space is here formed. If now we pour 

 into the tube some cold water we can study the 



Effects of Cold. 



At once a downward current is started, and this downward current carries with 

 it the clear air under the tube ; the two currents of dustless air from the sides 



* In the figs, the white surface represents the light-reflecting dusty air, while the black represents 

 the transparent air, free from reflecting particles. 



t The only reason I can imagine for this difference between Dr Lodge's results and mine is that 

 lie worked with more powerful sources of illumination than I did. He used either the sun's light, an 

 oxyhydrogen lamp, or a Seerin arcdamp, while I only used gas. Now one result of this difference 

 would evidently be that the illuminating beam used by him would bave a much greater heating effect 

 than the one used in my experiments, and would therefore heat the surfaces under examination. I 

 found this effect even with gas. If the body had a small capacity for heat, it was only necessary to 

 keep the light focused on it for a short time to heat it sufficiently to cause a clear space to form over 

 the part where the light acted. 



