240 MR JOHN AITKEN ON THE 



surface the cold thickened this haziness into a dense cloudiness, which extended 

 for some distance from the plate and showered down from it. But between 

 the fog and the test surface there was a well-marked dark space. To what was 

 this due ? I had already satisfied myself that cold did not tend to drive away 

 the particles. Then why did these particles conduct themselves differently 

 from the others % While the test surface was vertical, the motion of the 

 particles was too quick to be followed with the magnifying glass. The surface 

 was, therefore, placed nearly horizontal, with a slight slope towards the light 

 (see fig. 4). Still the dark space remained, and the current flowed on, but the 

 particles did not come close up to the plate, though gravitation was acting on 

 them. The cold could surely not be repelling the particles and keeping them 

 off the plate. A short examination with the strong magnifying glass, which it 

 was now possible to use as the particles were moving slowly enough, showed 

 that this was not the case. The particles were seen flowing along in the 

 current, but at the same time they were seen falling into the dark space and 

 disappearing when they came within a certain distance of the surface. The 

 explanation was evident. The surface, by its very low temperature, had robbed 

 the air close to it of its moisture, which it deposited on itself in ice crystals. 

 Into this cold but drier air the particles evaporated as they fell, and in this 

 case the dark plane would contain the dust of the atmosphere, which, however, 

 is black, compared with the brilliancy of the surrounding fog. In this case the 

 dark plane was produced by 



Evaporation, 



and this explains why it is not visible when artificial dusts are present, the 

 larger particles of the artificial dusts not being sufficiently reduced by evaporation 

 to make them comparatively invisible. We shall now pass on to consider the 



Effects of Heat. 



For this purpose let us remove the flat test surface from the smoke-box, 

 and put in its place a round tube of metal or glass. A glass one is preferable, 

 as it permits the illuminating beam to pass through it, and we are thus enabled 

 to see what is taking place all round. After the box is filled with dust, leave it 

 for some time, till the tube has acquired the same temperature as the air. On 

 examination we shall find, as before, the particles evenly distributed, and coming 

 close up to the surface of the tube on the top and at the sides, while underneath 

 we shall see the clear space produced by gravitation. We shall first examine 

 what the effect is of a slight difference of temperature. For this purposi 

 we shall pour some slightly heated water through the tube, so as to raise its 

 temperature a very little — a degree or two. When this is done the equilibrium 

 is destroyed, and currents begin to form. The clear space formed by gravita- 

 tion under the tube rises up, closely clasping and encircling the tube in 



