252 MR JOHN AITKEN ON THE 



very near the centre of rotation of the air. On examination, however, it was 

 seen that this was caused by an eddy, due to the upward channel in which 

 the air was confined being wider than the space under the plate. In the eddy 

 so formed the particles were soon sifted out by gravitation, and a clear space 

 formed. On contracting the breadth of the upward channel, and making it 

 equal to the passage under the plate, this eddy disappeared, and the clear space 

 was no longer formed. In this experiment, though the air was caused to curve 

 through a considerable angle, yet there was no satisfactory evidence of any 

 distributing action due to centrifugal force. It seems probable that, even 

 under these conditions, a certain amount of sifting action does take place, 

 though not enough to make it observable ; and though there are reasons 

 for supposing that if the particles were heavy enough, and the velocity of 

 the current great enough, there would be a visible effect, yet it is evident 

 that centrifugal force plays no part in the formation of the dark plane, in the 

 experiments with heat and cold. The fact that the dark plane has a sharply 

 defined boundary is proof that centrifugal force is not the cause of the distri- 

 bution, as this force would not give such a result. Its tendency would be 

 to throw the heaviest particles furthest out, and thus give rise to a shaded 

 outline. 



Effect of Electricity. 



Electricity is another force which might be supposed to play some part in 

 the formation of the dark plane. It was difficult to believe that the attraction 

 of the particles was a thermal effect when making the experiments with the hot 

 and cold surfaces placed opposite each other, and observing the way in which 

 the particles were repelled by the one plate and attracted to the other ; and on 

 making other experiments, which will be presently described, in which the dust 

 rising in the current from the heated platinum wires was attracted to, and 

 deposited itself on, the surfaces of bodies placed in its path. The dust particles 

 conducted themselves in a way strongly suggestive of electrical disturbance. They 

 seemed to be attracted by the cold surfaces in exactly the same way as if they 

 had become electrified at the hot surface. It was, therefore, thought advisable 

 to make experiments to ascertain whether electricity had anything to do with the 

 formation of the dark plane. Experiments were first made to see if the hot sur- 

 face became electrified in the dust-box by the passage of the air over it, or from 

 other causes. For this purpose I used a small cylindrical conductor of solid 

 metal, about 1 cm. in diameter, and with rounded ends. This conductor was 

 fixed to the end of a glass tube, and a conducting wire connected to it and 

 carried through the tube. The conductor was then introduced into the dust- 

 box through an opening in the back, after which its connecting wire was joined 

 to a gold-leaf electroscope. Before the conductor was put in its place it was 



