38 



Mr. W. Crookes on 



[Nov. 21, 



was hoped that some of the more obscure phenomena shown by the 

 deep cups with movable screens in front (referred to above) might be 

 intensified if set in action by a hot wire. Several kinds of apparatus 

 and experiments with them are described, but the results are too com- 

 plicated to be given in abstract. One experiment proves that the 

 direction of pressure is not wholly normal to the surface on which, it 

 is generated, but that some of it is tangential. 



The author then describes the turbine radiometer, early specimens 

 of which were exhibited before the Royal Society on April 5, 1876. 

 In the ordinary form of radiometer the number of disks constituting 

 the fly is limited to six or eight, a greater number causing interference 

 one with the other and obstruction of the incident light. In the 

 turbine form of fly there is no such difficulty, the number of vanes 

 may be considerably increased without overcrowding, and with corre- 

 sponding advantage. In the earlier turbine radiometers the flies were 

 made of mica blacked on both sides, and inclined at an angle like the 

 sails of a windmill, instead of being in a vertical plane. This form of 

 instrument is not sensitive to horizontal radiation, but moves readily 

 in one or other direction to a candle held above or below. A vertical 

 light falling on the fly gives the strongest action, but rotation takes 

 place, whatever be the incident angle, provided the light is caught by 

 one surface more than by the other. Ether dropped on the top of the 

 bulb to chill it causes rapid negative rotation. If the turbine radio- 

 meter is floated in a vessel of ice-cold water, and the upper portion 

 exposed to the air of a warm room, it rotates rapidly in the positive 

 direction, acting as a heat engine, and continuing so to act until the 

 rotating fly has equalised the temperature of the upper and lower 

 portions of the bulb. By reversing the circle of operations — by 

 floating the turbine radiometer in hot water and cooling the upper 

 portion of the bulb — the fly instantly rotates in the negative direction. 



After describing experiments in which the same fly was made to 

 rotate first in a large bulb and then in a small one at the same degree 

 of exhaustion, the author proceeds to discuss the influence exerted by 

 the inner side of the glass case of the radiometer as a reacting surface. 

 A flat metal band was put equatorially inside a radiometer, and lamp- 

 blacked, so that the molecular pressure generated under the influence 

 of light should react between the fly and the black band, instead of 

 between the fly and the glass side of the bulb. It was found that the 

 maximum speed with the band present was 40 revolutions a minute, 

 against 8^ revolutions when the band was absent. 



The rotation of the case of a radiometer, the fly being held immovable 

 by magnetism, is next described. A preliminary note on this subject 

 having already appeared in the " Proceedings,"* it need not be again 



* " Proc. Roy. Soc," No. 168, March 30, 1876. 



