AMERICAN 
JOURNAL OF SCIENCE AND ARTS. 
[THIRD SERIES.] 
Art. XLUL.--Experiments on the nature of the force involved 
m Crookes’ Radiometer ; by Prof. O..N. Roop, of Columbia 
College. 
Ir 1s impossible for a physicist to regard the little mstrument 
devised by Mr. Crookes with other than a feeling of unusual 
interest, based partly on the performance of the apparatus itself, 
and partly on possible applications which immediately suggest 
themselves, ‘The explanation of these curious phenomena 
offered by Ronalds, and afterward more in detail by Stoney,* 
together with the confirmatory experiment of Schuster, led me 
'0 devise two new methods for still farther testing the theory 
thus advanced, and to make at the same time an examination | 
* tte phenomena when the suspended discs were under the 
ordinary atmospheric pressure. . 
he explanation offered by Stoney is based on the mechani- 
ee sory of gases, and reaches the final result that a reaction 
es place between the blackened sides of the movable vanes 
and the glass envelope, so that there is a tendency for them to 
* eg from each other. I first arranged an experiment so that 
"eg at will destroy the possibility of this reaction taking 
Place, aia interfering with the other sg tf Spee 
_ -'ption of the apparatus,—Two dises of thin aluminium 
foil, A and B, fig. Gee same in size, were prepared, and 
ts - 
1876,” Crookes’ Radiometer; G. Johnstone Stoney, Phil. Mag., March and April, 
AM Jour. Scr., Tam SzRres—Vo . XII, No. 72.—Dzc., 1876. 
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