THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1876. 



XXII. On Crookes's Radiometer. By G. Johnstone Stoney, 

 M.A.^F.R.S., Secretary to the Queen's University in Ireland. 



To the Editors of the Philosophical Magazine and Journal. 



Hampstead, February 19, 1876. 

 Gentlemen, 



AS Mr. Crookes's Radiometer is at present deservedly at- 

 tracting a large share of attention, it will probably not 

 be without interest to many of your readers, and may perhaps 

 dispel some illusions, to point out that the excess of pressure 

 which he has observed is within the limits of what may be 

 accounted for by the known properties of the material agents 

 that are present. 



I am, Gentlemen, 



Yours faithfully, 



G. Johnstone Stoney. 



1. The Radiometer, as I have seen it in Mr. Apps's shop, 

 consists of four vanes supported at the ends of slender plati- 

 num arms, and movable in a horizontal circle on a cap rest- 

 ing on a steel point at the centre. I was informed that each 

 vane consists of three folds of thin platinum foil. The faces 

 of the vanes are presented horizontally, and are each about 

 two square centimetres in area. One face of each vane is 

 blackened. The whole is mounted within a small glass cham- 

 ber, some six or seven centimetres across, which I was told 

 had been exhausted by the prolonged action of a Sprengel 

 pump. 



2. When this apparatus is exposed to light falling horizon- 

 Phil. Mag. S. 5. Yol. 1. No. 3. March 1876. 



