276 Mr. W. Crookes on [Feb. 10, 



February 10, 1876. 



Dr. J. DALTON HOOKER, C.B., President, in the Chair. 



The Eight Hon. Lord Aberdare and the Bight. Hon. George Sclater- 

 Booth were admitted into the Society. 



The Eight Hon. Benjamin Disraeli, whose Certificate had been sus- 

 pended, as required by the Statutes, was balloted for and elected a Fellow 

 of the Society. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



I. "On Repulsion resulting from Radiation.-" — Part III. By 

 William Crookes, F.R.S. &c. Received January 5, 1876. 



(Abstract.) 



This paper contains an account of experiments on the action of radia- 

 tion on bodies the surfaces of which have their radiating and absorbing 

 powers modified by various coatings. The difference between a white 

 and a lampblacked surface in this respect was at first not very decided ; 

 and experiments have been instituted with the object of clearing up 

 some anomalies observed in the actions. Two pith disks, one white and 

 the other black, were suspended on a light arm in a glass bulb by means 

 of a fine silk fibre ; after perfect exhaustion the white and black disks 

 were found to be equally repelled by heat of low intensity, such as from 

 the fingers, warm water, &c. A copper ball was then tried at gradually 

 increasing temperatures. Up to 250° C. it repelled both equally, above 

 that the black was more repelled than the white, and at a full red heat the 

 repulsion of the black disk was very energetic. A lighted candle acts 

 with more energy than the red-hot copper. 



The presence of even a small quantity of aqueous vapour in the 

 exhausted apparatus almost, if not quite, neutralizes the more energetic 

 action which luminous rays appear to exert on a blackened surface. 



After describing several different modifications and some new forms 

 of apparatus devised to facilitate experiment, the author gives a drawing 

 of an instrument which enables him to get quantitative measurements of 

 the amount of incident light falling on it. It consists of a fiat bar of 

 pith, half black and half white, suspended horizontally in a bulb by 

 means of a long silk fibre. A small magnet and reflecting-mirror are 

 fastened to the pith, and a controlling magnet is fastened outside so 

 that it can slide up and down the tube, and thus increase or diminish 



