and liejjuhion resulting from Radiation. 249 



repel the black surface more energetically than they do the white 

 surface. Taking advantage of this fact, the author has constructed 

 an instrument which he calls a radiometer. This consists of four 

 arms, suspended on a steel point resting on a cup, so that it is 

 capable of revolving horizontally. To the extremity of each arm 

 is fastened a thin disk of pith, lampblacked on one side, the black 

 surfaces facing the same way. The whole is enclosed in a glass 

 globe, which is then exhausted to the highest attainable point and 

 hermetically sealed. 



The author finds that this instrument revolves under the influence 

 of radiation, the rapidity of revolution being m proportion to the 

 intensity of the incident rays. 



Several radiometers, of various constructions as regards details, 

 but all depending on the above-named discovery, were exhibited by 

 the author at the Soiree of the Royal Society on the 7th inst., and nu- 

 merous experiments were shown with them. The following Table, 

 which gives the result of some experiments tried with one of the 

 first-made radiometers (and therefore not so sensitive as more recent 

 instruments), is copied from a card which was distributed during 

 the evening : — 



" Time required for One Eevolution. 



" Source of radiation. Time in seconds. 



" 1 candle, 20 inches off 182 



10 „ .. 45 



5 „ 11 



2 candles, 5 „ 5 



4 „ o „ 3 



8 „ 5 „ . . 1-6 



1 candle, 5 „ behind green glass . . 40 



„ 5 „ „ blue „ . . 38 



5 „ „ purple „ .. 28 



5 „ „ orange „ .. 26 



5 „ „ yellow „ .. 21 



5 „ „ lightred „ . . 20 



Diffused daylight, dull 2-3 



bright 1-7 



Full sunshine, 10 a.m 0*3 



2p.m 0-25" 



These experiments are not mentioned in the paper of which the 

 above is an abstract, as it is intended to make the radiometer the 

 subject of a future communication to the Society. 



