1876.] Repulsion resulting from Radiation. 281 



glass, and water screens, so as to cut off the whole of the ultra-red or 

 dark-heat rajs. The ray of light which was thus freed from dark heat 

 was allowed to fall on the pith surface of the torsion-apparatus, when it 

 produced a deflection of 105°. On interposing a solution of iodine in 

 disulphide of carbon the deflection fell to 2°, showing that the pre- 

 vious action was almost entirely due to light. With a candle tried 

 under the same circumstances, the light filtered from dark heat produced 

 a deflection of 37°, which was reduced to 5° by interposing the opaque 

 solution of iodine. 



In order to obtain comparative results among disks of pith coated 

 with lampblack and with other substances, a torsion-apparatus was con- 

 structed in which two or more disks could be exposed one after the 

 other to a standard light. One disk always being lampblacked pith, the 

 other disks could be changed so as to get comparisons of action. If 

 the action of radiation from a candle on the lampblacked disk be taken 

 as 100, the following are the proportions obtained : — 



On Lampblacked pith 100 



Iodide of palladium ] 87*3 



Precipitated silver 56 



Amorphous phosphorus 40 



Sulphate of baryta 37 



Milk of sulphur 31 



Red oxide of iron 28 



Scarlet iodide of mercury and copper 22 



Lampblacked silver 18 



White pith 18 



Carbonate of lead 13 



Rock-salt 6-5 



Glass 6-5 



In consequence of some experiments tried by Profs. Tait and Dewar, 

 and published in ' Nature,' July 15, 1875, the author fitted up a very 

 sensitive apparatus for the purpose of carefully examining the action of 

 radiation on alum, rock-salt, and glass. The source of radiation was a 

 candle. Perfectly transparent and highly polished plates of the same 

 size were used, and the deflection was made evident by an index ray of 

 light. Taking the action on the alum at 100, that on the rock-salt in 

 five successive experiments was 81, 77*3, 71, 62*5, 60*4. This increasing 

 action on the alum was found to be caused by efflorescence, which took 

 place rapidly in the vacuum, and rendered the crystal partially opaque. 

 A fresh alum plate being taken, this and the rock-salt were coated with 

 lampblack and replaced in the apparatus, the black side away from the 

 source of radiation, so that the radiation would pass through the crystal 

 before reaching the lampblack. The action of radiation was in the 

 proportion of blacked alum 100 to blacked rock-salt 73. 



vol. xxtv. v 



