150 Royal Society : — Mr. W. Crookes on 



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 



E-ed oxide of iron 28 



Scarlet iodide of mercury and copper 22 . 



Lampblacked silver 18 



White pith 18 



Carbonate of lead 13 



Kock-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 examin- 

 ing 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 de- 

 flection 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. 



Rock-salt and glass were next tested against each other in vacuo 

 in a torsion-balance. Professors Dewar and Tait say that rock-salt 

 is inactive when the beam from a candle is thrown on it, wmile a 

 glass disk is active. The author has failed to corroborate these 

 results ; he found the mean of several concordant observations to 

 be — rock-salt 39, glass 40. 



The Measurement of the Force. 

 The author describes a torsion-balance in which he is enabled to 

 weigh the force of radiation from a candle, and give it in decimals 

 of a grain. The principle of the instrument is that of W. Ritchie's 

 torsion-balance, described in the Philosophical Transactions for 

 1830. The construction is somewhat complicated, and cannot be 

 well described without reference to the diagrams which accompany 

 the original paper. A light beam, having two square inches of 

 pith at one end, is balanced on a very fine fibre of glass stretched 

 horizontally in a tube, one end of the fibre being connected with a 

 torsion-handle passing through the tube, and indicating angular 



