and Mechanical Condition on Radiant Heat. 297 



Amorphous phosphorus and sulphide of iron gave the follow- 

 ing results : — 



Radiation. 

 Amorphous phosphorus .... 63*6 



Sulphide of iron 81*7 



Sugar and salt were reduced in a mortar to the state of ex- 

 ceedingly fine powders. In point of cohesion and physical 

 aspect these substances closely resemble each other; their radia- 

 tive powers, however, are as follows : — 



Radiation. 



Salt 35-3 



Sugar 700* 



In his last, interesting paper on emission at a red heatf, M. 

 Desains mentions oxide of zinc as a body which at 100° C. has 

 the same emissive power as lampblack. This is nearly true for 

 the hydrated oxide ; with the calcined oxide the following is the 

 relation : — 



Radiation. 



Lampblack 84*0 



Hydrated oxide of zinc 80*4 



Calcined 53'2 



Two red powders have been already compared together ; I will 

 now compare two black ones. With black platinum and black 

 oxide of iron the following results were obtained : — 



Radiation. 

 Black platinum (electrolytic) . . . 59*0 

 Black oxide of iron 81*3 



The black platinum here employed was obtained by electro- 

 lysis, a sheet of platinum-foil being coated with the substance. 



Let us now compare two white powders. Chloride of silver 

 and carbonate of zinc gave the following results :— 



Radiation. 



Chloride of silver 32- 5 



Carbonate of zinc 77'7 



As in all the other cases, the influence of chemical constitu- 

 tion makes its appearance here. 



When held upon its cube by the sulphur cement, the chloride 

 of silver soon darkens in the diffuse light of the laboratory. It 

 first becomes lavender, and passes through various stages of 



* I have of course no intention of adducing experiments on sugar and 

 salt, or on powdered rock-salt and alum, as opposed to the results of Mas- 

 son and Courtepee. They would regard their results as unaffected by such 

 experiments. It is the deportment of the chemical precipitates employed, 

 and not that of bodies reduced to powder by mechanical means, that invali- 

 dates their conclusions. 

 L f Comptes Rendus, July 3, 1865; Phil. Mag. August 1865. 



