88 PEOFESSOE TYNDALL ON THE INELUEXCE OF COLOUE 



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 : — 



Eadiation. 



Black Platinum (electrolytic) . . . 59-0 

 Black oxide of Iron 81 '3 



The black platinum here employed was obtained by electrolysis, 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 : — 



Eadiation. 



Chloride of Silver 32-5 



Carbonate of Zinc 77-7 



As in all the other cases the influence of chemical constitution 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 brown to black. During these changes, which may be associated with 

 a chemical reaction between the chloride of silver and the sulphur in which it is im- 

 bedded, the radiation steadily augments. Beginning in one instance with a radiation of 

 25, the chloride ended with a radiation of 60. 



We have thus far compared two red surfaces, two black surfaces, and two white sur- 

 faces together. The comparison of a black and white svirface gave the following result : — 



Radiation. 



Black Platinum 59-0 



White hydrated oxide of Zinc . . . 80*4 



Here the radiation from the white body far transcends that from the black one. 

 Again, comparing black and white, we have the following result : — 



Eadiation. 



Oxide of Cobalt 76-5 



Carbonate of Zinc 77-7 



Here the black radiation is sensibly equal to the white one. 



Finally, comparing black and white, we have the following result : — 



Eadiation. 



Lampblack 84-0 



Chloride of Lead 55 "4 



Here the radiation from the black body far transcends that from the white one. 



We have thus compared red powders with red, black with black, white with white, 

 and black with white ; and the conclusion to be drawn from the experiments is, I think, 

 that chemical constitution, so far from being of vanishing value, is the really potent 

 influence in the experiments. 



