298 Prof. Tyndall on the Influence of Colour 



brown to black. During these changes, which may be asso- 

 ciated with a chemical reaction between the chloride of silver and 

 the sulphur in which it is imbedded, the radiation steadily aug- 

 ments. 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 sur- 

 faces, and two white surfaces together. The comparison of a 

 black and white surface gave the following result : — 



Radiation. 



Black platinum 590 



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 : — 



Radiation. 



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 : — 



Radiation. 



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 conclu- 

 sion 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. 



Were the radiative power of these substances determined by 

 the state of division, I think it must make itself sensible even in 

 a case where the division is effected by the pestle and mortar ; 

 but I do not find this to be the case. A plate of glass was fixed 

 against the polished surface of a Leslie's cube, and on the plate 

 the powder of glass, rendered as fine as the pestle and mortar 

 could make it, was strewn. It was caused to adhere without 

 cement of any kind. The cube was filled with boiling water and 

 presented to the thermo-electric pile until a permanent deflection 

 was obtained. Permitting the cube to remain in its position, 

 the powder was removed with a cameFs-hair brush. An incon- 

 siderable augmentation of the radiation was the result, the in- 

 crease being such as might be expected to follow from the slight 



