^2 Professor Powell an the Influence of Colour on Heat. 



mate relations between differently coloured botiies in their ab- 

 sorptive and radiating powers; yet it will be impossible to 

 distinguish whether such relations are dependent on colour^ as 

 suchy or only on certain states of the body which are the conco- 

 mitants of a particular colour. 



In some cases such a distinction is readily made, even with 

 the degree of knowledge we already possess on the subject. For 

 example, coatings of lamp-black, or of the smoke of a candle, 

 have been used by all experimenters as highly efficacious ; but 

 in the case of simple heat it is more than questionable whether 

 it is the blackness which is the cause of the increased energy. 

 Where light is concerned, we have evidence of another kind that 

 it is so. But after the distinctions which, I conceive, I have 

 established (Phil. Trans. 1825) in the case of terrestrial heat, 

 we must recognise the highly absorptive texture as acting an 

 important part, and in the instance of simple radiant heat, with- 

 out light, the wliole increased efficacy is due to this property. 



Perhaps the most valuable information yet obtained on this 

 point is the conclusion of MM. Nobili and Melloni, when qua- 

 lified by the considerations which I have ventured to suggest 

 (Report, p. 9>QQ), that the radiating powers are inversely as the 

 condticting. If this remarkable conclusion be admitted as_suf- 

 ficiently established, it will go far to explain many effects appa- 

 rently connected with colour. In particular, all those very nu- 

 merous cases in which carbon enters as an ingredient into black 

 or dark coloured pigments. This being one of the worst known 

 conductors, will, by the above law, be one of the best radiators; 

 and thus, lamp-black, soot, &c. &c. radiate and absorb simple 

 heat with great energy, not as being black, but as being carbon. 



I cannot help observing, that, throughout this paper, its able 

 and ingenious author does not appear sufficiently to bear in mind 

 the distinctions just referred to between the different species of 

 heating eff^ects, if I may so term them ; or, more properly, the 

 different modes or channels by which, as it were, the same com- 

 mon effect, estimated by us in producing the same sensation of 

 heal, is conveyed. All these have been confounded together 

 under the one term Radiation, or radiant caloric, and, as I con- 

 ceive, no small confusion of ideas has sometimes resulted. In my 

 report on radiant heat, I took particular pains to place these dis- 



