230 Professor Powell on the Influence of' Colour on Heat. 



or spread on the surface. But as approximate results, in the 

 absence of any better, they are of the highest value. Now these 

 results extending through a considerable range of substances (as 

 I have remarked in my report) do seem to afford one or two 

 principles of classification as to increased energy of effect ; but 

 colour is evidently not one of them. We have some white sub- 

 stances near the head of the list, and some dark ones very low 

 in it. But to complete our view of the matter, let us only con- 

 trast the differences observable among such coatings in reference 

 to simple Jieat, with those which subsist when we expose them 

 to the heat accompanying, or belonging to, or excited by, light. 

 Here the case is clear, and not capable of misapprehension ; the 

 dark coloured substance (unless the effect be completely dis- 

 guised by its badly absorbing texture, as metallic polish, &c.) 

 absorbs the light, and thus greater heat is excited or developed. 

 I believe the most accurate method we at present possess for 

 making such comparisons, is that which I have adopted, viz. to 

 coat two thermometers with the substances to be examined, and 

 to observe the ratio of their risings when exposed to luminous, 

 and to nonluminous heat, or the comparative effects of the ra- 

 diation from a luminous source upon them, and their compara- 

 tive rapidity of radiating again the heat they have acquired. 

 An extensive series of such comparisons (which I have never yet 

 had leisure to make with the requisite accuracy,) would put us 

 in possession of most important data on a subject of which at 

 present we know hardly any thing. The main difficulty is that 

 of^ determmmg precisely the circumstances and properties wherein 

 the coatings differ. A very extensive induction perhaps, is the 

 only means open to us of ascertaining this, considering how to- 

 tally ignorant we are of the intimate nature and structure of 

 bodies, and above all, of the peculiarities on which their colour 

 depends. 



In fact, this consideration has long appeared to me to present 

 a formidable, perhaps insuperable difficulty, in all experiments of 

 the kind ; and I will merely say further, that so fully have I been 

 myself impressed with its formidable character, that though at 

 one time I had devoted much labour and attention to extensive 

 inquiries of this kind, I felt compelled to relinquish them as 

 hopeless. Yet, as far as my observations have extended, they 



