1825.] Mr. Poioell on Solar Light and Heat. 205 



From these and many more examples which might be adduced, 

 it is evident that heat accompanying solar light must be com- 

 pletely altered in its properties by the connexion subsisting 

 between them. 



(66.) If we had any experimental proof of the materiality of 

 light, and should observe heating effects accompanying it, we 

 should not hesitate to say that they were nothing more than an 

 ordinary effect of a combination of heat with the material sub- 

 stance in question. But in the absence of such proof can we be 

 permitted thus to describe the phenomenon? Did the question 

 involve no other difficulty than this, I should reply that as we 

 can define matter by no other tests than its observed properties, 

 it would be the proper course for the experimentalist to deduce 

 the nature of light from its observed properties, and not to 

 describe those properties merely in conformity with its supposed 

 nature. And observing real effects of ordinary heat, and finding 

 them coextensive with the luminous beam, I do not see any real 

 difficulty on this ground which should hinder us from describing 

 the phenomenon as a^ombination of heat with the luminous 

 particles. 



It may be objected that to attribute such an union with heat 

 to light is to assume the materiality of hght, and thus to adopt 

 gratuitous suppositions. 



It is never objected, however, that we make hypothetical 

 assumptions when we talk of ordinary matter possessing a 

 sensible temperature or latent heat, &c. and yet what assumption 

 do we make in the case of light which is not made here '! 



We conceive it allowable to say that ordinary matter is com- 

 bined with heat, yet if we come to consider the matter accu- 

 rately, it is only that we perceive a certain degree of solidity, 

 extension, &c. united with a certain figure, and at the same time 

 we find the sensation or effects of heat jjroduced coextensively 

 with those other properties cognizable by our other senses. 

 Why then is it not allowable in the instance of light where we 

 perceive a certain colour, extension, direction, &c. and heating 

 effects concomitant and coextensive with the display of those 

 properties, to say that light has heat in a similar sort of union 

 witli it ? 



(67.) In the preceding parts of these remarks, various proofs 

 have appeared of the close connexion subsisting between the 

 luminous rays and the heating effects accompanying them, and 

 of the exact proportion followed so long as the light is of the 

 same colour, and derived from the same source^ 



If then we can show by experiment that heating powers 

 belong to light ; if these effects accompany light in a manner 

 and degree strictly analogous to a given class of those pheno- 

 mena which arise from what we call an union of heat with ordi- 

 nary matter ; why should we not be permitted to describe the 

 facts by expressions framed upon such analogy? 



