from Terrestrial Sources. 5$ 



and is never rendered sensible but when the light itself undergoes 

 some change in its state. It disappears when light is formed or 

 given off from bodies ; and re-appears when light is absorbed, or 

 enters into combination. The principal part, at least, of the phe- 

 nomena cannot be described as belonging to what we might call 

 the temperature of light ; for then a black and a white surface 

 would be equally heated by its impact upon them. 



21. The relations of light and heat have justly been considered 

 as involved in much obscurity ; but I cannot help thinking that the 

 difficulties of the subject have been regarded as greater than they 

 really are. 



Of the nature of these two grand agents in physical phenomena 

 we are completely ignorant, that is, we have not been able to 

 detect any certain particulars in which they can be reduced by 

 analogy to their place in the arrangement of bodies : from this 

 cause it seems to have been tacitly taken for granted that since 

 their wa^wre is probably altogether sm' ^rcwem, so their mode of 

 action upon, or connexion with each other must be also equally 

 beyond the dominion of ordinary natural laws. 



To this inference, however, I think we are far fi'om being neces- 

 sarily led. With the powers of light as a chemical or physical 

 agent upon the bodies, we are but little acquainted ; all that we 

 know may be reduced to a few insulated facts. On the other 

 hand, with respect to heat, the case is widely different ; not only 

 do we recognise its action more or less in almost every pheno- 

 menon which nature presents, but nearly all the instances of its 

 action have been reduced to general laws, and explained on regular 

 theories. 



Hence in attempting to investigate the relations subsisting be- 

 tween these two agents, it appears to me the course most likely to 

 afford satisfactory information, that we should in the first instance 

 take tliat of the two with whose eflPects we are best acquainted, 

 and observing the laws of its connexion with ordinary matter, 

 inquire whether any of those laws will apply to its connexion with 

 light: this would be the course which on sound principles we 

 ought surely to prefer before that of framing new suppositions to 



