370 Mr. Powell on Terrestrial Light and Heat. [May, 



The experiments of Mr. T. Wedgwood also tend to the same 

 point. (Phil. Trans. 1792, No. 3.) He considers the light pro- 

 duced by attrition to be evolved by means of the heat generated. 



The agency of heat in causing the evolution of light is clearly 

 recognised by Count Rumford, in his experiments on the coales- 

 cing of flames. He conceived the increase of light to be owing 

 to the proximity of the flames "so as to communicate heat." (See 

 Mr. Brande's paper on Combustion, Phil. Trans. 1820, Part I. 

 and Sir H. Davy's Chem. Phil. p. 224.) 



But there are cases in which light is extricated, where it does 

 not appear that any elevation of sensible temperature is neces- 

 sary to its production. Such are the instances of phosphorescent 

 animals, of the light generated during putrefaction, &c. Any 

 theory of the subject ought, therefore, to be sufficient to explain 

 not only how the beat acts in evolving the light in the former 

 cases, but how the same cause can produce the effects as in this 

 latter case when the temperature is not increased. If then any 

 theory should at once embrace these two apparently very oppo- 

 site cases, it would probably be considered a strong argument 

 in favour of it. 



(20.) We have not any precise ideas as to the mode in which 

 the heating eff'ect which takes place whenever light is absorbed, 

 is produced. The theory which asserts that the light is trans- 

 formed into heat is a wholly gratuitous assumption ; it lays a 

 great claim to simplicity tsf principle ; but this is, perhaps, more 

 apparent than real. The simplicity of any hypothesis, considered 

 as an explanation of phenomena, depends not solely on the 

 absence of complicated combinations, but also on its analogy to 

 some well established principles on which other similar classes 

 of phenomena are explained. Thus it is easy to sav, and to 

 conceive, that light on absorption is converted into heat, or 

 exists under a different form ; but this, besides being a mere 

 assumption answers very little purpose ; and scarcely brings us 

 one step nearer to an explanation of the phenomena than we 

 were before ; in other words, it does nut exhibit them in any 

 such point of view as makes them analogous with any other class 

 of phenomena. Not only, however, is this hypothesis wanting 

 in the characteristic just mentioned, but it also appears to me 

 to be fairly chargeable with being positively at variance with all 

 established analogy. 



According to this theory light is a peculiar and extremely 

 subtle species of matter which, in its ordinary state, is distin- 

 guished only by the property of illumination, but on being 

 absorbed by bodies, enters into combination with them, and is 

 changed into a new substance, or continues to exist under the 

 form of another sort of matter, which is heat. 



Thus we have an extremely subtle sort of matter undergoing 

 an immense degree of condensation, and becoming a component 



