406 Mr. Powell on Light and Heat. [June, 



in which the effects seem to pass through two different orders of 

 chanoes, but both receive their completion at the same point ; 

 and this difference would seem to depend on some peculiar 

 difterence in the nature of the sources from which the light 

 emanates. 



If we attempt to enumerate the various sources of light which 

 comprise the two classes described, the only distinction which, 

 as far as 1 know, we can fix upon, is that of the one class con- 

 sisting of metals and carbon ; the other of sulphur, phosphorus, 

 and hydrogen. 



(11.) To Sir H. Davy we are indebted for the most important 

 acquisitions which have been made to our knowledge Ot the 

 nature of flame. The observation of the different peculiar tem- 

 peratures required in order to produce luminosity in different 

 species of inflammable gas, and the constant maintenance of 

 that temperature, while the emission of light of the same inten- 

 sity is continued, are circumstances clearly indicating the 

 employment of heat in some v\'ay in the production of light. A 

 body of gas must be raised to a particular temperature to enable 

 it to combine with oxygen, and to evolve light and heat. But 

 one of the most curious circumstances connected with this 

 inquiry is the different proportion which is maintained in differ- 

 ent instances between tlie degree of heat required for combus- 

 tion, and that produced by the combustion. (See Sir H. Davy's 

 paper, Phil. Trans. 1817, Part I. pp. 48, 52.) These differences 

 would seem very difficult to explain or account for on any 

 known principles ; but if I rightly apprehend the author's 

 meaning, it would seem by no means an improbable conclusion, 

 that a portion of the heat disappears as heat, and becomes the 

 latent heat of the light : of this inference, however, I only speak 

 doubtfully. 



(12.) I have made these various brief remarks, being fully 

 aware that they give nothing like a complete view of the subject; 

 but I am desirous of laying them before the readers of the 

 Annals, in the hope that some persons possessing the requisite 

 chemical knowledge will be induced to give a more complete 

 examination to that very interesting topic of inquiry, the con- 

 nexion between the colour and heating power of the light, the 

 radiant heat, and the chemical or electro-chemical nature of the 

 process which evolves them, and of the substances from which 

 they are produced. 



