1825 .J Mr. I'owetl (HI Light and Heat. 405 



universally true. Upon these topics I shall probably offer some 

 remarks at a future opportunity. 



(9.) The following remarks of M. Biot in reference to the 

 colour of flames are interesting in connexion with the present 

 inquiry. 



(Biot, Traite de Physique, vol. iv. 617.) " Enfin, puisque, 

 selon les observations de De la Roche, le calorique obscur, 

 emane d'un corps que Ton echauii'e graduellement, approche 

 aussi graduellement des conditions etdesproprietes que possede 

 le calorique luuiineux, on concoit que, lorsque I'emanation com- 

 mence a, devenir visible, elle doit etre d'abord analogue a la 

 partio la moins calorifique du spectre, qui se trouve a. I'extremite 

 violette. Aussi observe-t-on que toutes les flammes, lorsqu'elles 

 commencent a naitre, sent d'abord violettes on bleues, et 

 n'atteignent la blancheur que lorsqu'elles out acquis un plus 

 haut degree d'intensite. 



Note. " Cette progression de teintes a meme lieu pour la 

 lumiere que I'etincelle eleclrique exerce dans I'air. Je m'ensuis 

 assure en tirant ces etincelles a diverses distances, eutre une 

 pointe mousse et une sphere metalliques : disposition qui per- 

 mettait d'obtenir un jet continu, dont on moderait a volonte 

 I'intensite par Teloignement." 



According to the theory I have proposed, v/e may make the 

 following remark upon the points just specified. 



Violet rays have less latent heat than red ; and if two flames be 

 equal in other respects, but one of a red, and the other of a blue 

 colour, the temperature of luminosity for red rays will be less 

 than that for blue rays. Less of the heat will be occupied in 

 converting the matter into blue than into red light. Or again 

 when the chemical action is less, less heat can be afforded for 

 the formation of light, and blue rays will be formed. When 

 more heat is generated by an increased action, yellow, red, &c. 

 rays may result. 



(10.) The incandescence of metals may clearly be regarded as 

 the lowest stage of combustion : a combination with oxygen is 

 evidently going on, which, in the instance of iron at a white heat, 

 becomes very perceptible, from the oxide breaking oft" in scales. 

 Thus taking the whole range of thephfenomenon from a dull red 

 heat up to the most intense combustion in oxygen gas, we may 

 observe the metal giving out light which passes through all the 

 tints successively from the deepest and almost invisible red to 

 nearly perfect whiteness. 



The observation of the process of combustion in some other 

 cases, as in flames, exhibits a different succession of appear- 

 ances. Here at the lowest or most imperfect stage, the rays 

 given off are violet or blue, and these gradually pass with the 

 mcreasing completeness of combustion into whiteness. 



Thus we have two classes of the phaenomenon of combustion, 



