1825.] Mr. Powell on Terrestrial Light and Heat. 369 



certain stage of combustion continue at the degree of ignition 

 which belongs to that point without further increase, that degree 

 being different for different combustibles. 



It appears probable, if we extend the analogy from what we 

 already know, that the general law is, that in proportion to the 

 completeness of the combustion, more light and proportionally 

 less heat are radiated ; and it seems natural to suppose that a 

 greater energy of action would rather cause the heat to be em- 

 ployed in evolving light, than simply to radiate away. 



In the sun's rays it has been shown that the light produces 

 the whole heating effect ; hence if the origin of the solar rays 

 be from any process similar to combustion, it must be analogous 

 to the most perfect kind of combustion. 



Mr. Brande has shown (Phil. Trans. 1820, Part I.) that the 

 galvanic light approaches more in its chemical properties to the 

 nature of the solar light than that from any other source. 



(18.) If any doubt should remain as to the actual disappear- 

 ance of a portion of heat, let us only advert to the instances 

 afforded in the above experiments. In increased intensify of 

 combustion, a proportional increase of heat must be generated ; 

 but from the more intense combustion a greater increase of heat- 

 ing power is communicated to the light than is exhibited in the 

 radiant heat. This increase, therefore, consists of heat derived 

 from the hot body no longer forming heat of temperature, and 

 no longer radiating as heat ; but combined in a peculiar way 

 with light. 



Again, solid particles volatilized in a flame acquire tempera- 

 ture from it ; but they hence give out much more heating light, 

 but not as much more radiant heat. 



On the uniting of different flames, the same thing is most 

 palpably shown. The two flames united give out less than the 

 sum of their separate heats, and more than the sum of their 

 separate heating powers of light ; the latter must be increased 

 at the expense of the former. The heat disappears either as 

 temperature or as radiant heat. 



From the experiments on incandescent metal, we might 

 deduce exactly the same conclusion. 



(19.) That the extrication of light is in most cases owing in 

 some way to the agency of heat has been long an established 

 opinion. Thus Mr. Morgan (Phil. Trans. 1785, No. 11) consi- 

 ders light as a substance united to other bodies by peculiar 

 attraction, and separated by having that attraction overcome by 

 heat. Blue rays he conceives to have the least, and red the 

 greatest affinity, and consequently the former are first separated 

 by the heat, and it is not till the last stage of combustion that 

 red light is given oft". He considers the increased generation 

 of heat to produce a corresponding increase in the evolution of 

 light. 



New Series, vol, ix. 2 b 



