from Terrestrial Sources. 49 



placed no reliance upon them alone ; and only mention them as ad- 

 ditional variations, possessing considerable simplicity in their prin- 

 ciple. 



3. Having, as I conceive, in the investigation alluded to, 

 established the distinct existence and joint operation of simple 

 radiant heat, and the other heating agent, which for distinction I 

 call the heating power of light, in the emanation from luminous 

 hot bodies, it may not be uninteresting to examine the application 

 of this doctrine to the results of former experimenters. In 

 general it will be sufficiently obvious that the distinction thus 

 established must apply to many well-known facts. Tlius Mr. 

 Brande in his paper on combustion, ^c. (^Phil. Trans. 1820, Part 1, 

 Sect. 2,) states, that he obtained a considerable heating power 

 on a blackened thermometer from the light of a flame of olefiant 

 gas, collected by a lens, which did not become heated. This was 

 evidently the effect of that portion of the radiant matter which 

 is simply light possessing a power of communicating heat when 

 absorbed : whilst all the other portion, namely the simple radiant 

 heat was stopped by the glass ; and was tending, if the experiment 

 had been continued, to heat it. 



I need not proceed to state the application of the same mode 

 of explanation to various other results, as it must immediately 

 occur, but there are several particulars respecting the ratio 

 obtaining between the two parts of the total effect radiated from 

 different sources, which it appeared to me very desirable to 

 examine, in reference to the phenomena attending the evolution 

 of radiant matter. 



Upon the principle laid down, we may view the important 

 results of M. De La Roche ♦ from these : if we estimate the light 

 by the effect through the glass screen, and the heat, by the total 

 effect minus this last, we may obtain the following values for the 



ratio — in the different cases : 

 k 



Iron at 427° centig. — = — nearly 

 h 7. 



* Biot. Traiti de Physique, Vol. IV. p. 640, 

 Vol. XIX. E 



