1-38 Mr. B. Powell's experimental Inquirif into the Nature 



periments it apfjears, that a greater effect is produced on a 

 blackened thermometer when a glass screen is interposed, hi 

 proportion as the body under trial approaches nearer its point 

 of Inminositv, or becomes more intensely luminous. (Biot^ 

 Traite de P/it/s. torn. iv. p. 638. Ann. of Philos. O.S. vol. ii. 

 p. 163.) 



Both M. De La Roche and M. Biot (see Biot, iv. 612) 

 seem disposed to view the results obtained by the former, upon 

 the supposition of one simple agent, the priiiciple both of light 

 and heat. This is at first radiated as heat ; at a certain point 

 it begins to assume the form of light, when the interceptive 

 power of glass decreases in proportion to the increase of lu- 

 minosity. 



(3.) As long as the hot body continues below the tempera- 

 ture of luminosity, the partial or total interception of the effect 

 is precisely the same phaenomenon as that described by Pro- 

 fessor Leslie in his experiments on screens, and explicable in 

 the same way. (Phil. Trans. 1816, Part L " On newProperties 

 of Heat," Prop. 40.) And the apparent transmission of a por- 

 tion of the effect must be referred to the same principle, as is 

 clearly shown by Dr. Brewster, who has established, apparently 

 beyond contradiction, the impermeability of glass to simple 

 radiant heat upon quite independent principles. 



(-t.) Above the temperature of luminosity we must have re- 

 course to further considerations. The hypothesis of MM. De 

 La Roche and Biot appears to be nearly the same as that of 

 Professor Leslie. (Inquiry, p. 162.) And it certainly has the 

 merit of simplicity and satisfactory explanation of the phse- 

 nomena. But it is an opinion which has not received direct 

 proof; and it is also obvious that the phsenomena may be 

 explained without it; for we may just as well account lor the 

 facts, by supposing two distinct heating influences, one asso- 

 ciated in some very close way with the rays of light, carried 

 as it were by them through a glass screen without heating it ; 

 the other being merely simple radiant heat, affected by the 

 screen exactly as the radiant heat from a non-luminous body. 



(5.) In order to ascertain which of these suppositions is 

 true, it will not be sufficient to observe the effects produced 

 by the intervention of a screen alone. We must combine this 

 method with an examination of the relations of different sorts 

 of heat to surfaces. These relations have been shown to dif- 

 fer according as the body is luminous, or not : in the one case, 

 the direct heat affects bodies in proportion to the darkness of 

 their colour, without regard to the texture of their surface ; in 

 the other, the magnitude of the effect depends solely on the 

 absorptive textnre, without reference to colour. I use the term 



" absorptive 



