RADIANT HEAT. 307 



state of the thermometer before it has been Effected by the secondary 

 radiation from the screen. 



"The method employed by M. De la Roche, of observing the differ- 

 ence of effect when a blackened glass screen and a transparent one 

 were made successively to intercept the radiant heat, is liable to an 

 obvious error. The radiant heat would find a quicker passage through 

 the transparent screen, and therefore the difference of effect was not 

 due to the transmitted heat, but to the heat radiating from the 

 anterior surface. The truth contained in M. De la Roche's fifth 

 proposition is almost a demonstration of the fallacy of all those that 

 precede it. He found that a thick plate of glass, though as much or 

 more permeable to light than a thin glass of worse quality, allowed a, 

 much smaller quantity of radiant heat to pass. If he had employed 

 very thick plates of the purest flint glass, or thick masses of fluid that 

 have the power of transmitting light copiously, he would have found 

 that not a single particle of heat was capable of passing directly 

 through transparent media." 



7.) I have further attempted a direct experimental examination of. 

 the question in a paper inserted in the Phil. Trans., 1826, Part III, 

 p. 372. 



The substance of my observation is as follows : 



De la Roche found that if radiant heat be intercepted by two trans- 

 parent screens, the additional diminution of effect occasioned by the 

 second is proportionally much less than that produced by the first ; 

 and the same conclusion is extended Fo any number of screens. This 

 was explained by the supposition that the heat in its passage through 

 the first glass undergoes a certain modification, in some respects 

 analogous to polarization, by which it is enabled to pass with very 

 little diminution through the second and subsequent glasses. 



In those cases where the source of heat is luminous, such phenomena 

 would receive an obvious explanation on the principle investigated in 

 my other paper. — Vide infra. . 



But if the same effect is still observable below the point of lumi- 

 nosity, we must have recourse to some other principle of explanation 

 That deduced by De la Roche appears at least plausible; and though 

 it should be considered proved that, in general, heat is incapable of 

 being radiated directly through glass, it perhaps would not necessarily 

 follow that it might not, under peculiar circumstances, have a power 

 of doing so communicated to it. Though, on the other hand, it must be 

 confessed that in the present case some difficulty would attend such a 

 supposition. 



It certainly would not be easy to conceive such a property to be 

 communicated to the heat by the mere act of being conducted through 

 the first glass. Again, a new property of heat is thus introduced, 

 which, it must be conceded, is not absolutely and exclusively 

 established. 



It appeared to me, therefore, a point of some interest to examine, in 

 the case of non-luminous heat, in the first place, the accuracy of the 

 fact, and secondly, if verified, whether fhere might not be circum- 

 stances observable in the conditions of the experiment by which it 



