184 Mr. Poivell on Terrestrial Light and Heat. [Sevt. 



heat, is liable to an obvious error. The radiant heat would find 

 a quicker passage through the transparent screen, and, there- 

 fore, 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." 

 (P. 107.) 



(5.) That a greater effect should be produced on the thermo- 

 meter beyond the plain, than beyond the coated screen, 

 appeared to me a curious circumstance. And when we consider 

 how very little the thickness of the glass is increased by a coat- 

 ing of China ink, it would not seem likely that this alone could 

 have been sufficient to produce the difference of effect observed. 

 But there is still a further circumstance to be considered. The 

 blackened side of the glass was towards the radiating body, and 

 between it and the thermometer in the focus of the reflector. 

 The coated screen must, therefore, have absorbed more heat, 

 and we should in consequence be prepared to expect that it 

 would radiate more to the reflector, and thus a greater effect be 

 produced on the focal thermometer ; which we know was not 

 the case. This difference of effect was observed by De la Roche 

 not only with luminous, but also with non-luminous hot matter. 



The inference which seems to have been drawn is this : — Even 

 when the hot body was non-luminous, the effect on the thermo- 

 meter, with the plain glass interposed, was greater than with the 

 blackened ; but the transmission of heat by absorption and sub- 

 sequent radiation must have been at least equal in the latter age 

 to that in the former; and upon the established laws of absorp- 

 tion and radiation, it would have been greater. Hence it seems 

 necessarily to follow that the effect through the plain glass must 

 have been, at least in part, a direct transmission of the radiant 

 matter, or influence, unaltered in its nature, and merely subjected 

 to a certain loss of intensity. 



(6;) Before Ave fully assent to the conclusiveness of this 

 reasoning, there is one point necessary to be considered, which 

 appears likely to affect, the inference, and to afford a satisfactory 

 explanation of the apparent difficulty. 



The glass screen would necessarily be more heated at its 

 central part than towards its edges ; and neither would its whole 

 area be exposed to the rays coming from the first mirror, nor the 

 whole of the opposite surface be employed in radiating its 



