466 THEORY OF HEAT. [CHAP. IX. 



in volume, is not the same at all temperatures : but in the problems 

 which we have discussed, these changes cannot sensibly alter the 

 precision of the results. In general, in the study of the grand 

 natural phenomena which depend on the distribution of heat, we 

 rely on regarding the values of the coefficients as constant. It is 

 necessary, first, to consider the consequences of the theory from 

 this point of view. Careful comparison of the results with those 

 of very exact experiments will then shew what corrections must be 

 employed, and to the theoretical researches will be given a further 

 extension, according as the observations become more numerous 

 and more exact. We shall then ascertain what are the causes 

 which modify the movement of heat in the interior of bodies, 

 and the theory will acquire a perfection which it would be im 

 possible to give to it at present. 



Luminous heat, or that which accompanies the rays of light 

 emitted by incandescent bodies, penetrates transparent solids and 

 liquids, and is gradually absorbed within them after traversing an 

 interval of sensible magnitude. It could not therefore be supposed 

 in the examination of these problems, that the direct impressions 

 of heat are conveyed only to an extremely small distance. When 

 this distance has a finite value, the differential equations take a 

 different form ; but this part of the theory would offer no useful 

 applications unless it were based upon experimental knowledge 

 which we have not yet acquired. 



The experiments indicate that, at moderate temperatures, a 

 very feeble portion of the obscure heat enjoys the same property as 

 the luminous heat ; it is very likely that the distance, to which is 

 conveyed the impression of heat which penetrates solids, is not 

 wholly insensible, and that it is only very small : but this occasions 

 no appreciable difference in the results of theory ; or at least the 

 difference has hitherto escaped all observation. 



CAMBRIDGE: PRINTED BY C. J. CLAY, M.A. AT THE UNIVERSITY PRESS. 



