RADIANT HEAT. 375 



red heat, the flame of alcohol, and a metallic cylinder heated by W i 

 placed over the flame of a lamp, as before: 



(i.) The reflections were first made from that series of substances 

 which had displayed the gn in the former instance. 



The results are given in a similar tabular form. — (Tables XXXIII to 

 XXXVIII.) 



The author's general conclusion is, "that the modifications which 

 heat experiences on reflection are very considerable in the case of the 

 heat emanating from an Argand lamp: that with the heat of red-hot 

 platinum they diminish; with the heat of the flame of alcohol they are 

 still less; and in the case of the heat emitted by a heated iron cylin- 

 der, of whatever temperature it may be, between 70° and about 234° 

 Fahrenheit, they absolutely vanish.''' — (407.) 



Or more generally, "the changes undergone by heat on diffuse re- 

 flection are occasioned both by the nature of the sources of heat and 

 the properties of the reflecting body." — (408.) 



(ii.) It remained, as the author expresses it, to determine "whether 

 those surfaces which exert a similar influence on the rays of the Argand 

 Lamp, I. (..which the}^ reflect in such a manner that the heat reflected 

 by the one is transmitted by the diathermanous media used for test- 

 ing in the same proportion as that reflecte'cl by the others, would also 

 reflect the heat from the other sources, so that' the rays reflected by 

 them would pass through these substances in the same manner." — 

 (409.) 



The results of these experiments are given in detail. — (Tables 

 XXXIX to XLIV.) 



The question then arose with regard to the explanation of these phe- 

 nomena: Are they owing to any change undergone by the rays in per- 

 meating the diathermanous substances, or were they "the consequences 

 of a selective absorption of the reflecting surfaces for certain rays of heat 

 transmitted to them, as appeared the most probable view from the 

 experiments of Baden Powell and Melloni?" — (415.) 



This question the author proceeds to examine by a detailed com- 

 parison of the foregoing results, exhibited in new tabular arrange- 

 ments of every case. — (Tables XLV to LI.) 



Upon a minute discussion of all these results, the author decide sin 

 favor of the second alternative; or "that the changes experienced by 

 heat on diffuse reflection are merely the result of a selective absorp- 

 tion of the reflecting surfaces for certain rays of heat transmitted to 

 them."— (423.) 



The author also adverts to some other inferences from these experi- 

 ments; as that, " excepting charcoal and the metals, it cannot be said 

 that any body reflects heat better or worse than any other, because 

 this relation varies with each kind of radiation." 



Again: certain bodies of the same color reflect different kinds of 

 heat, and others of different color the same kind; and this is con- 

 nected with the fact that "every luminiferous source of heat emits a 

 large number of invisible rays, which are susceptible of reflection and 

 affect the thermal pile."— (424.) 



These, and some other remarks on the dissimilarity in the diffuse 



