RADIANT HEAT. 371 



the 3d case certain screens gave results less than in case 2, -while an- 

 other and smaller number only gave them greater. — (Table IV.) 



The peculiar, and at first sight apparently anomalous circumstance, 

 that platinum, at a stage intermediate between red and white heat, 

 transmits through certain of the screens employed rather less heat 

 than -when at the lower stage of red heat, may however be explained, 

 if we suppose that the rays given off at this intermediate stage are of 

 such a wave-length as to be subject to a peculiar absorption by these 

 particular screens. He then shows, from the conditions observed, 

 that the effect of secondary radiation was fully guarded against. 



Hence the author draws the conclusion, "that the passage of ra- 

 diant heat through diathermanous bodies is not in immediate con- 

 connexion with the temperature of the source, as was probable from 

 previous experiments; but is alone dependent upon the structure of 

 the diathermanous substance, which is penetrated by certain rays of 

 heat in a greater degree than by others, whether this occurs at a 

 lower or a higher temperature." 



In all this there appears nothing to remark, except, perhaps, the 

 observation that previous experiments might make a contrary result 

 probable, which does not appear to be the case, since (as already ob- 

 served) the temperature of luminosity has been long known to be very 

 different for different substances. It were to be wished, that when 

 the author speaks of "certain rays" being transmitted, he had more 

 distinctly indicated the species to which they belong, but which seem 

 to conform to the classification before noticed. 



Some further experiments were made on sources of different form 

 and size; cubes and cylinders of hot water of several magnitudes, and 

 small and large flames, having successively different screens inter- 

 posed between them and the thermoscope. The results were very 

 uniform for all the sources, proving that the differences in question 

 produced none in the transmission, as indeed might have been ex- 

 pected.— (Table V.) 



Section II is " On the Heating of Bodies by Radiant Heat." 



Here, after observing in general as a well-known fact, that the effect 

 is greatly influenced by the structure of the surface, he observes : 



"More recent experiments by B. Powell and Melloni have shown 

 that one and the same body is not uniformly heated by rays of heat 

 emanating from different sources, which exert the same direct action 

 upon a thermoscope coated with lamp-black." — (p. 205.) 



And he details some experiments, (Table VI,) showing that with a 

 lamp a greater effect is produced on a surface coated with black paper 

 than one with carmine, but with dark heat a less — a result which 

 might indeed have been expected from what was well understood 

 before the date of the researches alluded to, and which it was by no 

 means their object to establish. 



A set of experiments (Table VII) proved that for small thicknesses 

 of coating (within the limit of those employed by Leslie and Melloni) 

 the absorption of heat is proportional to the thickness. 



Also, that "the temperature of a body, when the thickness in- 



