RADIANT HEAT. 357 



complete polarization by reflection is very nearly the same for light 

 and for heat. 



(5.) If any diathermanous substance be interposed between the 

 luminous source and the piles, the index of polarization does not vary 

 with the substance employed. 



This, he contends, proves that the nature of the heat docs not alter 

 its polarizability. 



But also from direct experiment with the radiations from different 

 sources he makes the same inference, employing, instead of a lamp, 

 incandescent platina, metal heated to 400°, or boiling water, with the 

 same results of uniform polarizability. 



He maintains that the difference of polarizability by refraction, 

 arising from the different refrangibility of the rays of heat, is too mi- 

 nute to be sensible. 



And for all experiments on obscure heat he proposes to substitute 

 as the source a black glass heated by flame. 



(G.) On the depolarization he refers to Forbes' s experiments, in 

 which he contends the difference in the rectangular position is very 

 small, but nearly equal with different sources. 



He repeats the experiment, with black glass interposed, and finds 

 the effects much greater, and nearly equal in the different cases. 



He endeavors to explain Forbes' s result of the difference with dif- 

 ferent sources, by secondary radiation. 



Further, by the same method, (of interposing a black glass,) he 

 finds the equal depolarkabiliiy of every kind of heat. 



In an attempt to pursue the analogy of the tints of depolarized light, 

 he acknowledges a failure, and thence considers the interference of 

 calorific rays as not yet proved. 



Upon these investigations the following remarks may be offered: 



Under the first head, it should be recollected that Professor Forbes's 

 first memoir was avowedly only directed to ascertain general facts, 

 not numerical values; while, with regard to the more precise results 

 of the second memoir, it would appear from the details there given 

 that the secondary radiation could not affect the results. The screen 

 between the source and the piles was removed only during the few 

 seconds required for observing the first or impulsive arc of vibration, 

 the time of which was wholly insufficient for the conduction of heat; 

 besides, such an effect was disproved by direct experiment, as men- 

 tioned above. — (p. 352.) 



Of the second point we shall presently have to notice a complete 

 investigation by Professor Forbes. 



As to the third, with this construction, the heat absorbed by the 

 mica was' very trifling; but by the more improved process since used 

 by Professor Forbes, (p. 355,) we have seen this source of error is 

 wholly got rid of. The employment of a pencil of parallel rays does 

 not seem, upon consideration, materially to increase the intensity. 



The fourth point is no more than what has been already established 

 by Professor Forbes. 



With respect to the fifth head, (including the most important part 

 of these researches,) it must be observed, that the differences in the 



