150 



Anniversary Meeting. 



[Nov. 30, 



screen, and therefore was transmitted by it in greater proportion than the 

 heat which radiated from the front portion. 



Similar conclusions were obtained from experiments on glass and mica, 

 though the numerical results were not so striking, in consequence of the 

 comparatively great opacity of those substances for obscure radiant heat. 



It thus appeared, 1st, that the heat radiated by a body is not confined 

 to that which comes from the immediate neighbourhood of the surface, 

 but emanates from various, in the case of rock-salt considerable, depths ; 

 2ndly, that there is a relation between the quality of the heat radiated 

 and that absorbed by any given element of a body, and consequently by a 

 sufficiently thin plate of a body, of such a nature that the kind of heat 

 most freely radiated is also most freely absorbed. 



These results and others were comprehended by Mr. Stewart in a 

 definite theory, by means of his extension of Prevost's theory of ex- 

 changes. According to this extension, the stream of radiant heat within 

 a uniformly heated enclosure is the same throughout in quality as well 

 as quantity ; i. e. the uniformity of radiation exists for each kind of heat in 

 particular of which the total flux is made up. 



Few now can doubt the identity of nature of radiant heat and light ; 

 and, accordingly, the application to light of the extension of Prevost's 

 theory was an obvious step. This step was taken by Mr. Stewart, who 

 verified by experiment that which theory predicted — that a coloured glass 

 when heated, as compared with an opaque body glowing at the same tem- 

 perature, gives out by preference rays of the kind which it absorbs, and 

 consequently tends to glow with a colour complementary to its own. For 

 a similar reason a plate of tourmaline cut parallel to the axis, when 

 heated, and viewed in a direction perpendicular to the axis, is seen to 

 glow with light which is partially polarized in a plane parallel to the axis. 



It is right to mention that, in regard to the extension of Prevost's theory 

 in its application to light, Mr. Stewart was slightly anticipated by Professor 

 Kirchhoff, whose brilliant application of the theory to the lines of the 

 spectrum has attracted general attention, whose researches, however, 

 had hardly, if at all, reached this country when Mr. Stewart's papers 

 were presented. As regards Eadiation, however, without specifying of 

 what kind, the priority in the extension of Prevost's theoiy belongs to 

 Mr. Stewart, whose papers on Heat were published before those of Professor 

 Kirchhoff, to whom, however, they were not known when he published 

 his earlier papers. 



Me. Stewart, 



I have particular pleasure in presenting to you this Medal, because it 

 will testify to you that all that really conduces to the advance of our 

 knowledge meets sooner or later with its due recognition — and because I 

 hope that this tribute to your earlier labours will be especially agreeable to 

 you now that you are engaged in work of high public value, but which 

 must necessarily leave you little leisure for such original researches. 



