422 



sensible quantity is transmitted. He next placed the two panes with the-ir 

 coated surfaces in contact, the uncovered surfaces being outside. A part of 

 the radiant heat was now transmitted, and the effect on the thermometer was 

 observed to be 18. Thus about one fifth of the radiant heat incident on the 

 screen was transmitted. In fact, nearly as much heat was thus transmitted by 

 the two panes of glass with the tinfoil between them, as would have been 

 transmitted by a pane of uncovered glass. From this result it would appear 

 that the tinfoil loses its power of reflecting heat when the rays of he<xt have 

 previously passed through a medium of glass instead of a medium of air ; and ) 

 that, instead of reflecting them, it transmits them. 



The idea of investigating the effects which different temperatures in a radi- 

 ant body produce on the power of the radiated heat to penetrate screens of dif- 

 ferent substances, does not seem to have suggested itself to Sir John Leslie. 

 Later experiments, instituted by M. de la Roche, prove that the power of cal- 

 orific rays to penetrate bodies increases with the temperature of the radiator. 

 This heat radiating from a surface at a certain temperature, fails to penetrate 

 glass, except in a very limited degree ; but if the radiating body be considera- 

 bly elevated in its temperature, then the rays penetrate the glass in much 

 greater quantities. In fact, the degree of transparency of glass relatively to 

 the rays of heat would seem to depend on the temperature of the radiating 

 body, and to increase with that temperature. 



'l^he results of the preceding experiments, and, indeed, all the phenomena 

 connected with the radiation of heat, are satisfactorily explained by the theory 

 of exchanges, first proposed by Prevost of Geneva. According to this theory, 

 every point at and near the surfaces of bodies is regarded as a centre from 

 which rays of heat diverge in all directions. The surfaces also reflect rays of 

 heat incident upon them, in a greater or less degree, rays of heat striking on a 

 body, arid reflected or radiated by the other bodies around. Thus every body, 

 so far as regards heat, is constantly under the operation of three distinct pro- 

 cesses it radiates, reflects, and absorbs: it follows, from this, that betrveeii 

 bodies which are placed in each other's neighborhood, there must be a coijscant 

 interchange of heat. The heat which is radiated by one body strikes on oth- 

 ers ; part of it is absorbed by them, and is retained within their dimensions, so 

 as to raise their temperature, while another part is reflected, and scrikes on 

 other bodies, where it is subject to like effects. The body which radiates 

 heat in this manner is, at the same time, receiving on its surface rays of heat 

 which proceed from other bodies in its neighborhood ; and these rays of heat 

 are subject to the same effects on its surface as the rays which, proceeding 

 from it, encounter on the surface of other bodies they are partly absorbed and 

 partly reflected. 



If a body raised to a high temperature be placed in the neighborhood of other 

 bodies at a lower temperature, it will radiate a greater quantity of heat than the 

 bodies which surround it ; consequently the heat which it receives from them 

 will be less than the heat which it transmits to them. They will receive more 

 heat than they give, and it will give more heat than it receives ; the temperature, 

 therefore, of the hot body, will gradually fall, while the temperature of the sur- 

 rounding bodies will gradually rise. This will continue until the temperatures 

 of the bodies are equalized. Then the heat radiated by each of ihem will be 

 exactly equal to the heat absorbed, and the temperature will remain stationary. 



It has appeared from the result of direct experiments, that the bodies which 

 are the best radiators are also the best absorbers of heat. This would follow 

 as a necessary consequence of the theory which has been just explained. If 

 a body which is a powerful radiator were at the same time a bad absorber, the 

 consequence would be that it would radiate heat faster than it would absorb it ; 



