328 RADIANT HEAT. 



They are equally consistent with the facts, whether the curve of 

 heat terminate abruptly at the extremity of the red space, or continue 

 beyond the visible spectrum. 



Sir D. Brewster has, by particular methods of condensation, suc- 

 ceeded in detecting both heat and light at considerable distances' 

 beyond the maximum of heat with a Hint glass prism, that is, rays 

 undergoing very little refraction. 



He considers it highly probable that the deoxidizing rays will be 

 found to be subject to the same laws of absorption as those of heat and 

 light; the media we commonly use may absorb them copiously, whilst 

 others may be found which may transmit them more abundantly. 



Similarly with the magnetizing rays. And thus we may account 

 for the contradictory results hitherto obtained on this point by sup- 

 posing that some ingredient rendered one prism absorptive of these 

 rays, and another not so. 



6th. Sir D. Brewster extends these views to the analogies between 

 solar and terrestrial heat. 



He considers those rays ©f the solar spectrum just mentioned, which 

 undergo little refraction, to be analogous to those thrown off by bodies 

 slightly heated. The waves of heat are broad and slow in their 

 motion; as the temperature is raised they are thrown off with more 

 velocity, and become smaller and suffer a greater refraction. When 

 the velocity is such as to give them a refraction equal to that of the 

 red rays, then red light is produced; and successively the other colors' 

 are added, till at a very high temperature white light is radiated. 



He proposes to examine what transparent body transmits most heat, 

 and, by converting it into a lens, expects to find a series of foci at 

 different distances, beginning from that of the violet rays to that of 

 those corresponding to rays of very little refrangibility. 



7th. He applies these views as affording an explanation of De la 

 Roche's result before mentioned, viz: that a second screen intercepts 

 a much smaller proportion of the heat after passing a first than the 

 first did of the whole effect. This De la Roche ascribed to something 

 analogous to polarization. 



On the principle just stated the explanation is very simple. The 

 first plate intercepts those rays which it has a tendency to absorb, 

 and transmits the rest; the second, being of the same kind, of course 

 will transmit these with scarcely any further diminution. 



He observes that thick masses of colorless fluid or of glass transmit 

 scarcely any radiant heat in a way analogous to that in which thick 

 masses of colored glass are opaque to all rays of light. 



He conceives that substances may be found which are opaque to light, 

 and yet transparent to heat. These should be carefully sought for, 

 as they would be of great practical value. Red glass, for example, 

 which scarcely transmits any light, or one ray in 2,000, transmits all 

 the invisible rays of Herschel, G92 of the 1,000 red rays, 006 rays 

 out of 1,000 of solar heat, and 630 of "culinary" heat, according to 

 Sir W. Herschel. We may expect, therefore, to find an opaque me- 

 tallic glass, or thin plate of metal, which, though quite opaque for 

 light, may transmit heat copiously. 



