40 HEAT OR CALORIC. 



With him the heating power increased from the violet to the red 

 jay. The greatest heating power was in the red extremity of the 

 spectrum and not beyond it. His maximum of heat was where the 

 thermometer was still covered by the red ray. The fluid in the ther- 

 mometer sunk as it receded from the red ray, and entirely out of the 

 red ray, where Herschel fixed the maximum, its elevation above the 

 air around, was only one fifth of what it had been in the red ray. 



(A.) Red rays are considered as cheerful, because warmth and 

 therefore comfort, is found to be associated with them ; such rays are 

 emitted by burning charcoal and coke, and by a common wood fire ; 

 those from burning alcohol, especially if mixed with salt, are pale, 

 and have very little heat in them, and are therefore regarded as 

 gloomy. 



Mr. Seebeck has proved that the place of the greatest heat de- 

 pends very much upon the nature of the prism : thus, when it is of 

 crown or plate glass, the maximum effect is in the middle of the red 

 if of flint, it is beyond the red ; if a hollow glass prism be filled 

 with water, the greatest effect is in the yellow ; and if with sulphuric 

 acid, it is in the orange ; so that different substances appear to differ 

 in their power of refracting caloric.* Still the important fact is con- 

 firmed, that there are rays of caloric, that they are differently refran- 

 gible from rays of light, and that they possess unequal refractive 

 power. 



8. RAYS OF CALORIC ALONG WITH RAYS OF LIGHT ARE EMITTED 



FROM ALL BURNING BODIES, AS WELL AS FROM THE SUN. 



(a.) A plate of glass, presented to a common fire, intercepts the 

 heat, but permits most of the light to pass, while it becomes itself hot. 



(6.) A bright metallic plate reflects both the light and the heat, and 

 does not become hot. 



(c.) The same plate, if blackened with smoke, ink or paint, be- 

 comes hot, and then ceases to reflect either light or heat. 



(d.) A glass mirror reflects only the light of a common fire, for 

 it absorbs the heat and becomes sensibly 'hot ; the focus is therefore 

 luminous but not hot. 



In the sun's rays it forms both a luminous and a hot focus, and 

 therefore reflects both the heat and light. 



(e.) A metallic mirror acts in the same manner, and also with a 

 common fire, it reflects both the light and the heat ; if blackened, it 

 reflects neither, but becomes itself hot. 



(/.) A lens, before an artificial fire, becomes hot, and forms only a 

 luminous image ; presented to the sun, it concentrates both the light 

 and heat, and produces both a bright and a hot focus, while it scarcely 

 becomes heated at all. 



Edin. Jour, of Science, No. 1, pa. 358. 



