RELATION OF HEAT AND LIGHT. 



187 



RELATION OF HEAT AKD LIGHT. 



The whole body of natural phenomena in which the effects of heat and light 

 are concerned, demonstrate an intimate physical connexion between these 

 agents. Sunlight is warm, the light of red coals is warm, and the more bril- 

 liant light of flame excites still more intense heat. If every degree of light 

 were productive of heat, and, reciprocally, every degree of heat productive of 

 light, we should not hesitate to infer that heat and light are two distinct effects 

 of the same physical principle ; and such an inference would be corroborated 

 if it appeared that the energy of the luminous and calorific effects were pro- 

 portionate to each other, the most brilliant light always producing the most in- 

 tense heat, and the most fierce temperature always accompanied by the strong- 

 est illuminating power. 



Some of the more obvious phenomena countenance these views. All the 

 ordinary sources of light, are also sources of heat ; and by whatever artificial 

 means natural light is condensed, so as to increase its splendor, the heat which 

 it produces is at the same time rendered more intense. The direct rays of the 

 sun, playing on the bulb of a thermometer, will elevate its temperature to a cer- 

 tain extent ; but if a certain number of these rays be concentrated on the same 

 bulb by a concave reflector, or burning lens, then the elevation of temperature 

 will be much more sudden and extensive. These, however, are only the first 

 and more prominent effects which obtrude themselves on our observation. It 

 requires little attention to the phenomena of nature, much less to those which 

 are exhibited by the processes of science and art, to discover that the heat 

 which accompanies light is not always proportionate to the splendor of the 

 light ; and further, that heat of considerable intensity, both as regards the ther- 

 mometric effects, and the sensation it produces, may be either absolutely ac- 

 companied by light, or, at least, if it have light, the intensity of that light is so 

 small as to be below the limit of the sensibility of the eye. 



The fact of the existence of heat unaccompanied by any sensible degree of 



