EMISSION, ABSORPTION, AND REFLECTION OF OBSCURE HEAT. 215 



before doing this, I must mention that for these experiments it was necessary 

 to obtain the rays from the body examined unmixed with those of the flame 

 or of any substance by which the body is heated. I succeeded in this by 

 making use of a stream of heated air in which the body was suspended. 



I found that different substances heated to 150° C. emit different kinds 

 of rays ; some only one kind, or waves of one length, and others waves of 

 many different lengths. To the first class belongs pure Rock-salt. There is 

 an analogy between the heat emitted by this body and the light produced 

 by its vapours, or rather of the Sodium it contains. The light from this sub- 

 stance gives only one hue in the spectrum, and the heat also is only of one 

 wave-length. It is monothermic, as its vapour is monochromatic. 



Rock-salt absorbs the heat given out by Rock-salt, while it absorbs almost 

 none of that given out by other substances. 



There is another crystallized substance, the chloride of potassium, called 

 Sylvin, very hke rock-salt in its behaviour ; but it is not monothermic, because 

 it absorbs the heat from different substances, not in a very high degree, but to 

 a greater extent than rock-salt does. 



If our eyes would allow us to see the dark heat as we see light, and we 

 could project a spectrum of the heat of rock-salt, we should see but one line. 

 Rut in making use of the heat emitted by chloride of potassi^^m a longer 

 space would be iUuminated, but not so long as from lampblack or from a 

 flame. 



Here also is an analogy between the heat and light given off by chloride 

 of potassium. 



I then made experiments on the reflection of heat, and I foiind that Silver, 

 Glass, Rock-salt, Sylvin, and Fluorspar reflect nearly the same quantities of 

 heat coming from a flame, from Lampblack, Glass, or from other substances. 

 Rut of the heat from Rock-salt, the Fluorspar reflects five times as much as of 

 that from other siibstances. Of the heat from Sylvin the Fluorspar reflects 

 three times as much. 



It follows from these experiments that if obscure heat were visible, 

 and if Rock-salt were used as the source of heat, we should see the Fluorspar 

 brighter than all other bodies, as far as I know at present. With Sylvin as 

 the source of heat we should see the Fluorspar bright, but not so bright as in 

 the heat from Rock-salt. 



Although invisible to the eye there are millions of rays of heat passing 

 between difl'erent substances, being partly absorbed and partly reflected ; 

 and although we are surrounded by these motions, we cannot observe them 

 but by special experiments. 



The analogy between light and heat seems to me to be complete. 



