OF RADIANT HEAT THROUGH DIFFERENT BODIES. 



.-A 



to differently coloured lights? Lights of the same tint as the glass pass 

 abundantly, the rest are almost totally intercepted. 



These analogies lead us therefore to consider the radiations from dif- 

 ferent sources of heat as not being of the same nature. This seems in- 

 deed sufficiently established by the mere fact that the calorific trans- 

 mission of glass, Iceland spar, or any other diathermanous body varies 

 with the temperature of the radiating source. 



Thus boiling water, copper heated to 390°, incandescent platina, and 

 the flame of oil will be to us the sources of a heat that is more or less 

 coloured, that is to saj^, sources each of -which gives out a greater quan- 

 tity of calorific rays of a certain quality ; but the flame will furnish ca- 

 loric rays of everj^ kind as it furnishes light of all colours. 



We shall distinguish bodies into diathermanous and athermanous *. 

 The diathermanous we shall subdivide into universal and partial. The 

 first of these subdivisions, which is analogous to colourless media, will 

 contain but one substance, namely, rock salt; the second, Avhich cor- 

 responds with the coloured media, will contain all the bodies comprised 

 in our table, in addition to diaphanous liquids and diaphanous substances 

 in general. 



As to the class of athermanous bodies I had supposed at first that 

 every substance which completely intercepted light intercepted the whole 

 of the radiant heat also. This is found to be the fact in the greatest 

 number of cases. But subsequent experiments have shown me that flakes 

 of black mica and black glass, though they completely intercept the most 

 intense solar light, yet exhibit veiy strongly marked calorific transmis- 

 sions. The foUowinar are the results : 



• Athermanous, in contradistinction to diathermanous, evidently signifies the 

 absence of the power of transmitting heat. I adopt this term merely for con- 

 venience, without attaching to it a definite meaning; for, as there is no body 

 which, if reduced to an extremely thin plate, may not become in some degree 

 transparent, I think also that some rays of heat may pass through all substances 

 in a state of great tenuity. 



K 2 



