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 different sources of heat as not being of the same nature. This seems indeed sufficiently established by the mere fact that the calorific transmission 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 say, sources each of which gives out a greater quantity of calorific rays of a certain quality; but the flame will furnish caloric rays of every kind as it furnishes light of all colours.
We shall distinguish bodies into diathermanous and athermanous[1]. 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, which corresponds 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 very strongly marked calorific transmissions. The following are the results:
| Transmissions out of 100 rays issuing from | ||||
| a Locatelli lamp. | incandescent platina. | copper at 390°. | copper at 100°. | |
| Black glass(1 mm in thickness) | 26 | 25 | 12 | 0 |
| Ditto (2 mm ditto ) | 16 | 15.5 | 8 | 0 |
| Black mica (0mm.6 ditto ) | 29 | 28 | 13 | 0 |
| Ditto (0mm.9 ditto ) | 20 | 20 | 9 | 0 |
- ↑ Athermanous, in contradistinction to diathermanous, evidently signifies the absence of the power of transmitting heat. I adopt this term merely for convenience, 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.
