The brown colour of the crystal was so decided that when it was laid on a printed page in which the letters were very large, and placed in the fullest light, even the traces of the letters could not be distinguished. The paper and the printed characters became confounded together and presented the same dark hue. This crystal, however, transmitted 19°, while the thin plate of alum transmitted only 6°.
A body may then be very opake and afford a very easy passage to the rays of heat; or very transparent and intercept the greatest part of them. It is therefore necessary to distinguish those bodies which possess a capacity for calorific transmission from those which possess a capacity for luminous transmission, by giving them different denominations. The terms transcaloric and diathermanous[1] (transcaloriques ou diathermanes) seem to me to be best suited to this purpose, as being most analogous in form to the epithets transparent and diaphanous, applied to bodies endowed with the property of transmitting light.
After the statement made in respect to the smoky rock crystal, one might be tempted to ask whether there are any transcaloric substances totally opake. To that question no answer can be given until the effect of calorific radiation upon all known bodies has been tried, and this I am far from having done. I can only go so far as to say that pyroligneous acid in the rough state, and Peruvian balsam, though almost completely opake, afford perceptible transmissions of radiant heat. But all the diathermanous substances that I have subjected to experiment are comprised within that class of bodies which possess some degree of transparency. Those kinds of metal, wood, and marble which totally obstruct the passage of light obstruct that of heat also. Some other bodies, such as carburet of sulphur, rock salt, and Iceland spar, allow both kinds of rays to pass at the same time. It is therefore probable that calorific transmission cannot take place without a certain degree of transparency[2]; but it cannot take place abundantly without the cooperation of another quality, which varies as the bodies happen to be crystallized or without crystallization. We find, in fact, that in the different sorts of glass and liquids it follows the order of the different degrees of refrangibility; for flint-glass possessing a greater refracting power than crown-glass affords an easier passage to the caloric radiation. Carburet of sulphur is at the same time more refracting and
- ↑ The first of these terms requires no explanation. The second is derived from διά, through, and ξερμαίνω, to heat, as the word diaphanous is derived from διά, through, and φαίνω, to show.
- ↑ I have since found that the perfectly opake glass employed in the construction of mirrors designed to show the polarization of light transmits a considerable quantity of caloric rays. These obscure rays emerging from the dark glass may be employed in some curious experiments which we shall mention in the second Memoir.
