Differences so striking in bodies of the same aspect seem to arise rather from the particular structure of each crystal than from the chemical composition of the molecules; for a block of common sea salt being divided into flakes instantly arrests calorific radiation; and we perceive besides, by means of the second and third tables, that the transmissive power of pure water is increased nearly in the same degree whether we dissolve in it alum or rock salt, two substances which, in their solid state, transmit very different quantities of heat. But we perceive no relation between the power of transmitting heat and the primitive or the secondary form of crystallization.
M. Mitscherlich has found that the dilatation of crystals, when they are submitted to the action of heat, is not equal on all sides. Although such an effect may not proceed from the radiant heat, yet it might be thought that a difference in the direction in which the plates are cut out of the crystal would produce a difference of transmission. I have had plates of equal thickness cut out of rock crystal in all the principal directions relatively to its axes. The transmission varied in no case. I obtained the same results from Iceland spar.
Radiant heat is capable of passing through crystallized bodies of very considerable thickness. It may be affirmed, also, that the rays do not lose so much in the interior of these bodies as they do in the masses of glasses and of liquids. For we have seen that the deviation changed only from 21°·6 to 19°, though the smoky rock crystal first employed was replaced by one of fifty-seven or fifty-eight times its thickness.
I have exposed to the action of radiant heat a piece of Iceland spar 92 millimetres[1] in length. The deviation, which was 21°·8 through a flake of the same substance 2ᵐᵐ·6 in length, fell no lower than 18°·5; a circum-
- ↑ [A millimetre, it will be remembered, is equal to ·03937 of an English inch.—Edit.]
radiant heat that falls on its surface, by fixing vertically, on the same stand, a plate of this substance and a plate of glass or alum of the same dimensions, and by bringing the stand quite close to the fire of a stove. If it is allowed to remain in this state for five or six minutes, the glass becomes burning hot, while the rock salt, if applied to the most tender part of the hand, will produce no sensation of heat. These differences of temperature exist not merely in appearance, but are as palpable as those that are felt when we touch wood and marble that have been exposed to the sun. To prove this, we need only lay some pieces of wax or suet on the two bodies. Those laid on the glass will melt rapidly, but those laid on the rock salt will continue in their solid state. We may also demonstrate in a direct manner, and without the aid of a thermomultiplier, the great transmissiveness of rock salt as compared with other diaphanous substances. Let the two plates be brought close together in the same plane, and behind them let two metallic tubes be placed, with the blackened balls of two common thermometers of equal sensibility fixed at their further extremities. If we now place a red-hot bullet at a certain distance from the plates, the thermometer that is to indicate the transmissive power of the alum will ascend but 1°, while the other will ascend 8° or 10°.
