OF RADIANT HEAT THROUGH DIFFERENT BODIES. 53 



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 secon- 

 dary 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 cry'stal would produce a difference of transmission. I have had 

 plates of equal thickness cut out of rock crystal in all the principal di- 

 rections 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* in length. The deviation, which was 21°*8 through a flake 

 of the same substance 2""""6 in length, fell no lower than 1 8°"5 ; a circum- 



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 tlie most tender part of the hand, will produce no sensa- 

 tion 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 onlj' 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 de- 

 monstrate in a direct manner, and without the aid of a therm omul tiplier, the 

 great transmissiveness of rock salt as compared with other diaphanous sub- 

 stances. 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 thermo- 

 meter that is to indicate the transmissive power of the alum will ascend but 1°, 

 while the other will ascend 8° or 10°. 



• [A millimetre, it will be remembered, is equal to •03937 of an English inch. 

 —Edit.] 



Vol. I. — Part I. i* 



