OF RADIANT HEAT THROUGH DIFFERENT BODIES. 31 



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 denomina- 

 tions. The terms transcaloric and diathermanous* (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 crj^stal, 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 pyro- 

 ligneous 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 experi- 

 ment are comprised within that class of bodies which possess some de- 

 gree of transpareocy. 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 pro- 

 bable that calorific transmission cannot take place without a certain de- 

 gree of transparencyf ; 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 dif- 

 ferent sorts of glass and liquids it follows the order of the different de- 

 grees of refrangibility ; for flint-glass possessing a greater refracting 

 power than crown-glass affords an easier passage to the caloric radia- 

 tion. Carburet of sulphur is at the same time more refracting and 



* The first of these terms requires no explanation. The second is derived 

 from 8<a, through, and ^i^fictivu, to heat, as the word diaphanous ia derived from 

 ltd, through, and (pxii/a, to show. 



t I have since found that the perfectly opake glass employed in the con- 

 struction of miiTors designed to show the polarization of light transmits a con- 

 siderable quantity of caloric rays. Tliese obscure rays emerging from tlie dark 

 glass may be employed in some curious experiments whicli we shall mention 

 in the second Memoir. 



