^32 Transmission of Radiant Heat 



the same law holds good with respect to flint glass, crown glass, and 

 different kinds of glass. It is therefore extremely probable, that it 

 extends to all substances, solid and liquid, deprived of regular crystal- 

 ization. With respect to crystals the single comparison of the two 

 bodies which form the extreme limits of the second table is sufficient 

 to show that they are not at all subject to this kind of proportionality 

 between the faculty of refracting light and transraiting heat, for rock 

 salt possesses nearly the same refraction as alum, and transmits 

 eight times more radiant heat. These two bodies are found often 

 under the same crystalline form, and have nearly the same hardness 

 and specific gravity. The great difference in their action on rays of 

 heat seems not to depend either on chemical composition, since, by 

 dissolving them separately in water, they increase the transmitting 

 power, of that fluid in the same manner. Analogous considerations 

 apply to almost every substance of the same kind. In regularly 

 crystallized bodies, there is then no apparent relation between ca- 

 lorific transmission and the other known properties of matter. 



We have observed that the author had used an Argand lamp in 

 obtaining the results of the preceding tables. Now by employing 

 as radiating sources, incandescent platina, copper maintained at a 

 constant temperature by the flame of alcohol, and vessels full of 

 mercury or of water at the boiling heat, he found that the order of 

 calorific transmission was the same for all of them ; but the numer-f 

 ical value of each transmission underwent great diminutions. Thus, 

 rock crystal, Iceland spar, carbonate of lead, colorless topaz, which 

 gave by the lamp from 62 to 52 were reduced to 26.24 and 20 by 

 the platina. It was the same with other substances : tartrate of po- 

 tassa and soda, citric acid and alum, gave no sensible transmission. 



The quantity transmitted was still more reduced when the source 

 was metal heated to 400° or 500° C. It was reduced to zero when 

 the rays proceeded from boiling water. 



These experiments prove that the law which Delaroche found 

 with respect to glass, extends to the other diaphanous substances 

 mentioned in the preceding tables. But there is one extremely 

 remarkable exception to this general rule. Rock salt allows the 

 same proportion of radiant heat to pass, whatever the temperature 

 of the heating source. Thus, whether the rays proceed from the 

 most brilliant flame, from a red hot ball, from boiling water, or from 

 water heated only to 40° or 50° C, rock salt will always transmit 

 tW of the incident heat. A comparison with the analogous effects 

 of light will more clearly shew the importance of this result. 



