406 Intelligence and Miscellaneous Articles. 



In spite of this difficulty, MM. La Provostaye and Desains showed 

 in 1849* that, according as heat from a Locatelli's lamp has passed 

 through glass or through rock-salt, various quantities are reflected by 

 speculum-metal, silver, and platinum ; and in the case of all re- 

 flecting surfaces, less was reflected of that which had passed through 

 glass than of that through rock-salt. 



The same inquirers have subsequently published a comprehensive 

 series of experiments made with the heat of a lamp decomposed by 

 means of a glass prism, in which it was shown that heat from the 

 difTerent parts of the spectrum is variously reflected. But they 

 restricted their experiments to reflection from metallic surfaces, 

 doubtless on account of the feeble intensity of the incident heat. Now 

 that we possess in rock-salt a substance which only emits one or a 

 few wave-lengths, and we also know other bodies which at the tem- 

 perature of 150° C. radiate a limited number of wave-lengths, it is 

 possible to make experiments on the reflection of non-metallic sur- 

 faces. It has thus been found that from these the difTerent kinds of 

 heat or wave-lengths are reflected in very different quantity. Only 

 one of the most surprising examples shall be here mentioned. It 

 refers to the reflecting-power of fluor-spar. 



Of heat which very difTerent substances radiate, there are reflected 

 at an angle of 45° quantities which are indeed not equal, but which 

 do not differ much from each other. 



Silver, between .... 83 and 90 per cent. 



Glass „ 6 „ 14 



Rock-salt ,, 5 ,, 12 ,, 



Fluor-spar „ 6 ,, 10 ,, 



Of the heat from rock-salt, fluor-spar reflects 28 to 30 per cent., 

 while silver, glass, and rock-salt do not reflect larger proportions of 

 this than of the other kinds of heat. 



Here, as in the experiments on the transmission of heat, it has 

 been confirmed that sylvine emits a large quantity of rock-salt heat, 

 but at the same time emits other kinds of heat. And fluor-spar 

 reflects 15-17 per cent, of sylvine-heat, consequently less than 

 it reflects of rock-salt heat, and more than it does of that from the 

 other radiating bodies. 



If our eyes had the power of distinguishing the various wave-lengths 

 of heat as well as the colours of light, fluor-spar would appear brighter 

 than all other substances when the rays of rock-salt fell upon them. 

 If the rays came from sylvine, fluor-spar would also appear brighter 

 than all other bodies, but not so bright as with the radiation from 

 rock-salt. 



Melloni has taught us that various substances transmit very dif- 

 ferent quantities of heat, and that the source from which it originates 

 has great influence on its transmission. But the sources of heat 

 were only distinguished as to their degree of heat, and we knew that 

 with increasing temperature the diversity of the radiation increased. 



* Comptes Rendus, vol. xxviii. p. 501. 



