138 ANNUAL OF SCIENTIFIC DISCOVERY. 



EMISSION, ABSORPTION, AND REFLECTION OF VARIETIES OF 

 HEAT RADIATED AT LOW TEMPERATURES. 



Magnus has communicated an interesting and valuable paper 

 on the heat radiated at low temperatures, and on the absorption 

 and reflection of such rays. The* results of this investigation 

 the last, we suppose, made by the lamented author are, so far 

 as published, in his own words as follows : 



" (1.) Different bodies at 150 C. radiate different kinds of 

 heat. These kinds of heat are more absorbed by a substance of 

 the same kind as the radiating body than by others, and this ab- 

 sorption increases with the thickness of the absorbent. 



*' (2.) There are substances which emit only one or a few kinds 

 of heat, others which emit many kinds. 



" (3.) To the first of these belongs rock salt when quite pure. 

 Just as its ignited vapor, or that of one of its constituents, sodium, 

 radiates but one color, so rock salt, even at a low temperature, 

 emits but one kind of heat. It is monothermic, as its vapor is 

 monochromatic. 



" (4.) Rock salt, even when quite clear, emits, together with its 

 peculiar rock-salt heat, heat which is not more absorbed by a 

 plate of rock salt 80 mm. in thickness than by one 20 mm. in 

 thickness. 



" (5.) Rock salt absorbs very powerfully the heat Avhich it 

 radiates. It therefore does not, as Mellari supposed, allow all 

 kinds of heat to pass through it with equal facility. 



" (6.) The great diathermancy of rock salt does not depend 

 upon its less power of absorption for different kinds of heat, but 

 upon the fact that it radiates only one kind of heat, and conse- 

 quently absorbs only this one, and that almost all other substan- 

 ces send out heat containing only a small fraction or more of the 

 rays which rock salt emits. But all rt\,ys which differ from those 

 radiated by any substance are not absorbed by it, but pass 

 through with undiminished intensity. 



" From this we may infer that any substance is diathermanous, 

 only because it radiates but few waves of quite definite length, 

 and consequently absorbs only these, allowing all the others to 

 pass through. 



" (7.) Sylvin behaves like rock salt, but is not monothermic to 

 the same extent. In the case of this substance also, an analogy 

 exists with its ignited vapors, or those of potassium, which, as is 

 well known, yield a nearly continuous spectrum. 



"(8.) Fluor-spar completely absorbs pure rock-salt heat ; we 

 ought, therefore, to expect that the heat which it emits will be 

 equally absorbed by rock salt. Nevertheless, 70 per cent, of this 

 heat passes through a rock-salt plate 20 mm. in thickness. This 

 may doubtless be easily explained with reference to the quantity 

 of heat which fluor-spar emits in comparison with that of the rock 

 salt; still it is possible that fluor-spar at 150 emits rays other 

 than those which it absorbs at ordinary temperatures. This be- 

 havior is probably connected with the great reflecting power of 

 fluor-spar for rock-salt heat. 



