Physics and Chemastry. 105 
SCIENTIFIC INTELLIGENCE. 
I. PHYSICS AND CHEMISTRY. 
the emission, absorption and reflexion of varieties or 
heat pistitated at low temperatures. —Maenvs has communicated a 
ncrbings o and valuable paper on the heat radiated at low vette: 
ratures, and on the absorption and reflexion of such rays. The 
shiek ‘of this investigation,—the last we suppose made by the 
— author,—are so far as published, in his own words as fol- 
ow 
(i 1) Different bodies at 150° C. radiate different kinds of heat. 
These kinds of heat are more Paes ath y a substance of the same 
kind, as the radiating body, than by Se and this absorption 
increases with the thickness of the absor 
(2.) There are substances which emit sail one or a few kinds of 
heat, others, bebe emit man 
(3 ) To t of these belongs rocksalt when quite pure. 
monochroma 
(4.) Rocksalt even when quite pir! cats , together with its pe- 
culiar rocksalt-heat, heat which is n ore absorbed by a plate of 
rocksalt 80 in thickness than by ae "20% in thickness. 
(5.) Rocksalt absorbs very powerfully the heat which it radi- 
ates. It therefore does not, as Melloni supposed, allow all kinds of 
heat to pass through it with equal facility. 
6. The great diathermancy of rocksalt does not depend upon 
its less power of absorption for different en of heat, but upon 
the fact that it radiates only one kind of heat, and nonce 
absorbs only this one, and that almost all other substances sen 
out heat containing only a a small fraction or none of the rays which 
rocksalt emits. But all rays which differ from those radiated by 
any substance, are not absorbed by it, but pass through with un- 
iminished pepe B 
From this we may infer that every substance is diathermanous, 
se Samat it ie padiiten but few waves of quite definite length, 
rum. 
luor spar completely absorbs pure rocksalt heat. We 
sian therefore to expect that the heat which it emits will be 
equally absorbed by rocksalt. Nevertheless 70 eu cent of _ 
heat pass through a rocksalt plate 20™™ in thickness. This m 
doubtless be easily ay with reference to the rgpartion Oe 
the quantity of heat which ee emits in comparison with that 
