Intelligence and Miscellaneous Articles. 327 



machines, which, in proportion to their relatively small weight, de- 

 velope a far greater force than machines constructed by human in- 

 genuity. — Comptes llendus, June 3, 1867. 



INVESTIGATIONS ON THE ABSORPTION OF OBSCURE HEAT. 

 BY M. P. DESAINS. 



On May 27 I had the honour of laying before the Academy the 

 results of a series of experiments on the absorbing action which seve- 

 ral volatile liquids and their vapours exert on the heat from a lamp 

 with a glass chimney. 



These experiments show that, in the case of the heat in question 

 and the substances with which I worked, the absorption obeys a 

 very simple law — that a column of any of these liquids of a given 

 section and weight exerts an absorption which is independent of the 

 physical condition of the medium. The liquid column is very short, 

 and the gaseous one very long ; but they produce the same effect. 



It seemed important to ascertain whether this equality exists 

 when the rays are far more absorbable than those of a lamp with a 

 double draught, which have already passed through a considerable 

 thickness of glass — whether it obtains, for instance, in the case of the 

 radiation from a plate of copper heated to about 400°. 



But here a purely experimental difficulty at once presented itself. 

 This obscure heat is absorbed by glass to far too great an extent for 

 me to dream of employing the glass plates I used in my former ex- 

 periments. By the use of rock-salt Professor Tyndall was able to 

 obtain good results. But although we need not exaggerate the 

 fears which may arise from the facility with which this body absorbs 

 aqueous vapour, I preferred to get rid entirely of this source of error ; 

 and fortunately I obtained fromJVI. H. Soleil some very beautiful spe- 

 cimens of colourless fluor-spar, which were extremely useful to me. 

 Melloni's experiments have shown that a thickness of 2 millims. 

 of this substance transmits about 50 per cent, of the heat from a 

 blackened copper plate heated to 400°. 



I used it for closing my tubes, and for constructing my lenses and 

 troughs ; and, thanks to its use, I have been able to ascertain that 

 on the heat from a blackened copper plate heated to about 400° the 

 ether exerts absorptions which are always independent of the phy- 

 sical condition of the bodies used. This absorption is considerable, 

 amounting to about 92 per cent, in the case of a layer of liquid 

 ether of 0'0017 millim. in thickness at 25°. In all cases, whether 

 the ether be in the state of liquid or of vapour, the absorbent action 

 is the same, provided the number of active molecules which the ray 

 meets on its path is the same. 



This characteristic of the absorption which the diathermanous 

 bodies investigated in this research exert upon the heat is met with 

 under different forms. Thus, for instance, in working with illumi- 

 nating gas I ascertained that in a given tube a constant weight of 

 this gas exerts an action completely independent of the greater or 

 less quantity of air with which it is mixed. With the quantity of 

 air introduced, the pressure of the internal elastic fluid changes ; but, 

 provided nothing escapes from the tube, the absorption remains un- 



