101 



ft. An air-tiglit chamber between the tube a and the cube C. It 

 is kept constantly exhausted ; and the calorific rays, therefore, pass 

 from the radiating plate through a vacuum into the tube a, thus 

 retaining the quality which belonged to them at the moment ot 

 emission. 



To prevent the transmission of heat by conduction from the cube 

 C to the tube a, the chamber ft is partly embraced by an annular 

 space in which cold water continually circulates. 



3. A thermo-electric pile furnished with two conical reflectors, 

 and connected with an excellent galvanometer*. One of the faces of 

 the pile receives the rays which have passed through the tube a. 



4. A second copper cube C', also filled with boiling water, and 

 whose rays fall upon the second face of the thermo-electric pile. 

 The two cubes C and C' thus radiating upon the opposite faces of 

 the pile, tend, of course, to neutralize each other. 



Between the cube C' and the adjacent face of the pile, a screen S 

 is introduced, being attached to an apparatus of Ruhmkorff, capable 

 of extremely fine motion ; by the partial advance or withdrawal of 

 this screen the two sources of heat can be caused to neutralize each 

 other perfectly. 



The tube a and the chamber ft being both exhausted, the needle 

 is brought exactly to zero by means of the screen S. The gas or 

 vapour to be experimented with is now admitted into the tube a ; 

 and if it possess any sensible absorbing power, it will destroy the 

 previously existing equilibrium. The consequent deflection of the 

 galvanometer, properly reduced, is the measure of the absorption. 



In this way the action of eight gases and thirteen vapours have 

 been examined, and also the action of atmospheric air. 



Oxygen, hydrogen, nitrogen, and atmospheric air, respectively, 

 absorb about 0*3 per cent, of the calorific rays : this is the feeblest 

 action which has been observed. 



The most energetic action is that of olefiant gas, which at the 

 tension of one atmosphere absorbs 81 per cent, of the calorific rays. 

 Between those extremes stand carbonic oxide, carbonic acid, nitrous 

 oxide, and sulphuretted hydrogen. 



* The author points out the means by which a galvanic coil of any length and 

 of any degree of fineness, and possessing no trace of magnetism, may be obtained. 



VOL. XI. I 



