176 Prof. Tyndall on the Absorption and 



the contrary, as in the case of atmospheric air, both moisture 

 and carbonic acid were to be removed, the potash tube was in- 

 cluded. G G is a holder from which the gas to be experimented 

 with was sent through the drying-tubes, and thence through 

 the pipe pp into the experimental tube S S f . The appendage at 

 M and the arrangement atO may for the present be disregarded; 

 I shall refer to them particularly by and by. 



The mode of proceeding was as follows : — The tube S S'and 

 the chamber F being exhausted as perfectly as possible, the con- 

 nexion between them was intercepted by shutting off the cocks 

 m, m'. The rays from the interior blackened surface of the cube 

 C passed first aci'oss the vacuum F, then through the plate of 

 rock-salt S, traversed the experimental tube, crossed the second 

 plate S', and being concentrated by the anterior conical reflector, 

 impinged upon the adjacent face of the pile P. Meanwhile 

 the rays from the hot cube C fell upon the opposite face of the 

 pile, and the position of the galvanometer needle declared at 

 once which source was predominant. A movement of the screen 

 H back or forward with the hand sufficed to establish an approxi- 

 mate equality ; but to make the radiations perfectly equal, and 

 thus bring the needle exactly to 0°, the fine motion of the screw 

 above referred to was necessary. The needle being at 0°, the 

 gas to be examined was admitted into the tube ; passing, in the 

 first place, through the drying apparatus. Any required quan- 

 tity of the gas may be admitted ; and here experiments on gases 

 and vapours enjoy an advantage over those with liquids and 

 solids, namely, the capability of changing the density at plea- 

 sure. When the required quantity of gas had been admitted, 

 the galvanometer was observed, and from the deflection of its 

 needle the absorption was accurately determined. 



Up to about its 36th degree, the degrees of my galvanometer 

 are all equal in value; that is to say, it requires the same amount 

 of heat to move the needle from 1° to 2° as to move it from 35° 

 to 36°. Beyond this limit the degrees are equivalent to larger 

 amounts of heat. The instrument was accurately calibrated by 

 the method recommended by Melloni {Thermochrose, p. 59) ; so 

 that the precise value of its larger deflections are at once obtained 

 by reference to a table. Up to the 36th degree, therefore, the 

 simple deflections may be regarded as the expression of the ab- 

 sorption ; but beyond this the absorption equivalent to any deflec- 

 tion is obtained from the table of calibration. 



§ 5. The air of the laboratory, freed from its moisture and 

 carbonic acid, and permitted to enter until the tube was filled, 

 produced a deflection of about 1°. 



Oxygen obtained from chlorate of potash and peroxide of 

 manganese produced a deflection of about 1°. 



