of the Specific Heat of Gases under Constant Volume. 345 



as little distant from b as may be desirable. When it is intended 

 to stop the galvanic heating and to ascertain the final pressure 

 of the gas, it would be only necessary to allow the ingress of air 

 from some reservoir connected with a manometer and with C, its 

 pressure being adjusted beforehand, by means of preliminary 

 trials, so as to endow it with sufficient force to press down the 

 mercury column from d to b. If this first means should not 

 seem practicable, secondly, the tube B might have the shape 

 shown in the adjoining figure 2, the tube C Y\g. 2. 



abutting at d, and « representing a thin plate of ^ 



some suitable substance lying on the top of the / ^ 



mercury column. In this case, again, the range 

 of the mercury column may be made very small, 

 the plate a. acting as a stopper ; the volume of * 

 the gas will again remain constant, and the , 

 final pressure be capable of measurement, as I 

 before, by obvious means. 



4. The next important condition implied in the preceding- 

 illustration is the caloric impermeability or neutrality of the en- 

 closing vessel. In reality, this condition is impossible to fulfil, 

 the heat engendered by the spiral being actually communicated 

 in part only to the gas, whilst the remainder will be communi- 

 cated to the vessel, and thence again, in part, to the medium sur- 

 rounding the vessel. But, on the other hand, it is really not 

 required to fulfil this condition, but only properly to distinguish 

 and evaluate these several portions of heat. For this purpose, 

 in the first place, the globe A might be immersed in a vessel 

 containing water, whose temperature might be measured, and 

 might also be taken to indicate the temperature of the globe A, 

 though not of the gas contained in it. A better method, pro- 

 bably, would be to adapt to the globe in one or more places 

 small thermo-couples connected with a galvanometer, by means 

 of which its successive losses of heat by radiation and com- 

 munication to the air, as well as its final temperature, might be 

 determined after the manner resorted to in other similar cases. 



5. Besides the difficulties which it has just been attempted to 

 show how they might be overcome, there remain yet two others 

 which require to be noticed. In the first place, the extensibility 

 of the sphere A, both by increase of temperature and by interior 

 pressure, has to be taken into account, in order to allow for the 

 consequent variation of volume on the part of the gas, as also, 

 what would be more difficult, for the loss of heat due to the 

 resistance opposed by the elasticity of the sphere (and possibly 

 also of the tube B) whilst expanded by pressure. In the next 

 place, it would be necessary to determine also the heat commu- 

 nicated to the tube B and the mercury contained in it, so that 



