70 HEAT. 



was initially brought to some desired temperature, above or below that 

 of the calorimeter, in a vessel V contained in a constant-temperature 

 bath, placed close to the calorimeter, but screened from it by a badly 

 conducting partition. A pipe p, with a stopcock in it, led from this 

 vessel into the vessel v, within the calorimeter C, and when " mixture" 

 was to take place the cock was turned on and pressure applied through 

 the pipe P to the surface of the liquid, which was then forced into the 

 calorimeter. Besides the heat brought into the calorimeter by the 

 liquid some would be conducted by the connecting pipe, but this could 

 be determined and allowed for. 



Experiments on Gases. As we have seen, the expansion of a gas with 

 rise of temperature depends on the pressure to which it is subjected. In 

 the expansion, the surrounding material is pressed out, and heat has to 

 be given to the gas to do the work implied in this pressing out. The 

 heat thus required may be a very appreciable fraction of the whole heat 

 given, and so it is necessary to specify the pressure condition to which 

 the gas is subjected while its specific heat is being found. Regnault only 

 investigated the specific heat under one condition, viz., that of constant 

 pressure. His apparatus is represented in Fig. 55. The gas, carefully 

 purified and dried, was stored in a reservoir R, from which it was 

 allowed to flow through a gas- regulator worked by hand, so that its 

 excess of pressure over that of the atmosphere was constant. A water- 

 manometer M, connected to the gas channel by a very narrow tube, 

 indicated this excess. It was then conveyed through a spiral metal tube, 

 10 metres long and 8 mm. in diameter, coiled in an oil-bath, where its 

 temperature was raised. It then passed by a short tube surrounded 

 with non-conducting packing into the calorimeter, which consisted of a 

 series of brass boxes divided by spiral partitions inside, so as to lengthen 

 the path pursued by the gas ; and it finally emerged into the air. 



The gas was allowed to flow for ten minutes, and the quantity flowing 

 during that time was calculated from the observed fall of pressure in the 

 reservoir between the beginning and end of the experiment. By collect- 

 ing the gas in a subsidiary experiment in a globe, and weighing it, the 

 weight was found to correspond with the observed difference of pressure 

 in the reservoir. The spiral in the oil-bath was so long that the tempera- 

 ture of the gas on emerging from it was that of the oil, and subsidiary 

 experiments showed that, except when the velocity of the gas was 

 exceedingly small, it lost no heat between the oil-bath and the calori- 

 meter, and entered the calorimeter at the temperature of the oil. It 

 left it at the temperature of the calorimeter. Its pressure at entry and 

 emergence was shown by subsidiary experiments to differ by not more 

 than 1 mm. of water. Hence, the pressure was practically constant.* 

 We see then that a known weight of gas at constant pressure was 

 cooled in the calorimeter by an observed mean amount. This was 

 again virtually the method of mixtures. Knowing the capacity of the 

 calorimeter, the experiment enables us to determine the specific heat 

 of the gas. 



* Searle (Proc. Camb. Phil. Soc., xiii. Ft. V. p. 244) has shown that the heat 

 given up by unit mass of gas would be equal to specific heat at constant pressure 

 x temperature fall, even if there were a considerable difference of pressure between 

 entry and exit. 



