QUANTITY OF HEAT. SPECIFIC HEAT. 



75 



steam on all the surfaces, which very rapidly attain the temperature t v 

 There is, however, a slight continuous radiation from the suspended 

 part, and a corresponding continuous condensation and gain of weight 

 which must be determined and allowed for. In estimating w, correction 

 must be made for the difference in buoyancy of air at t l and steam 

 at # 2 . 



To keep the wire free from the sides of the hole through which 

 it enters the calorimeter, and yet to keep that hole sufficiently small to 

 prevent serious leakage of steam, the following construction is adopted. 

 The top of the calorimeter is conical, and is ground flat so as to 

 make a circular hole. On this rests a little copper disc weighing 

 about 22 mgm., and drilled centrally with a hole about mm. in 

 diameter. The suspending wire of platinum, which may be about 

 O'l mm. in diameter, passes down through the central hole in the 

 disc, and as the wire swings from 

 side to side it pushes the disc 

 about with it until the swings 

 have diminished to less than the 

 diameter of the hole, when the disc 

 is left and the wire finally hangs 

 centrally. Its weight suffices to 

 prevent lifting by the steam. 

 A platinum spiral in an electric 

 circuit surrounds the wire, and 

 during the experiment this is 

 made to glow just visibly. The 



o 



o 



FlG. 58. Joly's Differential Steam 

 Calorimeter. 



heat it gives to the suspending 

 wire suffices to prevent the con- 

 densation of steam on the wire. 



The Differential Steam 

 Calorimeter. Dr. Joly has de- 

 vised a form of calorimeter repre- 

 sented diagrammatically in Fig. 

 58. In this a platform or holder 

 and catchwater depends from each 



arm of the balance into a common steam chamber. The two holders 

 are made to have equal thermal capacities, so that if the substance to be 

 tested is placed on one, the excess of condensation on that side is due 

 entirely to the substance. The corrections for the holder and catchwater 

 arid also for the slow radiation from them are eliminated. 



This apparatus is the only one which has as yet been used to measure 

 the specific heat of gases at constant volume quite directly.* For this 

 purpose Joly used two equal copper globes, each 6'7 cm. in diameter and 

 about 160 cc. capacity. In the experiments on air, one of them was 

 filled with air which, at the lower temperature, was at normal pressure, 

 while the other was filled with air at a much higher pressure. In some 

 cases the mean higher pressure during an experiment was 26 atmo- 

 spheres. The excess of condensation on the one side was due to the 

 excess of weight of air on that side, the volume of which was constant, 

 except for the expansion of the copper with rise of x temperature and its 

 * Phil. Trans., A., 1891, p. 73. 



