74 



HEAT. 



n i. 



perfected a calorimeter in which the latent heat of steam is used some- 

 what as the latent heat of water is used in Bunsen's ice calorimeter.* 

 The instrument is represented in Fig. 57. The calorimeter C is a light 

 metal vessel, double walled and covered with cloth, placed beneath a 

 delicate balance which is not represented in the figure. A wire hangs 

 from one arm of the balance, and passing through a very small hole at 

 the top of the calorimeter sustains a light wire platform wp within it. 

 Beneath and attached to the platform is a catchwater cw of platinum 

 foil. The substance of which the specific heat is to be found is placed 

 on the wire platform and counterpoised. There is a wide pipe S enter- 

 ing at the top of the calorimeter through which steam can be introduced. 

 During the counterpoising, the steam pipe is not connected to the calori- 



meter and the entrance hole S is plugged 

 up. There is a large exit hole at the 

 bottom of the calorimeter, which is also 

 stopped at this stage by a cap c, shown 

 in the figure out of position. Before an 

 experiment a thermometer is inserted in 

 the calorimeter, and left till all has 

 come to a steady temperature t r Mean- 

 while steam is got up in the boiler, and 

 during the experiment it must be coming 

 off very freely. It is passed through the 

 coupling tube for some time before con- 

 nection to the calorimeter, to drive out 

 all air. Then the thermometer in the 

 calorimeter is read and withdrawn, and 

 the hole for its insertion is plugged up. 

 The exit at the bottom is unstopped, the 

 entrance for the steam is unplugged, and 

 the steam pipe connected. For 30 or 40 

 seconds the steam rushes freely down the 

 calorimeter, driving out all the air, and 

 FIG. 57. Joly's Steam Calorimeter, condensing on all the surfaces. Then the 



exit hole is nearly closed, and in from one 



to four minutes the whole of the inside of the calorimeter rises to the 

 temperature of the steam, when no more should condense. While the 

 steam is still slowly passing through the calorimeter, the balance is 

 again counterpoised, and the gain in weight gives the weight of steam 

 condensed by the platform, catchwater, and substance in rising from t^ 

 to the temperature t 2 of the steam. A previous experiment has given 

 the weight condensed by the platform and catchwater, and the excess 

 above this is due to the substance alone. 



If W is the weight of the substance, s its specific heat, w the weight 

 of steam condensed by it in rising from ^ to t 2 , \ the latent heat of steam, 

 then is found from 



Loss or gain of heat by radiation during the rise in temperature is 

 almost entirely eliminated by the sudden inrush and condensation of 



* The perfected form is fully described in Proc. R. S., xlvii., 1890 p. 218 



