CAPACITY FOR HEAT OF METALS AT LOW TEMPERATURES. 
329 
(6) Heat Capacity of Subsidiary Substances. 
The mean values of the specific heats of glass, petrol, alloy, &c., between 0° C. and 
liquid air temperatures were determined by means of the ice calorimeter. 
The arrangement was very simple ; a vessel with a central tube was fixed above the 
calorimeter and the annular space was filled with liquid air. 
The substance was suspended in the central tube alongside a platinum thermo¬ 
meter, and when it had settled to an approximately constant temperature it was 
dropped directly into the calorimeter. 
An approximate value of the coefficient of variation of specific heat with tempe¬ 
rature was obtained by determining the mean values of the specific heats over another 
range, usually 0° C. to 80° C., and assuming a linear variation the values given in 
Table IV. were deduced. 
Since the heat capacity of these substances entered only as a small correction, the 
approximate values were amply sufficient for our purpose. 
Table IV. 
Substance. 
Specific beat at 0° C. 
Specific heat at - 150° C. 
Glass (soft soda). 
0-180 
0-092 
Petrol (density 0 • 74) . . 
0 • 455 
0-358 
The calorimeter constant* was taken as 15‘4 8 6 mgr. mercury per mean calorie. 
(7) Method of Experiment. 
The constant attention necessary to maintain the surrounding enclosure at a fixed 
temperature imposed considerable strain on the observer attending to this part of the 
apparatus and precluded the possibility of experiments such as those described in our 
previous work. 
The procedure adopted in the present investigation was as follows :— 
The temperature of the enclosure was lowered progressively by utilizing the full 
supply from the compressor and controlling the flow so as to produce a steady pressure 
drop through the valve of 120 to 150 atmospheres. 
The temperature of the block would fall at a steady rate by radiation to the 
enclosure walls, and when its temperature had nearly reached the desired point the 
compressor was stopped. 
The temperature of the enclosure would then rise rapidly by conduction from 
without and soon pass that of the block which, in consequence of the slow transmission 
of heat by radiation, would lag behind that of the walls. 
* ‘ Proc. Phys. Soc.,’ London, vol. 26, p. 1, 1913, 
2 TJ 
YOL. CCXIY.—A. 
