82 HEAT. 



rises. In the range from 250 to 600", in which he did not experiment, 

 the diminution must have become much more rapid, for he found that 

 the specific heat in the range from 600 to 1000 only very slowly 

 increased towards -46. 



With graphite, the specific heat about may be represented nearly by 



= -152 + -0007*. 



The rate of increase falls off somewhat more rapidly than with diamond, 

 and the values for the two are not very different over the higher range, 

 as the following table shows : 



SPECIFIC HEATS OF DIAMOND AND GRAPHITE. 



606-7 806-5 985 '0 



Diamond . . -4408 '4489 -4589 



Graphite . . -4431 -4529 -4674 



With crystallised boron the specific heat rises in a similar way, and 

 between and 250 it is fairly, but not exactly, represented by 



8 ='22 + -0007 It. 



Here again the rate of increase falls off as t rises, and from the exact 

 results over the range Weber deduced a limiting value at high tempera- 

 tures of 0'50. 



With crystallised silicon the limiting value was nearly reached 

 in the lower range of experiments. We may take the following 

 values: 



Specific heat of crystallised silicon at - 50 . '13 



. -16 



50 . -18 



100 . -195 



200 . -202 



and the value to which the results tend as 0'205. We shall see the 

 bearing of these values directly. 



Dewar,* working at the same time as Weber, also found that the 

 specific heat of gas-carbon increased very considerably with rise of 

 temperature. Between 20 C. and 1040 0. he obtained a mean value of 

 0-32, and between 20 C. and 2000 a mean value of 0'42, concluding 

 that at 2000 it must be at least 0*5. 



Dewar t has also found the specific heat of diamond, graphite, and a 

 number of other substances at low temperatures by means of a liquid air 

 or a liquid hydrogen calorimeter. The calorimeter consists of a vacuum 

 vessel of 25 to 5l) c.c. capacity, containing liquid air or liquid hydrogen, 

 and it is immersed in a large vacuum vessel containing the same liquid. 

 From the calorimeter a long narrow tube rises up, and to its end a small 

 test tube containing the substance to be experimented on is attached by 

 a short rubber tube The rubber tube acts as a valve cutting off the test 

 tube from the calorimeter, except when the experiment is to be made. 



* Phil. May., xliv., 1872, p. 461. t Print. R.S., A. 76, 1905, p. 325. 



