466 On the Specific Heat of Carbon at High Temperatures. 



The following Table contains some of the results obtained in 

 working with this form of apparatus : — 



Table II. 



Mean Specific Heat of Carbon up to temperature of Oxyhy- 

 drogen Blowpipe (2000°) . 



Calorimeter and water equivalent to 523-4 gramme units. 



1 











Increase of 





Weight of 



Initial tem- 



Final tem- 



Increase of 



temperature, 





carbon. 



perature. 



perature. 



temperature. 



per gramme 

 weight. 

















o 



I. 



0747 



1754 



18-59 



105 



1-40 



II. 



0792 



17-28 



18-47 



119 



1-50 



III. 



0741 



17-38 



18-44 



106 



1-43 



IV. 



0-3915 



17-52 



18-12 



0-60 



1-53 



V. 



014 



17-92 



18-15 



0-23 



1-64 



Calculating from the highest result obtained at a temperature 

 of 2000° C-, the mean specific heat of carbon is about 0*42. 

 The true specific heat at 2000° must be at least 0*5 ; so that at 

 this temperature carbon would agree with the law of Dulong 

 and Petit. In general the rate at which the specific heat varies 

 in the case of the metals may be represented by a straight Ime ; 

 and the increment seems to be directly related to the rate of 

 variation of the coefficient of expansion. Now in the case of 



diamond^ graphite, and gas-carbon -r^ are as the numbers 



4*32 : 3*03 : 3-3, according to Fizeau; and as he has further 

 shown that diamond has a minimum volume at — 42°*3 C, 

 and that below this temperature it expands as the temperature 

 falls, we may anticipate some marked alteration in the specific 

 heat at very low temperatures, w^hich Dr. Weber proposes to in- 

 vestigate. Of the three varieties of carbon, graphite is certainly 

 the most stable at very high temperatures. Gas-retort carbon, 

 after being used as poles in a powerful electric arc, is in part 

 transformed into graphite ; and the diamond exposed to the tem- 

 perature of the voltaic arc passes also into graphite. Unless 

 graphite or carbon can pass into the form of diamond under cer- 

 tain conditions of pressure at comparatively low temperatures, 

 or is of vegetable origin, it is difficult to conceive how^ diamond 

 could occur if this earth ever had a temperature as high as 

 that of the voltaic arc. Starting from absolute zero, carbon as 

 graphite most probably increases regularly in specific heat, 

 whereas diamond probably diminishes until we reach — 42°-3 C, 

 and then increases regularly until it exceeds that of graphite, 

 which it continues to do until they agree at very high tempera- 

 tures. The excess of heat taken in by the diamond accumulates 



