Dr. H. F. Weber on the Specific Heat of Carbon. 253 



thus becomes extraordinarily great — so great that it is impossible 

 to account for it by the different methods used for the determi- 

 nation, by errors of observation, impurity of the substances, &c. 

 Kopp, in his comprehensive investigation of the specific heat 

 of sohd bodies (1865), subjected that of carbon to a new deter- 

 mination. By means of the mixing method somewhat modified 

 by him, he obtained for the specific heat of 



Gas-coal 0-185 



Native graphite . . 0*174 

 Furnace graphite . . 0'165, 



values from 9 to 16 per cent, less than those found by Regnault. 

 These considerably smaller values determine M. Kopp to the as- 

 sumption that carbon in all its modifications has only one and 

 the same specific heat, that of the diamond, 0*1469, and conse- 

 quently that carbon forms an indisputable exception to the law 

 of Dulong and Petit. In the numbers found by him for gas-coal 

 and graphite, rather greater than 0*1469, he sees the influence 

 of condensed gases and vapours; and Regnault^s still higher 

 numbers he accounts for by the heat of moistening which ensues 

 when the heated porous substance is immersed in the water of 

 the calorimeter. 



In order to test this latter assumption of M. Kopp, a new and 

 careful determination of the specific heat of carbon was recently 

 (1868) undertaken by Wiillner and Bettendorf, in the memoir 

 entitled " Experiments on the Specific Heat of AUotropic Modi- 

 fications." Their procedure was in substance that of M. Kopp ; 

 only it was carried out with greater accuracy, and the heat of 

 moistening was excluded. The specific heats obtained were : — 



for gas-coal .... 0*2040 



„ native graphite . . 0*1955 



„ furnace graphite . . 0*1961 



„ diamond .... 0-1483 



But these values almost exactly agree with those found by M. 

 Regnault. Hence Wiillner and Bettendorf conclude that '^in- 

 deed essentially different specific heats belong to the diflferent 

 forms of carbon ; and the heat of moistening of the porous forms 

 does not, as Kopp assumes, cause their specific heat to appear 

 too great." 



A slight error, however, has slipped into the calculation of the 

 above values, through the following circumstance. The sub- 

 stance under examination (1-5 grms.) was heated in a glass with 

 water (1-5 grms.) to about 70°, and cooled in the calorimeter 

 to about 20°. In the calculation it was assumed that the specific 

 heat of this water was constant and equal to 1 ; but the mean 

 specific heat of water between 20° and 70° is 1*001. The effect 



