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



twelve metals, the law that the product of atomic weight and 

 specific heat, the so-called atomic heat, has the same value for all 

 elements, about 6'3. In numerous trials, carried on from 1840 

 to 1862, M. Kegnault tested the universal validity of this law of 

 Dulong and Petit. The result was, that for most solid elements 

 it is approximately true ; only the specific heats must be deter- 

 mined at temperatures sufficiently below the melting-points of 

 the elements in question. For 32 solid elements the mean of 

 the atomic heats obtained was 6*3, with extremes of 6*76 for 

 sodium and 5*7 for sulphur; phosphorus and silicium have con- 

 siderably smaller atomic heats, namely 5-39 and 5*04, while 

 crytallized boron has only 2*67, and crystallized carbon even as 

 little as 1'76. Accordingly boron and carbon stand far outside 

 the sphere of the validity of Dulong and Petit^s law. 



This remarkable exceptional position of carbon induced Reg- 

 nault to determine comprehensively the specific heats of its 

 various allotropic modifications. In his second memoir on 

 specific heat (1841) he showed that the diflferent allotropic 

 modifications of carbon possess quite difterent specific heats, and 

 that not one of them fulfils the law of Dulong and Petit. That 

 is to say, the following were the specific heats obtained : — for 

 Animal Charcoal . . . 0-2608 



Wood charcoal 

 Coke. . . . 

 Gas-coal . . . 

 Native graphite 

 Furnace graphite 

 Diamond 



0-2415 

 0-2017 

 0-2036 

 0-2019 

 0-1970 

 0-1469 



juiamona U-I4uy 



A second series of experiments, made in 1862, gave the 

 values : — 

 0-1987] 



0"2020 > for three different pieces of Canadian graphite. 

 0-191lJ 



0*1988 for Siberian graphite. 

 0*2000 for pure gas-coal. 



Almost simultaneously with Regnault, De la Rive and Marcct 

 investigated the specific heat of carbon by the cooling method. 

 They found for charcoal from oil of turpentine 0*1801, for pure 

 sugar charcoal 0'140-0*159, and for diamond 0*119 as the values 

 of the specific heat ; but they are too great, from two causes : 

 first, the specific heat of copper, which served for comparison, 

 was set 4 per cent, too high ; and secondly, the values given by 

 the cooling method, for all substances which are not good con- 

 ductors of heat, are always too great. The difference between 

 the results obtained by Regnault and by De la Rive and Marcet 



