162 Dr. H. F. Weber on the Specific Heat of 



bon, as diamond, so low a number as 1*8 (the atomic weights of 

 these three elements being taken, in accordance with the results 

 of vapour-density determinations, as 28, 11, and 12 respectively) . 



Silicon accordingly stands considerably without the sphere to 

 which the law of Dulong and Petit applies ; boron and carbon 

 form unmistakable exceptions to this notably simple natural law. 



The exceptional position of these three elements induced Reg- 

 nault to subject their various allotropic modifications to a search- 

 ing inquiry, in order to determine their specific heats. In his 

 second communication on the specific heats of solid bodies'* he 

 showed that the different allotropic forms of carbon are possessed 

 of different specific heats, and that no one of these fulfils the 

 conditions of the law of Dulong and Petit. The numbers which 

 he gave (specific heats) are as follows : — 



Animal charcoal . . . 0*2608 



Wood charcoal . . . 0*2415 



Coke 0-2017 



Gas-coke 0*2036 



Graphite 0*2019 



Furnace-graphite . . 0*1970 



Diamond 0*1469 



In a research published in 1861, Regnault obtained analogous 

 results for boron and silicon : the following are the specific heats 

 of 



Graphitic boron . . . 0*2352 

 Crystallized boron . . 0*2574 

 Fused silicon .... 0*1661 

 Crystallized silicon . . 0*1733 



Contemporaneously with Regnault, De la Rive and Marcet 

 examined the specific heats of two modifications of carbon by 

 the method of cooling j\ These physicists also found that the 

 specific heat of the diamond is notably less than that of porous 

 amorphous carbon ; the specific heat of the former being given 

 by them as 0*119, while that of the latter is 0*165. These 

 results were an evident witness to the justice of the opinion that 

 the physical state played as great a part as the chemical nature 

 of the elements, as regards their specific heats, and that the law 

 of Dulong and Petit could not, therefore, be regarded as the uni- 

 versal expression of the law of specific heat. De la Rive and 

 Marcet believed that the great differences between their numbers 

 and the numbers of Regnault could be accounted for by the fact 



* Ann. de Ctfm. et de Phys. Ser. 3. vol. i. p. 202. 



t Ibid. Ser. ?. vol. Ixxv. p. 242, and Ser. 3. vol.ii. p. 121. 



