34 SCIENCE PROGRESS 



Here we have before us the striking fact that any possible 

 formula for carbon will fall into one of these three classes. 

 These three distinct classes are remarkable from the fact that 

 they would account for the three modifications of carbon, and 

 suggest the possible formulae for amorphous carbon, graphite, 

 and diamond. 



The following considerations may serve as a pioneer attempt 

 to assign to each modification its constitutional formula. 



The calorimetric measurements of the heat of complete 

 combustion of carbon per gram-atom give the following numbers 

 (Berthelot and Petit, A. Chim. [6] 18, 89-98, 1889). 



Amorphous Carbon 97'65o Cals. 



Graphite . 94*810 „ 



Diamond 94'3io „ 



These calorimetric measurements indicate the sum-total of 

 the energy liberated during the formation and degradation of 

 the complexes, plus that of the oxidation of CO to C0 2 . 



Taking equal weights of amorphous carbon, graphite and 

 diamond, and subjecting them to complete combustion, we find 

 that the amount of heat evolved is different for each form of 

 carbon, although the number of atoms taken and the number of 

 C0 2 molecules formed is identical. 



Looking for the cause of this dissimilarity, we are driven to 

 attribute it to the varying stability of the molecules in the three 

 cases, which must depend upon the mode of linkage of the units 

 inside the molecule. 



Returning to our table of classification, we expect that the 

 least stability will be shown by molecules whose units have the 

 power of free rotation ; the maximum stability will be found in 

 the molecule all the constituent units of which are in a state of 

 rigidity, the intermediate case being a molecule having some 

 units which are rigid and some free. Now, considering the fact 

 that the greater the stability the smaller will be the evolution 

 of heat on complete combustion (compare the case of phosphorus), 

 and correlating this with the calorimetric measurements quoted 

 above, we find that — 



Amorphous carbon is represented by Class I., where none of 

 the units are rigid ; 



Graphite, Class II., some units are rigid; and 



Diamond, Class III., all units are rigid. 



