PHYSICAL CHEMISTRY 539 



calories from that given in (3) ; thus it may be concluded that 

 the heat of formation of the — C — C — Hnking in the diamond 

 is nearly identical with that of the same grouping in aliphatic 

 compounds. 



From the measurements of Franck, Knipping, and Kriiger 

 on the ionisation potential of hydrogen it can be shown that 



H + iO, = iH,0(Li<,.) + 74 Kcal 

 .'. V = 74± 3 Kcal. 



Therefore a knowledge of the heat of sublimation of the 

 diamond is all that is necessary for the complete solution of 

 equation (i). 



Up to the present only approximate values have been 

 deduced for the heat of sublimation of carbon, Gruneisen has 

 calculated, from the relationship between the atomic heat Cv 

 and the coefficient of expansion of the diamond, that \o = 274 

 Kcal. Fajans obtains a similar value \ = 287 Kcal from a 

 consideration of the work of Lummer on the variation with 

 pressure of the temperature of the positive crater of the carbon 

 arc (3940-4200° abs.). Assuming that these temperatures re- 

 present the sublimation temperatures of carbon under the 

 corresponding pressures, and applying the Clausius Clapyron 

 equation to the vapour pressure curves which are obtained, the 

 heat of sublimation has been deduced. Kohn {Zeit. Phys., 

 1920, 3, 143), while confirming the general conclusions of Fajans, 

 finds that Lummer has made a miscalculation of the tempera- 

 tures of the carbon arc. After correcting these errors the heat 

 of sublimation is 168 Kcal, a value agreeing fairly well with 

 that deduced from the Nernst heat theorem and the chemical 

 .constant of carbon (140 Kcal). These values are, however, too 

 uncertain to make it possible to give the heats of formation of 

 organic compounds from the gaseous elements in the monatomic 

 state with any accuracy. 



That this line of attack is fruitful has been demonstrated 

 by Fajans {Zeit. Phys. Chem., 192 1, 99, 395) in an investigation 

 into the anomalies of the heats of formation of aliphatic hydro- 

 carbons. He finds it necessary to reject the idea that each 

 carbon and hydrogen atom is associated with only 4 and i 

 valencies respectively. The more important subsidiary valen- 

 cies which are exhibited in carbon compounds are those 

 along the edges of the van t' Hoff tetrahedron. These " edge " 

 valencies play a considerable part in the internal energies of the 

 compounds. He points out that, whereas pentane and isopentane 

 contain an equal number of principal — C — C — and — C — H 

 linkings, the number of " edge " valencies 'is dissimilar. The 

 energy in the - C - C - " edge " linking is quite appreci- 



