538 SCIENCE PROGRESS 



Each of these quantities, which can be derived separately 

 from experimental data, are connected by the relation : 



- Ei = X - Qhi - Di - Dh - Ih. 



The heat of formation of hydriodic acid from hydrogen and 

 iodine atoms will be given by Qhi + Dh + Dj calories. 



Calculations of the ionisation potentials made by Born and 

 Fajans from their lattice theory of crystal structure, and by 

 Franck from spectroscopic data are in good agreement with 

 the values obtained experimentally for hydrochloric, hydro- 

 bromic, and hydriodic acids. This concordance proves the 

 soundness of the theoretical basis on which these heats of dis- 

 sociation and ionisation have been calculated. 



It is necessary to adopt other methods of determination in 

 the cases of the — C — C — and — C — H linkings, on account of the 

 complexity of the reactions between carbon and hydrogen at 

 high temperatures. Fajans {Zeit. Phys., 1920, 1, loi) shows 

 that it is possible to derive the values for these linkings from 

 the heats of combustion of the hydrocarbons provided that the 

 heat of sublimation of carbon could be deduced. 



The dissolution of the carbon-carbon and the carbon- 

 hydrogen linkings, in the combustion of organic compounds, is 

 accompanied by the oxidation of the carbon and hydrogen, to 

 carbon dioxide and water. Thus the combustion of ethane can 

 be analysed into four separate processes, which can be repre- 

 sented as follows : 



(i) - 6x + 6v - y -\- 2z = 2i70'9 Kcal, 



where x represents the heat of dissociation of C — H linking 

 and y the value of the C — C linking, whereas v and z represent 

 the heats of oxidation of monatomic hydrogen and oxygen 

 respectively. 



Thomsen {Zeit. Phys. Chem., 1887, 1, 369) showed that 

 the heat of combustion of the CHg group in a homologous series 

 of carbon compounds was 156 Kcal. Thus : 



(i) — 2X+ 2v — y + 2 = iSS'^ Kcal, and from (i) and (2) it 

 follows that : 



(3) z — 2y = 96-5 Kcal (later values 101-105 Kcal). 



The investigations of W. A. and W. L. Bragg on the X-ray 

 analysis of the diamond supply data for the evaluation of 

 z and y, since the carbon atoms in the diamond are arranged 

 as in the aliphatic carbon compounds. If y is the heat of 

 formation of the — C — C — linking in the diamond we can 

 write for the heat of combustion of the diamond. 



(4) 3 - 2y^ = 94*4 calories. This value differs by only two 



