30 



CLARKE 



The apparent heat of formation of gaseous water from molec 

 ular hydrogen and oxygen has already been cited as equal to 

 57 934 calories. From this figure, with the aid of data from the 

 foregoing column, the heat of formation of the hydrogen mole- 

 cule, H — H, may be deduced. The equation is 



2[H,0] - 2[H,] - [O,] = 57934 x 2 = 115 868 ; 

 and by substituting the absolute values for O^ and HjO we find 



H^or H- H= 17840. 

 This quantity is of a different order from the henotherm, and 

 bears no simple relation to it. The henotherm, then, is prob- 

 ably not a universal thermochemical unit, and other constants 

 may govern other classes of compounds than those which we 

 have so far studied. 



Three of the numbers given in the column deserve special 

 attention ; namely, those which represent the heats of formation 

 of chlorine, bromine and iodine molecules. Chlorine, having 

 the smallest value, is easiest dissociated by chemical means, and 

 is therefore the most active of the three elements. Iodine, at 

 the other end of the series, is the most stable, and is hence the 

 most likely to separate from its compounds with reversion to the 

 free state. The value for bromine has already been vindicated 

 by the bromination data of Louguinine and Kablukoff ; but 

 that for iodine may at first sight seem to be questionable. The 

 iodine molecule is the one most easily dissociable by heat ; at 

 least it dissociates at the lowest temperature ; and this fact calls 

 for explanation. In the first place it must be remembered that 

 we are dealing with quantities which represent gramme mole- 

 cules and not equal weights of the several substances, and that 

 254 grammes of iodine, 160 of bromine and 71 of chlorine are 

 compared with one another. Secondly, iodine, having the lowest 

 specific heat, is the most easily raised to the dissociation tem- 

 perature, and between the latter factor and the quantity of heat 

 necessary to separate the iodine atoms there is no known rela- 

 tions. Quantity and intensity must not be confounded. 



There is, however, a justification of the three values of a 

 highly suggestive character. According to Thomsen's meas- 

 urements ^ the apparent heats of formation of gaseous hydro- 



* Therm. Unt., vol. 2, p. 398. 



