An example from the dissociation of water vapour

An Example from the Dissociation of Water Vapour.— As an example of the application of the above methods, I may here cite our results on the dissociation of water vapour. The heat of combustion of hydrogen is well known, and the specific heats required have been measured with sufficient accuracy by Pier and his followers by the explosion method. The Second Law may therefore be applied, as in hardly any other case, over a range of temperature in which there are no gaps. 

The numbers given under calc/ are taken from the paper by Siegel (97), who has repeated the calculations made by v. Wartenberg and myself (1906) in the light of the latest measurements of specific heats. 

These formulae rest on explosion experiments covering an interval of 2730 to 2700° for the first two gases, and of 2730 to 2900° for the last. They also reproduce excellently all the other available trustworthy measurements for the range of temperature from 2730 to 1600°. The extrapolation by some hundreds of degrees upwards may be made without any hesitation, as no sudden changes arc to be expected here, according to all other experience. 

On the other hand, the above formulae may on no account be extrapolated downwards ad libitum (cf. Chapter IX). It must be emphasized that the region of their validity, and hence also that of the equation given below, ends somewhere about T = 2000. 

The heat of formation of a molecule of water vapour at 290° abs. at constant volume amounts to 57,290 cals.,J or 114,580 cals, for the reaction 2H8 + 02 = 2Ha0 + UT. Then, according to the First Law, 

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