1210 





In Pa {corr) — bi P^ {corr) 



R 



This value Q^ is, theretbi-e, llie heat which would l)e required 

 for transformation, if ice had no vapour tension, and agrees, therefore, 

 in significance with the homonymous heat of § 3. It is the heat 

 that we want for the calculation; it is a function of the temperature, 

 but only a feeble one; the change of the heal of Iransformation 

 with the temperature is namely expressed by the algebraic sum of 

 the specific heats of hydrate, n molecules of ice, and one molecule 

 of sulphuretted hydrogen at constant pressure (see § 3). If the law 

 of Kopp is valid this sum of specific heats will be indicated by the 

 difference between that of one molecule of solid H^S and one mole- 

 cule of gaseous //,-S' (at constant pressure). As now the specific heat 

 of solid //,>S amounts to about 10 >), that of gaseous R^S to about 

 8.5, and the difference is therefore 1.5, it is clear that this correction 

 for a range of temperature of about 20° to a heat effect of about 

 5000 calories (see late)-) cojistilules a correction of about GYu,,, whicii 

 is negligible for our purpose. 



Hence the above calculation gives us the heat of the transformation : 



. H,S.nH,0:^n,S ^ nH,0 - E, . . . . . (5) 



(solid) (.7««) (solid) 



The change of energy £", may be found from Q^ by deduction of 

 the external work RT. (In the transformation evaporates one mol. 

 of gas) '). 



6. The three-phase line SL^G. 



In the transformation S"^ L, -\- G — Esl^g an appreciable devia- 

 tion from the corresponding case of ^ 3 occurs. For not only does 

 sulphuretted hydrogen dissolve in the aqueous liquid, but water has 

 also a definite not to be neglected va])0ur tension. The transfor- 

 mation at this equilibrium may be split up into : 



H,S.nH^O:^H,S^nH^O — E,, . . . . (6) 



(solid) (fjns) ' li'^uid) 



') See Nernst. Theor. Cbem. Gesetz von Dulong und Petit. 



2) In the same way as this has been done for the line hydrate-liquid-gas (see 



r ,. , dP Q 



further on) the value of E] may also be calculated trom T --, = — ^, in which I 



represents the volume gas formed by decomposition of one gramme molecule of 

 hydrate (sulphuretted hydrogen + water vapour) ; then the heat Q must be cor- 

 rected for the heat of sublimation of a small quantity of ice in order to find Q^ 

 and El with it. 



