1209 



fig. 2 by C. Through this quadruple j)oint passes also the three- 

 phase line SSbG, and we shall now show that the determination 

 of the said three-phase lines SSbO and SL,G is again sufficient 

 for the calculation of the composition of the hydrate; the sole 

 difference with the case of § 3 consists in this that a few 

 corrections must be applied, which, however, can be calculated for 

 the system sulphuretted hydrogen-water with sufficient accuracj' 

 from the data of the literature. The requii-ed cori-ections will be 

 mentioned separately for each of the two three-phase lines. 



5. The threephase line SSbG. 



Now the transformation on this three-phase line is not as in ^ 3 

 expressed by 



ABn-^A-^ nB—E,, 



[solid] (ffas' [solid) 



because the gas does not consist of the pure first component (A == ff,S), 



but ice (solid B) has always some, though little, vapour tension. 



Hence a modification only occurs in the gas phase. When from the 



observed pressures P we now subtract the tension of ice of the same 



temperature, we find the values P{corr), which accordingly denote 



the partial pressures of the sulphuretted hydrogen in the gas phase '). 



When we now determine lay P{corr) and the corresponding T—^ 



values, the graphical representation yields a curve which only little 



departs from a straight line. For this curve the following equation 



holds : 



dPirorr) Q, 



1 = Ficorr) 



dr RT ^ ' 



or integrated over a small range of temperature, where Q^ may be 



considered as constant: \ 



In Ficorr) = 1- C. . 



If the curve does not differ perceptibly from a straight line, this 

 expression may be directly applied to the whole line and Q^ can 

 be calculated from the inclination. 



If the curve has a perceptible curvature the value (2, can be 

 calculated for the small range of temperature in question from every 

 time two observations for temperatures that differ little (indices a 

 and b) by the aid of 



1) In this we, therefore, assume that the compound does not exist in gaseous 

 state. If it occurs for a small amount in gaseous state, its influence will certainly 

 remain within the errors of observation, the tension of water vapour being already 

 small itself. 



