and Attached Water. 449 



§ 141. A cryohydrate undergoing solidification may there- 

 fore be considered physically as the homologue of a saturated 

 salt solution in the act of boiling. For absolute comparison 

 we should have to eliminate pressure, or rather make auto- 

 matic pressure the measure of the effect of elimination. 

 Wiillner (Pogg. Ann. Bd. ex.) found that the vapour-tension 

 of a salt solution varied inversely with the amount of salt it 

 contained. Thus at 51°*8 the vapour-tension of aqueous so- 

 lutions of chloride of sodium, in mercury pressure, were found 

 to be : — 



H 2 0. NaCl. millim. 



100 100 



90 10 94 



80 20 88 



Wiillner also found that the diminution of the vapour-ten- 

 sion effected by a given percentage of the salt increases as the 

 apour-tension increases ; that is, the diminution is greater as 

 the temperature is greater. Looking upon vapour-tension as 

 the mechanical equivalent of effort at separation, we find that 

 it has its perfect counterpart in the left-hand branches of the 

 curves of fig. 1. Compare the quantity of vapour with the 

 quantity of ice. The left-hand branches are nearly straight 

 lines; but they are on the whole clearly concave downwards. 

 The variation in the nature of the salt also affects the vapour- 

 tensions; and on constructing the curves of vapour-tension of 

 different salts in solutions of different strengths, we see that 

 the ice-curve is algebraically continuous with the vapour-ten- 

 sion curve. The same may be also seen if we assume that the 

 atmospheric pressure, being constant, will not affect the order 

 of curvature of the curves representing the ratio of salt in a 

 olution and its boiling-point. 



§ 142. The fact that the separate constituents of a solution 

 of the proportion of a cryohydrate are both of one form of 

 matter, of course favours their combination. With boiling 

 aqueous solutions the dissimilarity of the educts is generally 

 unfavourable to their union. There is ample evidence, how- 

 ever, of a similar, though less definite (because more influenced 

 by pressure), reunion of the constituents of certain solutions, 

 f an indefinite quantity of dilute alcohol be rectified 100 

 times at a constant pressure in such a manner that the first 

 (say half) portion is redistilled, the water is never eliminated, 

 lor does the strengthening extend beyond a definite limit. 

 ~erhaps the experiments bearing most directly upon this 

 rain of thought are those of Roscoe and Dittmar on the 

 lydration of acids. Since, for instance, nitric acid contain- 

 ng 80 per cent, of HN0 3 is weakened on boiling (consider- 



Phil. Mag. S. 5. Vol. 1. No. 6. June 1876. 2.H 



