i 3 2 SCIENCE PROGRESS 



Any attempt to increase the vapour-pressure resulted in the 

 formation of a hydrate with a larger proportion of water, whilst 

 the removal of water vapour, instead of producing a reduction 

 in the pressure, merely caused a part of the hydrate to dis- 

 sociate. If water vapour was added to the anhydrous salt, the 

 vapour-pressure rose by a series of abrupt steps from zero to 



4*4 mm. (CuS0 4 partially converted into CuS0 4 . H.,0) 



30mm. (CuSO,. H.,0 „ „ „ CuS0 4 .3H.,0), and 



47 mm. (CuS0 4 .3H,0 „ „ „ CuS0 4 . 5H0O), 



intermediate pressures being possible only as a transient con- 

 dition prior to the establishment of equilibrium. Further addition 

 of water would produce a fourth " step " in the vapour-pressure, 

 owing to the partial conversion of the pentahydrate into saturated 

 solution, but the pressure at this stage was not measured. 



A similar state of affairs was postulated by Mendeleef in 

 reference to hydrate formation in the liquid state, and his 

 expectation that abrupt changes of curvature and direction 

 would be found in the curves representing the physical pro- 

 perties of a series of solutions was a logical result of this view. 

 It is indeed by no means obvious at first sight that there need 

 be any marked difference between the conditions which deter- 

 mine the formation and decomposition of solid and of liquid 

 hydrates. The hydrates of sulphuric acid are at least as stable 

 as those of copper sulphate, as is shown by the fact that con- 

 centrated sulphuric acid is capable of destroying the blue colour 

 of the pentahydrate. It was therefore not unreasonable to 

 suppose that when water was added to the anhydrous acid a 

 series of liquid hydrates would be formed one after the other 

 in just the same manner as in the case of solid copper sulphate, 

 the number of co-existing hydrates being limited to two. 

 Looked at from the modern standpoint, the difference between 

 the two cases is seen to depend mainly on the fact that liquid 

 hydrates are, as a rule, miscible in all proportions, whilst each 

 solid hydrate forms, as a rule, a separate "phase." If the 

 hydrates of copper sulphate were isomorphous with one another 

 so as to form only a single solid phase, the " stepped " vapour- 

 pressure curve which marks the transition from 



/"Vapour 

 the system i CuS0 4 to the systems 



1 CuS0 4 . H.,0 



{Vapour (Vapour (Vapour 



CuS0 4 . H.,0 \ CuSO, . 5H,0 and ] CuSO, . 5 H.,0 



CuS0 4 . 3H,0 ICuSO, . 5H,0 Uat. solution 



