i 3 4 SCIENCE PROGRESS 



dissociation need not be complete even in the case of the least 

 stable compounds, and on the other hand cannot be disregarded 

 even in hydrates of exceptional stability. 



The theory of solutions which results from considerations of 

 this sort differs widely from the hydrate theory as originally 

 formulated by Mendeleef, and the frank recognition that the 

 formation of hydrates in solution is an incomplete and re- 

 versible process has been one of the most important factors 

 in winning for it the general acceptance which it enjoys at the 

 present time. 



The form which the theory now takes may be illustrated by 

 referring again to the hydrates of sulphuric acid. That sulphuric 

 trioxide and water when mixed in molecular proportions unite 

 to form the compound S0 3 . ELO is evident from the fact that at 

 temperatures below 10*35° a crystalline product is obtained which 

 answers to all the ordinary tests for a chemical compound. 

 When the crystals are melted it is probable that dissociation 

 at once sets in, though only to a very slight extent. As the 

 temperature rises dissociation increases, so that at comparatively 

 low temperatures fumes of sulphuric trioxide are already liberated 

 from the liquid ; at 338° the liquid boils, but the vapour consists, 

 not of the hydrate H 2 S0 4 , but of the dissociation product S0 3 . 

 The change represented by the equation 



H,S0 4 JSO :1 +H 2 



is then of dominant importance, but its existence must be 

 recognised over the whole range between the freezing-point and 

 the boiling-point of the acid. 1 At low temperatures other types 

 of dissociation must be recognised, giving rise to various hydrates 

 containing larger and smaller porportions of water, e.g. 



3{SO :t . H,0} JS0 3 .2H 2 + 2S0;>.H,0 



m on oh yd rate T_ dihydrate + half-hydrate 



5{S0 3 . H,0}J S0 3 . 3H,0 + 2{2S0 3 . H,OJ 



monohydrate ^_ trihydrate + half-hydrate. 



These must all be regarded as possible constituents of the melt, 

 although it is impossible at the present time to say which are 



1 Compare the case of nitrogen peroxide, where the colourless ice melts to a 

 pale straw-coloured liquid owing to slight dissociation of N.C^ into NO,. 



