THE HYDRATE THEORY OF SOLUTIONS 135 



the main products of dissociation or to what extent they are 

 produced under any given conditions. 



The dihydrate S0 3 . 2H2O is in many respects similar to the 

 monohydrate. It is a beautifully crystalline compound melting 

 at 8'53° C, but appears to be less stable than the monohydrate. 

 It boils at 220 , but the vapour consists of water and not of 

 sulphur trioxide. When melted, it must be supposed to 

 dissociate in much the same way as the monohydrate, but to a 

 greater extent, so that the products of dissociation, instead of 

 forming only a minute percentage of the mixture, may be present 

 in very considerable quantities. The nature of the dissociation 

 products will be similar to those formed from the monohydrate, 

 but sulphur trioxide and the half-hydrate will only be present in 

 small proportions, whilst water and the more aqueous hydrates 

 will be important constituents of the mixture. 



The hydrates S0 3 . 3H2O and S0 3 . 5H 2 0, melting at — 39° and 

 — 25 respectively, are even less stable than the dihydrate, and 

 it is probable that by the time they have reached atmospheric 

 temperatures they are already dissociated to such a large extent 

 that the products of dissociation are already major constituents 

 of the solution, present in quantities comparable with those of 

 the undecomposed hydrate. 



No other hydrates have yet been separated from mixtures of 

 sulphur trioxide and water, though many others may exist 

 which are either too soluble or present in too small proportions 

 to crystallise out from the melt. In this category may be placed 

 the intermediate hydrate SO3.4H2O and a long series of 

 hydrates containing larger proportions of water. In the case 

 of the simpler hydrates it is probable that marked differences may 

 exist between the stabilities of adjacent members of the series, 

 so that if the tetrahydrate were a compound of small stability 

 a solution of the composition S0 3 . 4H0O might consist mainly 

 of the hydrates S0 3 .3H 2 and S0 3 . 5H0O in approximately 

 molecular proportions. 



2(S0 3 . 4H,0) ;> S0 3 . 3H.,0 + S0 3 . 5HA 



tetrahydrate "^ trihydrate + pentahydrate. 



But it is doubtful whether amongst the more complex hydrates 

 such marked differences exist, and it is very unlikely that in 

 a solution containing 3*5 per cent. H 2 S0 4 the main constituent 

 would be (as Mendeleef supposed) the hydrate S0 3 . 151H0O, 



