THE HYDRATE THEORY OF SOLUTIONS 139 



cent, of the maximum recorded above. The minimum is, more- 

 over, extraordinarily sharply defined, the conductivity being 

 increased by 100 per cent, when the solution is mixed with 

 0*17 per cent. H 2 or 0*23 per cent. S0 3 only. According to 

 W. Kohlrausch {IVicd. Ann. 1882, 17, 69), the minimum is 

 reached when the two substances are present in the molecular 

 ratio S0 3 : H 2 = 0*9975 : 1, a value which differs so little from 

 unity that the small deviation is probably within the limits of 

 analytical error. At the same concentration the temperature- 

 co-efficient of conductivity reaches a maximum at 4 per cent, 

 per degree ; this has the effect of making the minimum less sharp 

 when the temperature is raised, but of accentuating it when 

 the solution is cooled. It is an obvious inference from these 

 observations that the hydrate S0 3 . H 2 is probably an insulator 

 like the simple compounds from which it is derived ; the 

 fused hydrate is, however, dissociated to a small extent 

 even at the melting-point, and the slight conductivity which it 

 possesses may reasonably be attributed to its admixture with 

 the products of dissociation ; as the temperature is raised the 

 dissociation increases, and as a consequence the minimum con- 

 ductivity, which serves so emphatically to direct attention to 

 the existence of the hydrate, gradually becomes smoothed out. 



The half-hydrate 2SO3 H 2 has an even lower conductivity, 

 0*0008 ohm cm -1 , than the monohydrate ; but as mixtures of this 

 acid with an excess of sulphur trioxide are also insulators, the 

 minimum is less striking ; the curve merely drops to the axis 

 of zero conductivity and remains there until it reaches the end 

 of the diagram. 



The minimum at 69 per cent. S0 3 , corresponding with the 

 composition of the dihydrate S0 3 2H9 is of much greater interest, 

 especially in the contrast which it presents with that due to the 

 monohydrate. The actual conductivity of the liquid dihydrate 

 at 1 8° is 0*0980 ohm cm. -1 — a value twelve times as great as in 

 the case of the monohydrate ; and in order to increase the con- 

 ductivity by only 3 per cent, it is necessary to add 2\ per cent, 

 of S0 3 or H 2 0, the minimum being, therefore, 300 times less 

 sharp than in the previous case. The contrast between the two 

 compounds brings out in a very remarkable way the extreme 

 stability of the monohydrate, and it is evident that in this case 

 at least the conditions necessary for the successful application 

 of Mendeleefs method are likely to be amply fulfilled. 



