310 SCIENCE PROGRESS. 



of dissolution, heat capacity, refractive index, magnetic rota- 

 tion, and freezing point, and changes of curvature were found 

 at the same points for all. Even when the solutions become 

 very dilute, similar changes of curvature occur, and, although 

 it is impossible to say, in such cases, whether or not the points 

 correspond to definite molecular proportions, it seems fair to 

 conclude that the changes are due to the same cause as those 

 which appear in stronger solutions. As we go on adding 

 water to a solution of sulphuric acid, heat continues to be 

 evolved, so that it seems necessary to suppose that one 

 acid molecule is able to combine with, or, at all events, 

 to influence in some way, an enormous number of water 

 molecules, and this idea is confirmed by other facts. For 

 instance, the volume of a substance in solution is in general 

 smaller than its volume in the solid state, and, in some 

 cases, even appears to be negative, thus showing that 

 the water has been compressed. Again, the molecular 

 heat (i.e., the product of the specific heat and the mole- 

 cular weight) of a solution is sometimes actually less than 

 that of the water present, so that the thermal capacity of 

 the whole, or, at all events, a large part, of the water must 

 have been affected. 



There seems, then, considerable evidence to show that 

 hydrates exist in solution, or, at all events, that there is a 

 tendency towards the formation of such definite molecular 

 aggregates. It was soon noticed that it was in the case of 

 electrolytes that there was the clearest evidence of chemical 

 action between the solvent and the body dissolved. This 

 suggested the idea that chemical action was the condition 

 necessary for ionisation, and the formation of complex mole- 

 cules from which individual ions could be easily removed 

 by collision with other aggregates, the meaning of con- 

 ductivity. 



By approaching the subject from two different sides we 

 have, then, arrived at two conclusions. Firstly, that pairs 

 of opposite ions must be separated, and the ions exist free 

 from each other's influence, and, secondly, that there must 

 be a tendency towards the formation of more or less stable 

 molecular aggregates between salt and solvent. 



