1886.] on Suspended Crystallisation. 515 



From these experiments it will be seen that in the case of the 

 double salts formed by the halogen acids, certain of the component 

 salts are capable of inducing crystallisation, but in the case of the 

 salts formed from acids of higher basicity the component salts are 

 incapable of causing that particular disruption of the solution. 



Having now treated this question experimentally, it may be 

 of advantage to examine it shortly from a theoretical point of view to 

 see if any explanation may be offered at least with our present 

 knowledge of these sudden changes from liquid to solid. 



At the present time it would be rash to attempt a comj)lete answer 

 to the questions — 



(a) What fully takes place when a salt dissolves ? 



(6) Why some salts always separate out when their hot solutions 

 are cooled : and conversely why certain ones remain dissolved under 

 the same conditions ? 



In order that a salt may dissolve in water we must suppose some 

 attraction between the molecules of the salt undergoing solution, and 

 the molecules of the water. With most salts, the power of the water 

 to dissolve them is increased with rise of tem^ierature, and this rise 

 in temperature means increase in the active movement of the water 

 and the salt molecules, and therefore greater facility for them to 

 come near enough to one another for their mutual attractions to be 

 exerted. 



Then why do not all salts dissolve more in hot than in cold 

 water? all do not do so, as you have seen in my diagram during 

 lecture. This leads us to following up the completion of this 

 attraction ; namely, the combination of the water and the salt molecules 

 which I have alluded to in my lecture under the term hydration. 



We must suppose that some hydrates exist at a higher temperature 

 than others, and this is borne out by exj)eriment. In the case of salts 

 whose hydrates exist only at the lower temperatures, the effect of 

 raising the temperature would be merely to increase the vibratory 

 movements and so shake asunder the water and salt molecules, the 

 dehydrated salt naturally separating out, and therefore we could not 

 expect more to go into solution by merely heating the liquid. 



With regard to the second point. We must remember that there 

 exists a strong attraction between the individual molecules which 

 compose the salt. Taking then the case of a salt-like potassium 

 chlorate, which in the solid state contains no water attached to it. 

 We dissolve it in hot water, and on cooling, much of the salt sepa- 

 rates out. We can suppose that the attraction of the water for the 

 salt and the active movement produced by the rise in temperature 

 overcome this attraction of salt molecule for salt molecule, but as the 

 solution cools, this exercises its full force, and crystallisation ensues. 



Now, taking the instances where the salt remains in solution 

 even after cooling, but in much larger quantities than can be obtained 

 by treating the solid salt with water at that temperature ; this bein» 

 what I have called " suspended crystallisation." 



