ABSOEPTION SPECTRA OP SOLUTIONS. 29 



We have uow to ask how these hydrates will be affected by elevation of the tem- 

 perature of the solution. Aud, first, in the case of solutions of moderate strength, clearly 

 elevation of temperature, meaning increase of the mean velocity of the molecules, will 

 involve a diminution iu the number of the water molecules in the system of any salt 

 particle. For the outer water particles of any system, which the central salt particle is 

 just able to keep within its sphere, will by an increment of their velocity be enabled to 

 escape. Moreover, as increase of the velocities of the particles of the system will in- 

 volve an increase in the perturbing opportunities of neighbouring molecules, there will 

 be a more frequent interchange of water molecules between any system and the sur- 

 rounding solvent. These will be the main effects of elevation of temperature ; and 

 they will be produced at all temperatures up to that at which the salt molecule is no 

 longer able to keep any water molecules revolving round it. 



Th.e immediate effect of increase of concentration will be the bringing of the hydrate 

 systems into closer proximity. It follows that, in the case under consideration, the re- 

 sultant force exerted on a water molecule of any system, by the solution external to that 

 system, will be increased, and therefore the resultant force on it towards the central 

 salt molecule of its system, will be diminished. Hence the outer molecules of a system, 

 which the central salt molecule was just able to keep within the system, will be enabled to 

 escape ; and consequently the number of water molecules in the hydrate will on the 

 average be diminished. Also any hydrate system coming more frequently, in these cir- 

 cumstances, into proximity to other systems, than when there is more water or less salt 

 present, will have its outer water particles more frequently wrested from it by per- 

 turbation, with the result of a more frequent interchange of water molecules, between 

 it aud the surrounding water. 



Thus, in the main, the effect produced on the hydrate systems of solutions of mod- 

 erate strength by elevation of temperature will be the same as that produced by increase 

 of concentration. 



Secondly, in the case of very dilute solutions, elevation of temperature will obviously 

 produce an effect of the same kind as in solutions of moderate strength. Increase of con- 

 centration, however, will not. For the resultant force exerted on a water molecule of any 

 system, by the solution external to that system, will not, when the systems are very far 

 apart, be appreciably increased by increase of concentration. And thus increase of con- 

 centration may be expected to produce on the hydrates of very dilute solutions no appre- 

 ciable effect whatever. 



Thirdly, in the case of very strong solutions, so strong that the hydrate systems them- 

 selves form the great bulk of the solution and are thus brought into such close contiguity 

 that they interfere with one another, forming, possibly, complex hydrate systems contain- 

 in"- each two or more molecules of salt, the effect of elevation of temperature will be to 

 increase the amount of free water in the solution and thus to diminish the interference 

 of the systems, whereas the effect of increase of concentration will be to diminish the 

 amount of such free water, and consequently to increase the interference of the hydrate 

 systems. And hence in very strong solutions elevation of temperature and increase of 

 concentration will produce different effects on the hydrates they contain. 



So far we have been dealing with salts supposed to be incapable, in the circumstances, 

 of undero-oing dissociation. It is obvious, however, that as each salt molecule is to be 



