ABSOEPTION SPECTEA OF SOLUTIONS. 31 



made to account for their large molecular depression of the freezing point and vapour 

 tension as well as for other properties ; and for this purpose each of the molecules into 

 which a salt molecule dissociates is assumed to lower the freezing point or vapour tension 

 to the same extent as the uudissociated salt molecule. This leads us to assume that the 

 dissociated molecules exert attractions on the molecules of the solvent in the same way as 

 the uudissociated molecules. And it follows as above that the dissociated molecules also 

 will form hydrate systems, and that the effects of elevation of temperature and increase 

 of concentration on them will be related to one another as in the case of the hydrate sys- 

 tems of the salt molecules. 



As water has practically no selective absorption, at any rate for light of a very great 

 range of wave-leugths, the absorption of a solution throughout that range will, according 

 to the aboA'e hypotheses, depend wholly upon the hydrate systems formed either by the 

 salt molecules or by the products of their dissociation. 



If, therefore, there be no dissociation, or so little that it may be neglected, eleva- 

 tion of temperature and increase of concentration must produce in the main the same 

 elfect on the absorption of solutions of any salt which are of moderate strength. They 

 may be expected to produce opposite effects in very strong solutions. And in very 

 dilute solutions increase of concentration may be expected to produce no appreciable 

 effect. 



If, however, there be dissociation, the absorption spectrum of a solution must be 

 made up of absorption. lines or bands due to the hydrate systems of the salt mole- 

 cules and of others due to those of the dissociated molecules. If elevation of tempera- 

 ture and increase of concentration did not change the relative number of dissociated 

 and uudissociated molecules present in the solution, they would clearly produce on the 

 total absorption of the solution eftects related to one another in the same way as they 

 would be if there were no dissociation. But as elevation of temperature involves, in gen- 

 eral, increase in the degree of dissociation, and therefore diminution in the intensity of the 

 absorption lines due to the systems of the salt molecules, and increase in the intensity of 

 the absorption lines due to the dissociated molecules ; and as increase of concentration 

 involves a diminution in the degree of dissociation, and therefore an exactly opposite 

 change of relative intensity, it follows that the changes produced in the absorption by 

 elevation of temperature and increase of concentration may be expected to differ from one 

 another to a greater extent if the salt molecules really undergo dissociation, than if they 

 do not. It will be noted, however, that the direct effect on the hydrates of raising the 

 temperature or increasing the concentration of the solution, is felt by all the hydrates in 

 the solution and modifies the absorption of all, while the effect of these changes on the 

 degree of dissociation modifies the absorption of a comparatively small number. Hence 

 we may expect that in general the changes produced in the absorption of solutions of salts 

 which undergo dissociation, by variations of temperature and concentration, especially 

 if these variations be small, will be related to one another in the same way as if there 

 were no dissociated molecules in the solution. 



As elevation of temperature is well known to change the absorption of a solution, 

 the above conclusions are clearly inconsistent with Beer's law of absorption, which states 

 that two absorbing strata of solutions of the same salt will have the same absorption, 



