458 



EEPORT 1886. 



already combined in the acid and base from which the salt may be pro- 

 duced. Thomsen also considers that the chemical constitution of the 

 hydrates present in a solution is not altered by dilution of the liquid with 

 more water. 



But against these facts above stated we find that salts which are very 

 soluble, and which crystallise with a large amount of water, such as 

 calcium chloride, do not, as a rule, reduce the pressure of vapour of water 

 in which they are dissolved to a greater extent than salts which, like 

 sodium chloride and potassium nitrate, crystallise habitually without 

 water. There can be no doubt that every one of these and similar thermal 

 effects are, like the corresponding volume claanges, merely differential 

 results, which represent the resultant of several simultaneous or im- 

 mediately consecutive operations. 



A very important observation has been made by Dr. Nicol, which 

 bears directly on this question. In his study of the molecular volumes 

 of salt solutions (' Phil. Mag.' Sept. 1884) he finds that when a salt 

 containing water of crystalHsation is dissolved this water is indistinguish- 

 able by its volume from the rest of the water of the solution. In the 

 Report ' presented to the British Association last year the following 

 passage occurs : — ' These results point to the presence in solution of 

 what may be termed the anhydrous salt, in contradistinction to the view 

 that a hydrate, definite or indefinite, results from solution ; or in other 

 words, no part of the water in solution is in a position, relatively to the 

 salt, different from the remainder.' These two statements are not strictly 

 consequent upon each other. 



I feel inclined to take the view that, save perhaps in excessively dilute 

 solutions, the dissolved substance is attached in some mysterious way (it 



Fig. 4. 



-----r 



,T"MOL 

 J OBS . 



/MOi-ECUL^R VOLUME. OF n'riilOL: Or Hi^O 



IN SOLID Sulphates of magnesium group 



I I ( THOnPE «- WATTS ) I 



■ WOLECilLES 



matters not whether it is supposed to be chemical or physical) to the 

 whole of the water. We cannot otherwise get over the difficulties presented 

 by the bydrated salts which give coloured solutions, by the control of 

 the vapour pressure by the dissolved salt, and by the altered specific 

 heat. 



With regard to water of crystallisation, E. Wiedemann (' Wiedem. 

 Ann.' xvii. 1882, p. 561) has shown that hydrated salts in general expand 



' Report of Committee on Vapour Pressures, &c., of Salt Solutions, Dr. W. W. J, 

 Nicol, secretarj-. Presented at Aberdeen Meeting. 



