98 



Mr. W. W. J. Nicol on the 



The specific gravity of the diluted solution having been 

 determined, it was in its turn diluted with an equal volume of 

 water. This was repeated till further dilution was unattended 

 by change of volume. With some salts this was found to be 

 the case after the first dilution, whereas with others a second 

 and even a third dilution was required. It may be noted here 

 that, in the case of the less soluble salts, the dilution has to be 

 carried further than with the more soluble ones. I will return 

 to this later on. Now this change of volume on dilution can- 

 not be explained by the hypothesis that, on each dilution, the 

 salt combines with a further quantity of water to form hy- 

 drates richer in water ; for Gruthrie's experiments* show that 

 the cryohydrate is the salt which contains the maximum num- 

 ber of water-molecules with which the salt can combine; and, 

 as shown in Table IV., the solution which gives no change of 

 volume on dilution contains far more water than that required 

 by the corresponding cryohydrate ; in addition, the tempera- 

 ture 20° C. is in every case much higher than the temperature 

 of formation of the cryohydrate, consequently the dispropor- 

 tion of water to salt is much greater. 



Table IV. 



Salt. 



Approximate strength. 

 (Table III.). 



Cryohydrate. 



NaCl 



KC1 



1 molecule to 200 H 2 

 1 „ „ 200 H 2 

 1 „ „ 400 H 2 

 1 „ „ 400 H 2 

 1 „ „ 800 H 2 



1 molecule 10-5 H 2 

 1 „ 166 H 2 

 1 „ 813 H 2 

 1 „ 44-6 H 2 

 1 „ 222 H 2 



NaN0 3 



KN0 3 



K01O 3 



Berthelot's experimentsf are equally conclusive. He found 

 that the heat of solution of a salt varied with the temperature, 

 that the higher the temperature the greater the amount of heat 

 evolved. This appears quite irreconcilable with the hypothesis 

 that the salt combines with less water at a high temperature 

 than at a low one, while it is in accordance with the results of 

 my experiments on the molecular attraction of salts at various 

 temperatures. Further, Table V., which embodies my expe- 

 riments on the effect of dilution of salt-solutions at 20° and 

 40°, shows that the effect of a rise in temperature is to lessen 



* Phil. Mag. [4] xlix. p. 1 &c. 



t Annates de Chimie et de Physique, [5] 1875, iv. p. 28. 



