Nature of Solution, 93 



gives, I believe, the key to the problem of solution ; it is this 

 mutual attraction of the salt-molecules which, in conjunction 

 with that of the water for the salt, conditions the solubility or 

 insolubility of a salt. 



The solution of a salt in water is a consequence of the 

 attraction of the molecules of water for a molecule of salt ex- 

 ceeding the attraction of the molecules of salt for one another. 

 It follows, then, that as the number of dissolved salt-molecules 

 increases, the attraction of the dissimilar molecules is more 

 and more balanced by the attraction of the similar molecules : 

 when these two forces are in equilibrium, saturation takes 

 place. At the saturation-point the force tending to keep in 

 solution any single molecule of salt (attraction of dissimilar 

 molecules) is balanced by the force tending to produce sepa- 

 ration of that molecule from the solution (attraction of similar 

 molecules). Further, any external cause tending to alter the 

 intensity of either of these two forces or to modify both in 

 unequal degrees disturbs the condition of equilibrium, and 

 further solution or solidification ensues*. 



Before proceeding to a consideration of the experimental 

 results on which the above statement is based, an account of 

 the mode in which my experiments were made is necessary to 

 show the extent to which experimental errors affect the results. 



All the determinations of specific gravity of solutions were 

 made in Sprengel tubes ; and these were immersed in a con- 

 stant-temperature bath at 20° or 40° C, the variation in tem- 

 perature never exceeding one tenth of a degree. The solutions 

 containing definite quantities of salts were prepared by weigh- 

 ing a quantity of water, calculating the required amount of salt 

 and weighing that directly. The accuracy of this method is 

 shown by the following three determinations of the specific 

 gravity of a solution of sodium chloride containing 2 molecules 

 of salt to 100 molecules of water : — 



(1) September 21st, spec, gravity = 1*04387 



(2) October 4th, „ „ = 1*04393 



(3) October 26th, „ „ = 1*04388 



The solutions were made afresh each time ; and the maximum 

 difference is *00006. But in most cases the error is even less ; 

 for in the experiments on dilution the same solution was 

 diluted at 20° and 40°, thus eliminating error in the weight 

 of salt dissolved; and thus the limit of error is reduced to that 



* In addition to the attraction of water for salt, and of salt for salt, 

 there is that of water for water ; but as that for one temperature and for 

 the cases we are considering is practically a constant quantity, I have not 

 thought it necessary to introduce this third factor in the above account of 

 solution ; we shall see further on what part it plays. 



