474 Messrs. Wanklyn, Johnstone, and Cooper 



by some ammoniacal salts, where the volume of the solution 

 has been found to exceed the sum of the volumes of water and 

 dry salt. 



Finally, there is the very important and, we think, very 

 common- case where solution takes place without any change 

 of volume whatever. Such a case is afforded by solutions of 

 cane-sugar*, which dissolves in water in almost all propor- 

 tions, and the solutions of which occupy exactly the same 

 volume as the separate water and sugar which enter into 

 them. 



A characteristic example of solution is afforded by sugar and 

 water ; and we hold that solution is mutual permeation with- 

 out change of volume, as exemplified by sugar and water. 



The changes of volume so frequently observed, in the 

 instance of mineral salts, are due to chemical action, which is 

 often a concomitant of solution. 



In order to trace the connexion between the specific gravity 

 of solutions and the composition of solutions, the following 

 method may be followed with advantage. 



We regard solutions as being generated by the entrance 

 of successive units of weight into a large unit of volume. The 

 unit of weight is one gramme. The unit of volume is 100 

 cubic centim., or one litre. The unit of volume is taken to 

 be constantly filled with the solvent except in so far as it is 

 occupied by the thing dissolved. 



' i = the increment- coefficient ; that is to say, the increment 

 of weight occasioned by the entrance of one gramme of the 

 substance into 100 cubic centim., or one litre of solution. It 

 is found experimentally by weighing the 100 cubic centim. or 

 the litre of the solution, and subtracting the weight of 100 

 cubic centim. or a litre of the pure solvent. If the solution 

 contains more or less than one gramme of the substance in 

 the 100 cubic centim. or litre of solution, the number of 

 grammes must be ascertained and used as a divisor. 



When one gramme of a substance is inserted into 100 cubic 

 centim. occupied by a solvent^ one of three things must 

 happen: — 



(L) There may be absolutely no change in volume — 

 neither contraction nor expansion. When one gramme of 

 mercury is dropped into 100 cubic, centim. of water there is 

 neither contraction nor expansion, and the gramme of mer- 

 cury simply displaces its own volume of water, which over- 

 flows out of the 100 cubic centim. measure. Sp. gr. being 

 the specific gravity of mercury, the quantity of water which 



* Vide Chemical News (1891) vol. lxiv. p. 27, Wanklyn and Cooper. 



