MATTER FORMED OF SMALL PARTICLES 95 



The solution of a solid in a liquid differs, therefore, from the 

 mixing of two such liquids as alcohol and water, inasmuch as 

 it cannot proceed indefinitely. It resembles it, however, in the 

 peculiar thoroughness with which the solid permeates the 

 whole of the liquid. In other words, the solid, or the dissolved 

 portion of it, diffuses through the liquid, so that after a time 

 the whole is homogeneous. This may be proved either by 

 chemical means ; by the evenness of tint when the solid is 

 coloured ; by the evenness of density ; or by taking equal 

 volumes of the solution, evaporating, and weighing the solid 

 residue. The last process is generally applicable. 



The rate at which a solid dissolves depends upon the extent 

 of the areas in contact ; hence a powder dissolves more 

 quickly than the same quantity of the substance in one piece. 

 It also depends upon the quantity of the solid already dissolved. 

 A given liquid cannot hold dissolved more than a certain quantity 

 of a given solid. In considering the process of solution, it must 

 be remembered that the layer of liquid in contact with the 

 solid becomes saturated that is, it contains its maximum 

 quantity, except so far as it loses by diffusion. This process, 

 however, is slow. Hence movement quickens solution. For 

 the same reason solution is often quickened by suspending the 

 solid at the top of the liquid solvent, so that the denser 

 saturated portions may sink away from the solid. 



The limitation to the mixing of a solid with a given quan- 

 tity of liquid must be looked upon as due to the solid itself 

 rather than to any inability in the liquid. The knowledge 

 which we have previously gained of the different behaviour 

 exhibited by solids and liquids when acted on by stresses, 

 gives a probability to the view that a solid consists of small 

 particles which can only be separated with difficulty. The 

 actions between a solid and its solvent, whatever they may 

 be, which result in these firmly cohering particles being 

 separated, are gradually weakened as more and more of the 

 solid becomes mixed with the solvent. That the action is a 

 complex one is proved by the fact that solubility is in no way 

 proportional to tenacity. 



