2^8 THEORY OF SOLUTION AND CRYSTALLISATION. 



CRYSTALLISATION. 



14. Turning now to the circumstances under which crystals 

 may be produced from a liquid, we find two processes commonly 

 resorted to^ viz., {a) evaporation of part of the solvent, and (b) lowering 

 the temperature. A third method sometimes adopted consists in 

 adding to the solution some liquid which is capable of withdrawing 

 the solvent more or less completely, as for example, when water 

 is added to an alcoholic solution of iodine or camphor, or when 

 alcohol is added in sufficient quantity to the aqueous solution of a 

 salt such as sulphate of iron or sulphate of soda which is insoluble 

 or nearly insoluble in that liquid. With regard to the crystal- 

 lisation of " supersaturated '' solutions, some difference of opinion 

 seems to have existed with regard to particular instances, but by 

 one or other of the following agencies all supersaturated solutions 

 may be made to yield crystals of either the so-called normal salt, 

 or a salt modified in form and containing a different amount of 

 water of crystallisation. 



a. A supersaturated solution of a salt may always be made to 



crystallise by the introduction of a crystal of the same 

 salt. 



b. A supersaturated solution may often be made to crystallise 



by the introduction of a crystal of a truly isomorphous 

 substance. Thus common alum will crystallise round a 

 nucleus of chrome alum but not upon an octahedron of 

 pyrites, etc. (J. M. Thomson, Chemical Soc, March, 

 1879.) 



c. Crystallisation may be induced by the action of certain 



absorbents, such as paper, plaster, earth, added in such a 

 manner and in such quantity as not at once to become 

 saturated with the liquid. ( Jeannel, Liversidge, Grenfell, 

 these Proceedings, Part II.) Some hght seems to be 

 thrown on the action of absorbents from the recent 

 experiments of Mr. Bay ley already referred to (§ 12.) 



