CRYSTALLISATION. 215 



dish.' Any capacious glass vessel, not too high, will also answer 

 the purpose, if care be taken not to heat it suddenly and thus to 

 break it ; this may be prevented by placing the vessel into a larger 

 one about 6 cm wide, filling the latter a few centimetres high with 

 water, and heating the whole on the hob or on a stove until the nitre 

 is dissolved. That the cooling may not be delayed too long, the 

 vessel containing the solution should be removed from the larger 

 vessel and placed in a quiet spot, for example, a window sill; one 

 or two hours afterwards the liquid portion is poured away, and the 

 interior of the vessel will be found 



lined with crystals. /\\ 



Another salt which crystallises / \J\ 



readily from hot solutions is alum. 

 The relative quantities of alum and 

 water for producing crystals may 

 be the same as in the case of salt- 

 petre, but the form of the crystals is 

 different in the two cases : fig. 152 A, 

 represents a single perfectly formed 

 crystal of alum ; fig. 152 B, one Fl - 152 - 



of saltpetre. Such crystals, however, perfectly developed in all 

 directions, are only obtained with difficulty. 



Liquids which mix when shaken together, as water 

 and alcohol, water and vinegar, or water and saline 

 solutions, will also mix gradually when simply brought 

 into contact, without being* stirred or shaken. This 



' O 



mutual interchange which takes place between the 

 molecules of two different liquids in contact, without 

 the application of any external force, is called diffu- 

 sion. Diffusion proceeds at various rates, according 

 it is favoured or retarded by the difference in the 

 ific gravities of the two liquids. Two test-tubes, 

 ach being large enough to contain about 30 CC of water, 

 are filled with water. Into one of the test-tubes about 

 3^r Q f t ^ ue yftriol ' (cupric sulphate) are thrown; this 

 substance -forms in the solid state beautiful blue crys- 

 tals, and, when dissolved in water, a blue solution 



