508 Professor Thomson [May 14, 



WEEKLY EVENING MEETING, 



Friday, May 14, 1886. 



Henry Pollock, Esq. Treasurer and Vice-President, in the Chair. 



Professor John Millar Thomson, F.C.S. 



Suspended Crystallisation, 



The phenomena attending the ordinary solution of metallic salts 

 in water have been so often and ably brought before the considera- 

 tion of this audience, that I have determined to confine myself this 

 evening to certain considerations relating to the formation of and de- 

 position from so-called supersaturated solutions of these salts. At the 

 same time it is necessary for me to remind you of one or two points 

 connected with the ordinary solution of a salt in water, which I may 

 enumerate as follows. 



The solubility of a salt in water depends : 



(a) On the mass of salt presented to the water for solution, and 

 the state of aggregation in which that mass may be at the time of 

 solution. 



(6) The temperature at which the solution of the salt is carried 

 out ; rise in temperature generally producing an increase in the 

 solubility of the salt, although there are certain exceptions to this 

 rule. 



(c) Each salt has its own definite rate or amount of solubility ; 

 some being extremely soluble, as calcium chloride or sodium acetate ; 

 others having a very low amount of solubility, as calcium sulphate. 



{d) On the cooling of a hot solution containing large quantities 

 of salt, a deposition of the salt takes place, which deposition is known 

 under the term " crystallisation." 



Certain salts, however, which generally present abnormal pheno- 

 mena in their solution show no tendency to be deposited from their 

 solutions on cooling, provided such solutions are kept covered from 

 the access of the outside air. Such solutions are said to have become 

 supersaturated. 



This branch of the subject is the one which will engage our 

 attention. 



It may be divided into two classes. (1) That which occurs in 

 the presence of the undissolved salt ; and (2) That which occurs in 

 the absence of the undissolved salt ; this latter class being the one 

 we shall examine. 



Glauber's salt (sulphate of soda) affords a very good example of this. 

 If the crystallised sulphate be added to boiling water in a flask, as long 



