616 Professor Thomson on Suspended Crystillisation. [May 14, 



Let us consider the case of sodium sulphate as perhaps the most 

 familiar instance. You have seen a large volume of that salt 

 suddenly solidify on the introduction of the proper nucleus. Now 

 why did not that salt behave like the potassium chlorate instead 

 of remaining in solution in the liquid after cooling ? 



It has been suggested that this szfper-saturated solution is merely 

 a saturated solution of the anhydrous salt. That may or may not 

 be so. If it be the case, we can suppose that the molecules of the 

 salt and water are prevented from arranging themselves in their 

 normal order and proportions, and so there is a kind of strain 

 throughout the liquid. This can only be overcome by something 

 which will disturb the molecules sufficiently to bring about the neces- 

 sary rearrangement. 



Taking the instance of the susj)ended solidification of water cooled 

 below its freezing point ; we know that only the disruptive effect of 

 shaking is required ; but witli sodium sulphate in water and many 

 others, no amount of shaking, as we have seen, is sufficient. Some 

 stronger force is required ; this stronger force is found so far as we 

 know at present only in the attraction for the salt of the body itself, 

 or of some substance having the same crystalline form and a similar 

 chemical composition, as has been already shown. 



I say advisedly at j)resent, because in my opinion it has not yet 

 oeen conclusively proved, that no form of what we should call simply 

 mechanical disturbance may not bring about sudden crystallisation in 

 these so-called supersaturated solutions. Indeed, there is an interest- 

 ing experiment which seems to foreshadow such a possibility. By 

 dropping a single carefully washed crystal of alum into a super- 

 saturated solution of that salt, we notice a very interesting phenomenon. 

 The whole surface of the solution is covered with small crystals 

 separated by definite and considerable intervals, and it appears 

 as if the mere mechanical disturbance produced by the first crystal 

 attracting to itself similar molecules, had caused the union of 

 other molecules to form crystals in the remoter parts of the liquid. 

 This is, I think, a very interesting case, which if studied with other 

 similar instances may throw additional light on the causes of such 

 crystallisation. If we suj)i)ose that the salt exists in solution as a 

 hydrate, that is, in actual combination with water, we can imagine that 

 each individual molecule of the hydrate attracts each other one, and 

 is attracted by it equally ; so if one molecule were to move towards 

 another, it would be restrained by its neiglibour, and that in its turn 

 by those near it, and so a state of equilibrium would come about. 

 However, whatever may be the condition of the salt in solution, the 

 same cause, namely, the attraction of similar molecules, aj^pears 

 always to render the equilibrium unstable. 



[J. M. T.] 



