1878.] the Function of the sides of the Vessel, Sfc. 



193 



dated itself to the new circumstances and remained liquid when 

 cold. 



The results of these experiments seem to point to the conclusion 

 that adhesion to the side of the vessel is one of the conditions under 

 which the state of supersaturation is maintained ; and that what- 

 ever interferes with this adhesion, must prevent supersaturation or 

 destroy it. 



This conclusion was tested by another mode of experiment, starting 

 from the idea that if the solution were made to expose an amount of 

 free surface, about equal to the surface of contact with the vessel, the 

 adhesive force would scarcely be sufficient to maintain the state of 

 supersaturation, and the solution in cooling would crystallise as in an 

 open evaporating dish. This is what really did happen in the case of 

 ammonia alum (5 to 3). The solution in three covered shallow 

 vessels deposited massive crystals in cooling, and when cold, the 

 mother liquor was no longer supersaturated. A solution of the same 

 degree of strength was boiled in a flask and the opening' plugged with 

 cotton-wool : this deposited no salt in cooling, because the area of 

 adhesion was greatly in excess of that of the free surface. The solu- 

 tion under such conditions will remain unchanged for months. In all 

 cases the re-boiling of the solution in the flask into which it is filtered 

 greatly promotes adhesion, and, consequently, the duration of the 

 solution in the state of supersaturation. 



But to return. A similar crystallisation took place in the case of 

 sodic sulphate ; but that arose chiefly from the kind of vessel used and 

 the mode of covering it, as will be noticed presently. I must first 

 refer to supersaturated solutions of sodic sulphate (3 to 1) in shallow 

 foot glasses, four inches in diameter and one inch deep in the middle. 

 The boiling solution was poured into these glasses, which were 

 immediately covered with shallow glass vessels, inverted over them so 

 as to fit nicely and rest on their edges, or to fit with friction to the 

 sides. The result was curious and interesting. Instead of crystallis- 

 ing in cooling, the solution relieved itself by throwing down large 

 quantities of the seven- watered salt in finely shaped masses, while the 

 liquid portion remained supersaturated.* Next day these solutions 

 were taken into the open air and uncovered. From a point at the 

 extreme edge of each, crystallisation set in and spread like a fan over 

 the surface and through the solution. This was not a case of nuclear 



* A three-ounce flask was coated with, resin, and into it was poured a boiling 

 solution of magnesia sulphate (3 to 1) . The flask was covered with a small beaker 

 and set on the window ledge to cool. A fine display of crystals of the modified salt 

 sprang from the bottom nearly to the surface of the solution, which remained super- 

 satuated. Here again, as in the case of sodic sulphate, the solution relieved itself by 

 this deposit and there was sufficient adhesion on the part of the remaining liquid 

 to maintain a moderate degree of supersaturation, 



