1870.] 



Supersaturated Saline Solutions. 



535 



rated solution. In an experiment described, a drop of pale seal-oil formed 

 a well-shaped film, with a display of iridescent rings, and immediately 

 from the lower surface of the film there fell large flat prisms with dihe- 

 dral summits of the 10-atom sodic sulphate. The prisms were an inch or 

 an inch and a half in length, and three-eighths of an inch across. The 

 crystallization proceeded from every part of the lower surface of the film, 

 and as one set of crystals fell off, another set was formed, until the whole 

 solution became a mass of fine crystals in a small quantity of liquid, an 

 effect quite different from the usual crystallization which takes place when 

 a supersaturated solution of Glauber's salt is subjected to the action of a 

 nucleus at one or two points in its surface, as when motes of dust enter 

 from the air, or the surface is touched with a nuclear point. In such case 

 small crystalline needles diverge from the point and proceed rapidly in 

 well-packed lines to the bottom, the whole being too crowded and too 

 rapid to allow of the formation of regular crystals. 



Similar experiments were made on solutions of Glauber's salt of dif- 

 ferent strengths, with drops of ether, absolute alcohol, naphtha, benzole, 

 the oils of turpentine, cajuput, and other volatile oils, sperm, herring, olive, 

 linseed, castor, and other fixed oils of animal and vegetable origin, with 

 this general result, that whenever the liquid drop spread out into a film, 

 it acted as a powerful nucleus ; but when the oil formed a lens there 

 was no separation of salt, even when the flasks were shaken so as to break 

 up the lens into small globules. If, however, a sudden jerk were given to 

 the flask so as to flatten some of the globules against its sides into films, 

 the whole solution instantly became solid. A similar effect was produced 

 by introducing a clean inactive solid for the purpose of flattening a por- 

 tion of oil against the side of the flask. 



Stearine from sheep's tallow that had been exposed to the air produced 

 immediate crystallization, but by boiling the solution and covering the 

 flasks, the stearine, now catharized, had lost its nuclear character on the 

 cold solution. Similar observations were made with the fixed oils that 

 form lenses or globules in the solution. So also volatile oils containing pro- 

 ducts of oxidation, dust, &c. are nuclear, but when catharized by being 

 redistilled they are inactive in the globular state, active in the form of films. 



Supersaturated solutions of potash alum, ammonia alum, sodic acetate, 

 and magnesia sulphate were also operated on with results similar to 

 those obtained with solutions of Glauber's salt. 



When a liquid forms a film on the surface of a supersaturated solution, 

 the surface-tension of the solution is so far diminished as to bring the film 

 into contact with the solution, when that differential kind of action takes 

 place whereby the salt of the solution adhering more strongly to the 

 film than the water of the solution, the action of separation and crystal- 

 lization, thus once begun, is propagated throughout. A similar action 

 takes place with solid bodies that have contracted filmy nuclei by being 

 touched or drawn through the hand, or merely exposed to the airj 



