Mr. C. Tomlinson on Supersaturated Saline Solutions. 223 



clean flasks, and covered with watch-glasses. When cold, the watch- 

 glass being lifted off, a drop of oil is deposited on the surface of the 

 supersaturated 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 dihedral 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 

 different strengths, with drops of ether, absolute alcohol, naphtha, 

 benzole, the oils of turpentine, cajeput, 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 

 portion of oil against the side of the flask. 



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

 duced 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 products of oxidation, dust, &c. are nuclear ; 

 but when catharized by being redistilled, they are inactive in the glo- 

 bular 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 so- 

 lution, 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 crystallization, thus once begun, is propagated 

 throughout. A similar action takes place with solid bodies that 

 have contracted filmy nuclei qy being touched or drawn through 



