224 Royal Society. 



the hand, or merely exposed to the air ; they are active or nuclear 

 by virtue of the films of matter which more or less cover them. 



On the other hand, when a drop of oil (or many drops) is placed 

 on the surface of a supersaturated saline solution, and it assumes 

 the lenticular form, or even flattens into a disk, such lens or disk is 

 separated from actual contact with the solution by surface-tension. 

 That the adhesion is very different from that of a film may be shown 

 by pouring a quantity of recently distilled turpentine, for example, 

 on the surface of chemically clean water, and scraping upon it some 

 fragments of camphor ; these will be immediately covered with a 

 solution of camphor in the oil, which solution will form iridescent 

 films, and sail about with the camphor, vigorously displacing the 

 turpentine, and cutting it up into smaller disks and lenses. So in 

 the case of supersaturated saline solutions ; the oil-lens is not suffi- 

 ciently in contact with the surface of the solution to allow of the 

 exertion of that differential kind of action whereby salt is separated. 

 Even when, by shaking, the oil is broken up into globules, and these 

 are submerged, they are still so far separated from the solution by 

 surface-tension as to prevent actual contact. 



In the second section it is shown that solutions of certain salts 

 which remain liquid and supersaturated at and about the freezing- 

 point of water, by a further reduction in temperature, to from 0° F. 

 to —10° and in the absence of a nucleus, rather solidify than crystal- 

 lize, but on being restored to 32° recover their liquid state without 

 any separation of salt. 



A solution of ferrous sulphate, for example, at 0° Fahr. formed 

 tetrahedral crystals at the surface, which spread downwards until the 

 contents of the tube became solid. In snow-water at 32° the frozen 

 mass shrank from the sides of the tube, formed into a smooth rounded 

 mass, and gradually melted, leaving the solution clear and bright 

 without any deposit. On removing the cotton-wool from the mouth 

 of the tube, small but well-shaped rhomboidal crystals soon filled 

 the solution. 



A similar experiment was tried with the double salt formed by 

 mixing in atomic proportions solutions of the zincic and magnesic 

 sulphates. A supersaturated solution of this salt at about — 8°F. 

 became solid, and it melted quickly at 32°. Such a solution may 

 be solidified and melted a number of times, provided it be protected 

 from the action of nuclei ; but if the cotton-wool be removed from 

 the tube, even when the contents are solid, and be immediately re- 

 inserted, there will be a separation of the salt during the melting, in 

 consequence of the entrance of nuclear particles from the air. 



Solutions of such a strength as to be only saturated at ordinary 

 temperatures, and therefore not sensitive to the action of nuclei, 

 become very much so by reduction of the temperature below 32° Fahr. 

 Salts that contain a large amount of water of crystallization, such as 

 the zincic and magnesic sulphates, require only a small portion of 

 added water in order to form supersaturated solutions, and they be- 

 come more sensitive to the action of nuclei as the temperature falls, 



