536 



On Supersaturated Saline Solutions, [June 16, 



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 len- 

 ticular 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 sufficiently in contact with the surface of the solution to allow of th e 

 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 sub- 

 merged, 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° Fahr. to —10° and in the 

 absence of a nucleus, rather solidify than crystallize, 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 tetra- 

 hedral 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° Fahr. 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 reinserted, there will be a separa- 

 tion 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 tempe- 

 ratures, and therefore not sensitive to the action of nuclei, become very 

 much so by reduction of temperature below 32° Fahr. Salts that con- 

 tain a large amount of water of crystallization, such as the zincic and 

 magnesic sulphates, require only a small portion of added water in order 



