Mr. C. Tomlinson on Supersaturated Saline Solutions. 223 



a modified salt, as in the ease of zinco-sulphate and sodic sulphate; 

 the fifth contains an inquiry as to whether anhydrous salts form 

 supersaturated solutions ; and the sixth and last part is a summary, 

 with a classified list of the salts examined. 



1. Definition. — When water at a high temperature is saturated 

 with a salt, and, on being left to cool in a closed vessel, retains in 

 solution a larger quantity of the salt than it could take up at the 

 reduced temperature, the solutionis said to be supersaturated. 



2. History. — During many years the phenomena of supersatura- 

 tion were studied with reference to solutions of Glauber's salt. In 

 1809, Ziz of Mayence* showed that the sudden crystallization of 

 these solutions is not due to agitation ; that the vessels containing 

 the solutions do not require to be hermetically sealed, but if put 

 under a bell-glass, or loosely covered (as with a capsule), they can be 

 preserved during a long time ; that solids brought into contact with 

 the solutions act as nuclei and produce instant crystallization, but 

 that such solids act best as nuclei when dry ; if wet or boiled up 

 with the solution, they become inactive. The most efficient nucleus 

 is a crystal of the salt itself. Air, if artificially dried, ceases to be a 

 nucleus. Three varieties of the sodic sulphate are noticed, i. e. the 

 anhydrous and the ordinary 10-atom hydrate, and also a peculiar 

 salt formed when supersaturated solutions in closed vessels are left to 

 cool down. This x salt, as it is termed, contains less water of crys- 

 tallization than the ordinary salt, and is more soluble. If the vessel 

 in which it is formed be suddenly opened, or the mother-liquor 

 touched with a nucleus, the mother-liquor instantly solidifies into 

 the 10-atom hydrate, and the x crystals become opaque, like the 

 boiled white of an egg. 



In 1819 Gay-Lussacf referred the state of supersaturation to the 

 inertia of the saline molecules, the molecular condition of the sides of 

 the vessel, and other causes. He also showed that solutions of some 

 other salts exhibit the phenomena of supersaturation. In 1832 the 

 number of such salts was shown by Dr. Ogden J to be not less than 

 twenty-one. 



In 1825 Faraday § published some experiments on the supersa- 

 turated solutions of Glauber's salt. Graham ||, Turner U", Ure, and 

 others also contributed new facts ; but the most elaborate inquiry 

 was by M. Lowel between the years 1850 and 1857, the results of 

 which are contained in six memoirs**. According to this writer, the 

 ordinary 10-atom sodic sulphate increases in solubility from 32° to 

 93°* 2 F., at which latter temperature it begins to fuse in its water of 

 crystallization, and to deposit the anhydrous salt. This salt follows 

 an inverse order of solubility as compared with the 10-atom hydrate, 

 its solubility diminishing as the temperature rises ; or, what is the 



* Schweigger's Journal, 1815, vol. xv. 

 t Annates de Chimie et de Physique, 2nd ser. vol. xi. 



% Edinb. New Phil. Journ. § Quarterly Journal of Science, vol. xix. 



|| Trans. Roy. Soc. Edtiib. Tf Elements of Chemistry. 



** Annales de Chimie et de Physique, 3rd ser. vols, xxix., xxxiii., xxxvii.,xliii., 

 xliv., xlix. 



