404 Mr. C. Tomlinson on Supersaturated Saline Solutions, [May 28, 



X salt, as it is termed, contains less water of crystallization than the ordi- 

 nary salt, and is more soluble. If the vessel in which it is formed be sud- 

 denly opened, or the mother-liquor touched with a nucleus, the mother- 

 liquor instantly solidifies into the 10-atoni hydrate, and the x crystals be- 

 come opaque, like the boiled white of an egg. 



In 1819 Gay-Lussac* referred the state of supersafuration 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 exhi- 

 bit the phenomena of supersaturation. In 1832 the number of such 

 salts was shown by Dr. Ogdenf to be not less that twenty-one. 



In 1825 Faraday 1 published some experiments on the supersaturated 

 solutions of Glauber's salt. Graham §, Turner 1|, Ure, and others also con- 

 tributed 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 sul- 

 phate increases in solubility from 32° to 93°'2 F., at which latter tempera- 

 ture it begins to fuse in its water of crystallization, and to deposit the an- 

 hydrous salt. This salt follows an inverse order of solubihty as compared 

 with the 10-atom hydrate, its solubility diminishing as the temperature 

 rises; or, what is the same thing, from 218°, the boihng-point of a satu- 

 rated solution, down to 64° the solubihty increases ; but at 64° the solution 

 undergoes a new molecular modification, and begins to form crystals of the 

 7-atom hydrate (the x salt of Ziz) . This salt is much more soluble at or- 

 dinary temperatures than the 10-atom hydrate, its maximum solubihty being 

 at 80°*6. Thus the sodic sulphate has three maxima of solubility; viz. 

 93°'2 when it is under the molecular constitution of the 10-atom hydrate, 

 78°-8 to 80°' 6 when it is under the molecular constitution of the 7-atom 

 hydrate, and 62°*6 to 64°'4 when it is under the molecular constitution of 

 the anhydrous salt. At these three maxima the saturated solutions are 

 about equally rich in salt. The 7'atom hydrate and the anhydrous salt 

 can only maintain their molecular constitution when in contact with the 

 mother-liquor in closed vessels, in which they are sheltered from the air 

 and from other bodies that act upon them as nuclei. No sooner are they 

 exposed to the air than they become opaque and warm, and assume the mo- 

 lecular constitution of the 10-atom hydrate as well as its solubility. Hence 

 the conclusion is that supersaturated solutions of the sodic sulphate are not 

 really so, since they hold a salt of much greater solubility at ordinary tem- 

 peratures than the normal 10-atom salt. Lowel extends his inquiry to 

 sodic carbonate and magnesia sulphate, and endeavours to show that in their 

 supersaturated solutions salts of a lower degree of hydration and of greater 

 solubility than the normal salts are formed ; and his general conclusion is 



" Annales de Chimie et de Physique, 2nd ser. vol. xi. 1' Edinb. New Phil. Jouvn. 

 X Quarterly Journal of Science, vol. xix. § Trans. Roy. Soc. Edinb. 



II Elements of Chemistry. 



% Annales de Chimie et de Physique, 3rd ser. vols, xxix., xxxiii., xxxvii., xliii., xliv,, xlix. 



