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XXVII. On Supersaturated Saline Solutions. By Charles Tomlinson, F.R.S. 
Received April 21, — Read May 28, 1868*. 
I. History. 
The phenomena of supersaturation have been studied chiefly with reference to 
solutions of Glauber’s salt. In 1809, Ziz of Mayencef showed that the sudden crys- 
tallization of these solutions is not due to agitation ; that the vessels containing them 
need not 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 con- 
tact with the solutions act as nuclei and produce instant crystallization, especially when 
dry ; if wet or boiled up with the solution they become inactive. The best 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 , the ordinary 10-atom 
hydrate , and also a modified salt formed when supersaturated solutions are left to cool 
down in closed vessels. This salt contains less water of crystallization than the ordinary 
salt, and is more soluble. If the vessel in which it is formed be suddenly opened, or a 
nucleus touch the mother-liquor, this instantly solidifies into the 10-atom hydrate, and 
the crystals of the more soluble salt become opaque, like the boiled white of egg. 
In 1819 Gay-Lussac J 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 besides Glauber’s salt exhibit the phenomena 
of supersaturation. In 1832 the number of such salts was stated by Dr. Ogden § to be 
not less than 21. 
In 1825 Faraday [| published some experiments on the supersaturated solutions of 
Glauber’s salt. Graham % Turner**, Ure, and others also contributed new facts ; but 
the most elaborate inquiry was made by M. Lowel between the years 1850 and 1857, the 
results of which are contained in six memoirsf f . According to this writer, the modified 
salt is formed in consequence of a molecular change which takes place when a solution 
saturated at 218° Fahr. cools down to 64°. The modified salt is much more soluble at 
ordinary temperatures than the 10-atom hydrate, its maximum solubility being at 80 O- 6. 
The sodic sulphate has three maxima of solubility, viz. 93 0- 2 for the 10-atom hydrate, 
78 0, 8 to 80 o, 6 for the 7-atom hydrate, and 62 0, 6 to 64 0- 4 for the anhydrous salt. At 
* Rewritten by the author from the original paper in a more condensed form. 
f Schweigger, Journal for 1815, Band xv. J Annales de Chimie et de Physique, 2nd ser. vol. xi. 
§ Edinburgh New Philosophical Journal. || Quarterly Journal of Science, vol. xix. 
IT Transactions of the Royal Society of Edinburgh. ** Elements of Chemistry. 
ft Annales de Chimie et de Physique, 3rd ser. vol. xxix., xxxiii., xxxvii., xliii., xliv., xlix. 
MDCCCLXVIII. 4 X 
