Mr. C. Tomlinson on Supersaturated Saline Solutions. 227 



upon the edge of the finger-mark, crystallization sets in, and the 

 solution becomes solid. Solutions not filtered that begin to crystal- 

 lize at above 100° in open vessels, or even in closed flasks, may by 

 filtration be freed from nuclei, and so cooled down in the latter to low 

 temperatures without any separation of the salt. 



4. On the formation of a modified salt. — The readiness with 

 which sodic sulphate parts with its water of crystallization, and two 

 or three other considerations, make it more than probable that a solu- 

 tion of sodic sulphate at high temperatures is really a solution of the 

 anhydrous salt. But M. Lowel supposes that a supersaturated solu- 

 tion in cooling down below 60° assumes a new molecular constitution, 

 viz. that of the more soluble 7-atom hydrate which it then holds in 

 solution. The author gives an experiment to show that such cannot 

 be the case, but that the solution continues to hold the anhydrous 

 salt until a portion of it actually separates. If a boiling solution of 

 two parts salt to one part water be filtered into vessels made che- 

 mically clean by being washed out with spirits of wine instead of 

 sulphuric acid, and if these vessels, when cold, be placed in water 

 at 32°, or from that to 40°, a few octahedral crystals of the anhy- 

 drous salt will be thrown down. The temperature will slightly rise ; 

 and if the tube be now set aside in a moderately warm air, the anhy- 

 drous salt will enter into solution, forming a dense lower substratum, 

 from which the 7-atom hydrate will be produced in small quantity, 

 there not being sufficient water present to form the ordinary 10-atom 

 salt. The rest of the solution is still supersaturated ; and if the plug 

 be removed from the vessel, crystallization will set in from the sur- 

 face and proceed rapidly downwards, carrying down enough water to 

 convert the whole solution, as well as the 7-atom, into the 10-atom 

 hydrate. 



This process may be conveniently watched in the case of the zinc 

 sulphate. When a saturated solution of this salt cools down from the 

 boiling-point to about 70°, the monohydrated salt is thrown down 

 in quantity, and, as the solution cools, a portion of this dissolves and 

 a crop of acicular crystals is produced which readily melt down at 

 about 100°. On removing the cotton-wool from the tube, crystalli- 

 zation sets in from the surface, and the ordinary 6-atom hydrate is 

 produced. 



The author examines M. Lowel's experiments on solutions of the 

 sodic carbonate in which two modified hydrates are pointed out, viz. 

 the 7 H O a and the 7H O b, which differ in solubility from each other 

 and from the 10-atom salt ; but as M. Lowel attaches great import- 

 ance to the peculiar catalytic properties of the sides of his vessels in 

 determining the formation of these salts, the author cannot help 

 thinking that M. Lowel's results were due to portions of the sides of 

 his vessel, not chemically clean, acting as nuclei. In chemically 

 clean vessels M. Lowel's results have not been reproduced ; for on 

 reducing the temperature to a certain point depending on the strength 

 of the solution, the whole became suddenly solid, with a rise in tem- 

 perature of 35° or 40°. M. Lowel also points out two modifications 

 produced from supersaturated solutions of the magnesia sulphate. 

 The author has placed boiling saturated solutions, when cold, in 



Q2 



