408 Mr. C. Tomlinson on Supersaturated Saline Solutions. [May 28^ 



set aside in a moderately warm air, the anhydrous salt will enter into solu- 

 tion, 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 super- 

 Saturated, and if the plug be removed from the vessel, crystalhzation will 

 set in from the surface 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 sul- 

 phate. 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, crystallization 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 HO 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 importance to the peculiar cata- 

 lytic 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 re- 

 produced ; for on reducing the tem.perature to a certain point depending on 

 the strength of the solution, the whole became suddenly sohd, with a rise 

 in temperature of 35° or 40°. M, Lowel also points out two modifications 

 produced from supersaturated solutions of the magnesia sulphate. The 

 author has placed boihng saturated solutions, v/hen cold, in freezing-mix- 

 tures at 10° without producing any separation of the salt. 



The ammonia phosphate throws down from its supersaturated solution 

 an anhydrous powder, which, again entering into solution, form.s a dense 

 lower stratum in which a modified transparent crystallized salt is formed in 

 small quantity. 



The strontic nitrate also deposits an anhydrous salt in cooling down to 

 about 62° ; but as this salt is not soluble in the solution, the modified salt 

 is not formed. 



Some solutions on being cooled down in freezing-mixtures suddenly be- 

 come solid ; others freeze and sometimes thav^ again without any separa- 

 tion of the salt, as in the case of the cupric sulphate ; but if a boiling 

 saturated solution of this salt be prepared with strict attention to chemical 

 purity, it may be cooled down to near 0° F. without any separation of the 

 salt. 



5 . Anhydrous Salts. — The method adopted to ascertain whether an an- 

 hydrous salt forms a supersaturated solution was to make a solution of 

 known strength, as indicated by some good Table of solubilities, raise it to 



