1868.] Mr. C. Tomlinson on Svpersaiurated Saline Solutions, 407 



cooled down below 60°. Supersaturated solutions of various saltswere cooled 

 down to various temperatures from 32° to 0° F. without crystallizing. Sodic 

 acetate, for example, was kept for some hours at 14°, when on touching it 

 with a wire it became solid, and the temperature rose to 104°. Sodic 

 arseniate, sodic succinate, sodic borate, sodio-potassic tartrate, potash alum, 

 and other saline solutions were treated in this way. Some of these solu- 

 tions become viscid at a low temperature, and do not immediately crys- 

 tallize on removing the cotton-wool plug. If they be touched, or the 

 side of the flask scratched with a chemically clean wire, there is no ac- 

 tion ; but if the wire be not chemically clean, the scratches immediately 

 become chalky white by being covered with minute crystals of the salt, 

 and the action then spreads until the solution becomes solid. 



Some salts that are not very soluble in water, such as the plumbic 

 acetate, form highly charged supersaturated solutions, and retain their 

 liquid state below ordinary atmospheric temperatures. When at a certain 

 point they suddenly solidify. Other solutions merely deposit the excess 

 of salt above the condition of supersaturation, leaving the mother-liquor 

 saturated ; the cupric sulphate is an example of this. 



The memoir contains a number of details respecting the action of nuclei, 

 whether derived from the air, from the flask, from the salt itself, from the 

 filter, or the cotton-wool used in closing the vessels. If the solution touch 

 the wool, crystallization immediately sets in ; or if the upper part of a che- 

 mically clean tube be touched with a finger slightly greasy before filtering 

 into it the hot solution, the latter will cool down to the temperature of the 

 air without crystallizing, nor will there be any efl'ect if the tube be inclined 

 so as to touch the clean portions of the inner surface ; but the moment the 

 solution comes upon the edge of the finger-mark, crystallization sets in, 

 and the solution becomes solid. Solutions not filtered that begin to crys- 

 tallize at above 100° in open vessels, or even in closed flasks, may by filtra- 

 tion be freed from nuclei, and so cooled down in the latter to low tempera- 

 tures 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 solution of sodic sulphate 

 at high temperatures is really a solution of the anhydrous salt. But M. 

 Lowel supposes that a supersaturated solution 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 

 chemically clean by being washed out with spirits of wine instead of sul- 

 phuric acid, and if these vessels, when cold, be placed in water at 32°, or 

 from that to 40°, a few octohedral crystals of the anhydrous salt will be 

 thrown down. The temperature will slightly rise ; and if the tube be now 



