3B68.] Mr. C. Tomlinson on Supersaturated Saline Solutions. 409 



the boiling-point, and then note whether salt began to be thrown down 

 when the solution cooled down to the temperature indicated by the Table. 

 For example, according to Poggiale's Table, 100 parts of water at 158° will 

 dissolve 129*6 of sodic nitrate. This is the same thing as 622*22 grains of 

 the salt in 1 ounce water. Such a solution on cooling down from the boil- 

 ing-point began to deposit salt at 160°. 



^ In like manner, according to Gay-Lussac's Table, 100 parts of water at 

 150° F. contain 125 of potassic nitrate. A solution of 125 parts salt to 

 100 of water began to deposit salt at about 149°. The deposit first began 

 to be made on the side nearest the window, or the coldest side, when the 

 flask was suspended in air ; but if the flask were placed on metal, or any 

 other good conductor, a ring of salt was first formed at the bottom, some 

 6° or 8° earlier than if the flask stood on a block of wood. 



It has been frequently stated that the potassic bichromate forms a super- 

 saturated solution. According to Kremer, 200 of water at 140° F. dissolve 

 100 parts of the salt. Such a solution, on cooling from the boiling-point 

 began to throw down crystalhne flakes at 138°. The remarkable deepen- 

 ing in colour of this solution under the influence of heat is pointed out. 



Sal-ammoniac, potassic chlorate, and some other salts were also examined, 

 the conclusion being that anhydrous salts do not form supersaturated so- 

 lutions. 



6. Conclusion and Bummary. — The author refers to the prevailing theory 

 that supersaturation exists in appearance only and not in fact, since it is 

 supposed to be the modified and more soluble salt that is in solution. If 

 this were true, it ought to apply to all cases of supersaturation, and it has 

 only been claimed in the case of a very few salts, and in them much im- 

 portance has been attached to the active or the inactive condition of the 

 sides of the vessels containing the solutions. 



The author, while admitting, in the case of a very few solutions, that a 

 modified salt may be deposited, denies that it is due to any molecular change 

 that takes place in the solution, either from reduction of temperature or 

 any catalytic property of the sides of the vessel. His theory is that when 

 these modified salts are formed, it is the anhydrous salt that is held in so- 

 lution, a portion of which is thrown down as the temperature falls ; and 

 this anhydrous deposit, entering again into solution, forms a dense substra- 

 tum containing less water than the upper portions, so that when the modi- 

 fied salt forms in it, it is out of the reach of suflicient water to form the 

 normal salt. Wlien, on the contrary, under the influence of a nucleus, 

 crystallization sets in from the surface, the normal salt is formed, and the 

 crystals carry down sufficient water to convert the whole into the ordinary 

 hydrated salt. 



As to the action of nuclei or the sides of the vessel, when chemically 

 clean the solution adheres to them as a whole, and there is no separation 

 of the salt ; when not chemically clean there is a stronger adhesion between 

 the salt and the nucleus than between the salt and the solvent, and there 



