228 Royal Society : — 



freezing-mixtures at 10° without producing any separation of the 

 salt. 



The ammonia phosphate throws down from its supersaturated so- 

 lution an anhydrous powder, which, again entering into solution, 

 forms a dense lower stratum in which a modified transparent crystal- 

 lized 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 sud- 

 denly become solid ; others freeze and sometimes thaw again without 

 any separation 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 dow T n to near 0° F. 

 without any separation of the salt. 



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

 an anhydrous salt forms a supersaturated solution was to make a 

 solution of known strength, as indicated by some good Table of so- 

 lubilities, raise it to the boiling-point, and then note whether salt 

 began to be thrown down when the solution cooled down to the tem- 

 perature indicated by the Table. For example, according to Pog- 

 giale'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 boiling-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 G° or 8° earlier than if the flask stood 

 on a block of wood. 



It has been frequently stated that the potassic bichromate forms a 

 supersaturated 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 crystalline flakes at 

 138°. The remarkable deepening in colour of this solution under the 

 influence of heat is pointed out. 



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

 amined, the conclusion being that anhydrous salts do not form su- 

 persaturated solutions. 



6. Conclusion and Summary.— ■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 su- 

 persaturation ; and it has only been claimed in the case of a very few 

 salts, and in them much importance 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, 



