Mr. A. Liversidge on Supersaturated Saline Solutions. 71 



rod ; and when the last flask of all was proved, it had stood for rather 

 more than eighteen months. 



One explanation accounting for the activity of the thin film as 

 prepared by the eminent author of the paper referred to maybe this: — 

 That in order to place the oil upon the solution, the flask was opened 

 and exposed to the air, thus affording an opportunity for nuclei to gain 

 entrance ; and also they may have been carried in by the greasy rod 

 itself, for there would be plenty of time in its passage for it to pick 

 nuclei up : such nuclear bodies would probably float upon the surface 

 of the disk or globules of oil, and would not come into contact with 

 the solution itself; neither might they touch its surface even when 

 the disk was broken up into small globules, for these globules would 

 be immensely large in comparison with the dimensions of the nucleus 

 itself; but, on the other hand, when the disk was flattened out into 

 an iridescent film, and therefore one of excessive tenuity, the nuclei 

 might then easily fall through it, come into contact with the supersa- 

 turated solution, and start its crystallization. As it is probable that 

 several nuclei would enter at the same time, they would naturally 

 become dispersed by the jerk, and hence crystallization would be set 

 up at various points. 



That nuclei will pass through the substance of a thin film is shown 

 by the solidification which almost immediately takes place on expo- 

 sing to air the solution covered merely by a film of oil, turpentine, 

 &c. ; a thick coating of oil is, of course, one of the best means we 

 have of protecting a supersaturated solution from nuclei. 



The principal substances made use of by the writer for the forma- 

 tion of thin films were as follows: — Citronella-oil, olive-oil, Russian 

 tallow, castor-oil, camphor in alcohol, creosote, turpentine, benzole, 

 chloroform, ether, &c. 



Concerning the action of a crystal of the normal sodic sulphate 



upon a supersaturated solution of the same. 

 It is well known that there are three modifications of sodic sul- 

 phate crystals : — 



1. The anhydrous salt (Na 2 S0 4 ), crystallizing in octahedra, and 

 deposited from a supersaturated solution on further concentration ; 

 these crystals are inactive to a supersaturated solution. 



2. The modified salt (Na 2 S0 4 , 7H 2 O), containing 7H 2 0, formed 

 in a supersaturated solution by reduction of temperature and other 

 causes; these also are inactive, and admitted to be so by all. 



3. The normal salt (Na 2 S0 4 , 10H 2 O), crystallizing in prisms with 

 dihedral summits, and containing 10H 2 O. Usually regarded as the 

 best nucleus. Experiments relating to its behaviour as such will be 

 detailed. 



It is always the normal salt (Na 2 S0 4 , 10H 2 O) which is formed 

 when a solution is caused to crystallize by touching it with a dirty 

 rod or by exposing it to the air, &c. 



Experiments were made with recently generated crystals of the 

 normal salt. 



Exp. Two beakers, containing fully supersaturated solutions, were 



