502 Mr. A. Liversidge on Supersaturated Saline Solutions . [June 20, 



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 in contact with the supersaturated 

 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 exposing 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 super- 

 saturated solution from nuclei. 



The principal substances made use of by the writer for the formation of 

 thin films were as follows : — Citronella-oil, olive-oil, Russian tallow, castor- 

 oil, camphor in alcohol, creosote, turpentine, benzol, 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 sulphate 

 crystals : — 



1 . The anhydrous salt (Na 2 SO,), crystallizing in octahedra, and depo- 

 sited 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 0), containing 7H 2 0, formed in a 

 supersaturated solution by reduction of temperature and other causes ; these 

 also are inactive, and admitted so by all. 



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

 dral summits, and containing 10H 2 O. Usually regarded as the best nu- 

 cleus. 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 ex- 

 posing it to the air, &c. 



Experiments were made with recently generated crystals of the normal 

 salt. 



Exp. Two beakers, containing fully supersaturated solutions, were covered 

 with watch-glasses, and allowed to cool ; in one of the beakers a small 

 glass bucket, attached to a thread, had been placed and boiled up with the 

 solution. Next, both beakers were arranged under a large bell-jar, and the 

 silk thread from the bucket passed up between the stopper and the neck 

 of the jar. The sol ations were then uncovered, after waiting ten minutes 

 for any nuclei which might have been disturbed to fall ; a fine wire was 

 passed down into the beaker containing the bucket, and as far as possible 



