Geological Society. 225 



or, in other words, as they become more highly supersaturated. 

 Thus a very strong solution of calcichloride, which is not sensitive 

 to nuclei at 40° or 45°, becomes very much so at 24° or 34°. 



The sodio-zincic sulphate contains only 4 proportionals of water 

 of crystallization ; and hence its supersaturated solutions are not 

 stable at low temperatures. When freshly made, they may be re- 

 duced to 10° Fahr. without separation of the salt; but by repose, 

 even in clean tubes and in the absence of nuclei, long crystals of 

 the separated salts occupy the length of the tube, but they are in- 

 visible on account of having the same refractive index as that of the 

 solution in which they are immersed. In the course of time, pro- 

 bably from the escape of vapour of water through the porous plug, 

 they become visible. 



A solution of the ammonia zincic sulphate at 4° Fahr. formed 

 beautiful large feathery crystals of an opaque white, which gradually 

 filled the tube. They melted rapidly at 32°. 



A supersaturated solution of nickel sulphate resisted a tempera- 

 ture of 6° Fahr. Mixed with an equivalent weight of cupric sulphate, 

 the two salts separate if the solution is exposed to the air ; but in 

 closed tubes the solution at 0° Fahr. forms beautiful feathery crys- 

 tals, which melt rapidly at 32°, without any separation of salt. 



Similar phenomena are produced by a supersaturated solution of 

 zinc sulphate and potash alum in equivalent proportions exposed to 

 a temperature of 4° Fahr. A similar solution of the cupric and 

 magnesic sulphates at — 4° also became solid, and melted rapidly 

 at 32°. 



Experiments were also made with the sodic and magnesic sulphates, 

 cadmic, and some other sulphates. The addition of potassic sulphate 

 to other sulphates, in atomic proportions, forms double salts, which, 

 so far as they were examined, do not form supersaturated solutions. 



The effect of low temperatures was in some cases to throw down 

 a portion ^of the salts in the anhydrous form, upon which were 

 formed by repose crystals of a lower degree of hydration than the 

 normal salt. Some cases of this kind are described in the paper. 



GEOLOGICAL SOCIETY. 



[Continued from p. 142.] 



March 9th, 1870.— Warington W. Smyth, Esq., F.E.S., 

 Yice-President, in the Chair. 



The following communications were read : — 



1. " On the Structure of a Fern-stem from the Lower Eocene of 

 Heme Bay, and on its allies, recent and fossil." By W. Carruthers, 

 Esq., F.L.S., F.G.S. 



The author described the characters of the fossil stem of a Fern 

 obtained by George Dowker, Esq., F.G.S. , from the beach at Heme 

 Bay, and stated that in its structure it agreed most closely with the 

 living Osmunda regalis, and certainly belonged to the Osmundacca:. 



