on Supersaturated Saline Solutions. 297 



solution of sulphate of iron gave tetrahedral crystals, while the 

 sulphate of zinc and ammonia at 4° formed beautiful feathery 

 crystals. The solution of the last-named double salt consisted 

 of 80 grains of the salt, previously crystallized in an open dish 

 and dissolved in § of an ounce of water. In this case one of the 

 salts, although anhydrous, did not interfere with the formation of 

 a supersaturated solution ; but sulphate of potash, from its compa- 

 ratively small solubility, does so interferein all the cases I have tried. 



I think there can be little doubt that the tetrahedral crys- 

 tals referred to are hydrated compounds of the salts in solution ; 

 and when such salts form supersaturated solutions, they exist, 

 as I believe, in the anhydrous form. When such solutions are 

 considerably reduced in temperature, the water of the solution 

 cannot crystallize and form ice, because the saline molecules are 

 too numerous to be excluded from the aqueous molecules ; but 

 the saline molecules, being brought by the low temperature 

 within the range of their mutual attractions, form crystalline 

 masses enclosing the water which suspends and surrounds them. 



I have not been able to determine the amount of hydration 

 of these tetrahedral crystals. The problem is a difficult one, 

 how to examine crystals which apparently owe their existence 

 to a low temperature and the absence of nuclei. Perhaps 

 some member of the Section may be able to give me a hint on 

 the subject. I may mention that in one case, soon after the 

 tetrahedral crystals had begun to form in the middle of the tube 

 and attached to its side, the tube was taken out of the freezing- 

 mixture and held in air at about 50°. The crystals split up, 

 and threw down to the bottom of the tube a quantity of anhy- 

 drous powder, which immediately began to combine with water 

 and to rise in temperature, so that, on putting the tube into snow 

 and water, it immediately acquired a thick coating of ice, except 

 at the bottom, where, to the height of about a quarter of an inch, 

 the tube was quite free from ice. 



When a supersaturated solution of sulphate of soda is put 

 into a freezing- mixture, it throws down octahedral crystals of 

 the anhydrous salt, which take up water and form the abnormal 

 seven-atom salt. This cannot be exposed to the air without 

 heating and fixing three additional equivalents of water. Highly 

 supersaturated solutions begin to throw down the anhydrous 

 salt at various temperatures below 60°. But if undisturbed, it 

 may happen that such a solution will resist a temperature of 

 18° or 20° F. without any apparent change. During the last 

 winter I had a globular flask full of a solution (3 salt to 1 

 water), and a portion of it occupied the neck. It was placed 

 over night on the window-ledge, and next morning the register- 

 thermometer outside showed a minimum of 18°. There was no 



Phil. Mag. S. 4. Vol. 40. No. 267. Oct. 1870. X 



