on Supersaturated Saline Solutions, 299 



sulphate of soda, is of course quite different from the tetrahedral 

 crystals already noticed, being much more permanent, even at 

 atmospheric temperatures, and not changing so long as it is co- 

 vered by the solution. But, like the seven-atom sulphate of 

 soda, it cannot exist if exposed to the air ; for it immediately 

 fixes an additional quantity of water and becomes the normal salt. 



The sulphate of zinc and soda, as also the sulphate of zinc and 

 cadmium, also form supersaturated solutions which at about 

 10° F. form compounds resembling the tetrahedral crystals, of a 

 silky texture, which at about 50° rapidly melt, leaving the solu- 

 tions clear and bright as before. At a lower temperature the 

 sulphate of zinc and soda forms crystals of a peculiar character, 

 which will be noticed in another paper. 



By varying the strength of the solutions and the temperature 

 to which they are reduced, as well as the time of exposure to 

 cold, varied results may be obtained. Biidorff* showed some 

 years ago that, by employing saturated solutions of single salts 

 andreducingthem to low temperatures, ice is formed togetherwith a 

 hydrated compound of the salt in question. In my experiments 

 with supersaturated solutions in close vessels chemically clean, 

 the conditions are of course quite different, and no ice is formed. 

 In some cases the solutions became viscid like syrup ; but they 

 never froze. For example, sulphate of soda and potash-alum 

 in atomic proportions formed in an evaporating- dish an amor- 

 phous mass containing 55 per cent, of water of crystalliza- 

 tion. A portion of this wetted with a few drops of water can by 

 gradual heating be raised to the boiling-point without depositing 

 the anhydrous salt. When filtered into tubes it may be kept for 

 hours at about 0° F., when it becomes very viscid without any de- 

 posit of salt. Under the influence of a nucleus the solution crys- 

 tallizes, the soda- salt apparently separating from the alum in the 

 process. 



In some cases the results vary, unless the double salt is first 

 formed before a supersaturated solution is made with it. For 

 example, 246 # 3 grains of sulphate of magnesia in large crystals 

 and 322 grains of sulphate of soda, dry but not effloresced, and 

 -| oz. of water were heated to boiling and filtered into two tubes. 

 At about C the sulphate of soda crystallized separately in long 

 lines, after which the magnesia-salt attached itself to the sides 

 and grew upwards like a vegetable. The tubes were again heated to 

 near boiling, and the solution was poured into a small evapora- 

 ting- dish and placed on hot sand. Crystalline scales formed on 

 the surface, consisting of a central boss surrounded by a flat 

 ring with radial markings directed to the centre, and this by an- 



* Jahresber. der Chemie, 1862, p. 20. 

 X2 



