1876.] 



Supersaturated Saline Solutions. 



127 



quesces, forming a strongly supersaturated solution. The anhydrous 

 sulphate also forms a supersaturated solution when added to water, as 

 De Coppet pointed out. 



I touched a drop of the acetate with the point of a penknife ; a little 

 drop crystallized on the penknife, but the drop itself did not. I then 

 repeatedly touched the surface of the drop rapidly with the solidified 

 part and obtained a little rod, formed of separate layers and nearly 

 "I inch long. At last the rod broke in the drop, which instantly crystal- 

 lized. I have repeated this with carbonate of soda. The fact is interest- 

 ing as showing how very local the crystallizing force is. Earadayhad an 

 idea that this force might possibly be transferred by wires ; but I have 

 poured out part of a solution which was crystallizing into a test-tube, 

 where it remained supersaturated. 



Professor Tomlinson has long maintained with great ingenuity the 

 theory that the cause of crystallization in these solutions is adhesion. 

 To a surface covered with a film of greasy matter the salt adheres, while 

 the liquid does not, and therefore separation follows. I do not think 

 that theory can be sustained in the presence of the following facts : — 



1. E-ubbed the finger on the palm of the hand, and took up solution 

 of alum from a drop, and deposited on another part of the same plate ; 

 inactive. 



2. E,ubbed oil on the palm of the hand, and repeated : again inactive. 



3. Smeared oil over a glass plate : inactive to drops of alum. 



4. Eubbed oil on the finger; took up some sodium carbonate, and 

 rubbed it hard on the plate : inactive. 



o. Repeated this with sodium acetate. 



The mere fact, however, that the salts are, as a rule, perfectly insensible 

 to every kind of surface, wood, paint, paper, glass, and dust of all kinds, 

 seems to me fatal to this theory. 



A solution of one part of normal sodium sulphate in about six of sul- 

 phuric acid possesses some curious properties. This solution, which sets 

 quite firm, can be kept for a week in an open beaker, so that the air ap- 

 ' parently has no crystals to introduce ; and yet when dropped on to a 

 dirty surface in my laboratory it more often crystallizes than not. It is 

 thus much more sensitive than an aqueous solution of sodium carbonate 

 or acetate. The crystals are apparently a hydrate of the hyperacid salt 

 NaHa (80^)2 ; and it is almost inconceivable that the dust should con- 

 tain crystals of this salt. It is extremely deliquescent, and the excess of 

 acid should certainly be taken up by the dust, and very often by the 

 surface itself. 



The solution sometimes crystallizes suddenly in the test-tube as though 

 something had fallen in. The crystallized drops will not stand exposure 

 to air for more than 30 minutes or so. Hence, although there is plenty 

 q{ sulphuric acid in my laboratory, where I have often heated this 

 solution, I find it very hard to believe that the salt exists in this form 



