Mr. J. Gr. Grenfcll on Supersaturated Saline Solutions. 307 



Sodium sulphate crystallizes generally on a clean plate exposed 

 in my laboratory as 10-atom salt, whilst if protected by an inverted 

 beaker it dries up by evaporation, formiug the modi tied salt. So, 

 again, I have had two drops of sodium sulphate liquid all night, 

 and both crystallize within ten minutes of my entering the room 

 in the morning. In my bedroom, however, I left a test-tube 

 containing this solution open all night with the pipette on the 

 mantlepiece. In the morning the solution had not crystallized, 

 while the end of the pipette was covered with a white incrustation, 

 which was inactive in the liquid. The incrustation was again 

 left to dry up, and then contained plenty of water, being evidently 

 the 7-atom salt. 



JFor sodium acetate and carbonate it is quite useless to have any 

 cover on the flask or test-tube which contains them, and also for the 

 sulphate in an ordinary room. Care must be taken that crystals 

 are not formed near the mouth of the tube, so as to fall in ; but that 

 is the only precaution necessary. Carbonate of soda by evaporation 

 becomes oily like sodium and potassium acetates. I have not yet 

 investigated the composition of the films and crystals which these 

 solutions deposit. 



Normal sodium acetate when heated leaves a white mass which 

 deliquesces, forming a strongly supersaturated solution. The an- 

 hydrous 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 -| inch long. At last the rod broke in the drop 

 which instantly crystallized. I have repeated this with carbonate 

 of soda. The fact is interesting as showing how very local the crys- 

 tallizing force is. Faraday had an idea that this force might pos- 

 sibly 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 pre- 

 sence of the following facts : — 



1. Rubbed the finger on the palm of the hand, and took up solu- 

 tion of alum from a drop, and deposited on another part of the 

 same plate : inactive. 



2. Rubbed oil on the palm of the hand, and repeated : again in- 

 active. 



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



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

 and rubbed it hard on the plate : inactive. 



5. Repeated this with sodium acetate. 



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