121 



Mr. J. G. Greiifell on 



[June 15, 



VIII. " On Supersaturated Saline Solutions.^^ By J. G. Grenfell, 

 B.A., F.G.S. Communicated by C. Tomlinson, F.R.S. Re- 

 ceived June 6, 1876. 



In making experiments on the sensitiveness of supersaturated solutions 

 to air and greasy surfaces, I was much annoyed by the solutions so 

 frequently crystallizing on the removal of the cotton-wool, as this 

 necessitated boiling the flask again and waiting till it was cool. I 

 noticed that frequently part of the cotton-wool adhered to the mouth of 

 the flask ; and it struck me that in removing this some fibres must get 

 detached and fall in, carrying with them in all probability crystals of the 

 salt. I soon convinced myself that this was the case, and that cotton- 

 wool is perhaps the worst material that could be chosen for covering 

 these solutions. I now always use paper or tinfoil ; and I find that these 

 can be removed many times from the same solution without inducing 

 crystallization. I then found that even the most sensitive solutions 

 could be taken up in a clean glass tube and dropped on a clean glass 

 plate without crystallizing ; and that they will remain liquid exposed to 

 the air for a very long time, often, in fact, till they dry up by evaporation 

 in modified forms. Twenty drops on a plate give twenty experiments 

 on the effect of air, clean and unclean surfaces, and evaporation ; then 

 the plate is cleaned, and more drops are taken from the original solution 

 till this is used up. The trouble of boiling is thus reduced to a mini- 

 mum, and the drops can be put upon all kinds of surfaces to test their 

 activity. The slow growth of the modified salts can be watched for 

 hours ; and their forms are sometimes peculiar. Thus sulphate of soda 

 often gives a single, square, flat pyramid, or a broad well-shaped prism, or 

 occasionally small octahedra round the edge of the drop. The pyramids 

 and prisms change to opaque white when touched, and are apparently 

 the 7-atom salt ; the octahedra do not change, and are evidently the 

 anhydrous salt. This fact is interesting, from its supporting the view 

 that it is the anhydrous salt which is in solution. 



Or, again, a plate with drops may be dried over calcium chloride ; and 

 this sometimes modifies the results, as in the case of ammonia alum. 

 This salt, when allowed to evaporate in air, generally forms a shining 

 semitransparent film of greenish colour with a depression at the top, in 

 which is often a circular opening, while inside small globular concretions 

 of a dull, opaque, milky white colour are formed ; these will remain 

 moist inside for a couple of days or more. When touched with the 

 normal salt, the whole drop becomes brilliant opaque white, quite dry, 

 and apparently increases in volume, as the crust often breaks up and 

 curls outwards. 



This modified salt is apparently new. I put some drops over calcium 

 chloride : no film was formed, but the drops crystallized very slowly in 



