1876.] 



Supersaturated Saline Solutions, 



125 



• the globular forms mixed with little, clear, flat, very thin pointed plates 

 which reminded me much of a particular form of aluminium sulphate. 

 When dry all the drops were brilliant opaque white, and retained a good 

 deal of water. 



Potash alum forms similar films and globular masses. The mother- 

 Hquor of the ammonia alum sometimes slowly deposits short, fine, silky 

 needles with a faint milky tinge and small globular masses. I have only 

 recently adopted the method of using drops, and have not much leisure 

 for working ; but the field is so wide, and the results already obtained 

 have such an important bearing on the theory of the crystallization of 

 these solutions, that I have ventured to put them forward in their present 

 incomplete state. 



The most commonly received theory is that of which M. de Grernez is 

 the most prominent advocate — that only a crystal of the same salt causes 

 crystallization, and that these are introduced by the air, which is a vast 

 storehouse of crystals of all kinds. 



The following experiments seem to support the crystal theory ; but at 

 the same time they clearly show that the quantity of salts present in the 

 atmosphere is indefinitely less than we have hitherto been led to sup- 

 pose, and, in fact, they bring that quantity down T^ ithin the limits of 

 ordinary probability. 



1. Put drops of a very strong solution of sulphate of soda on a plate on 

 my laboratory table ; waved a newspaper over them for some time, pro- 

 ducing a strong current of air : most of them did not crystallize, and 

 one slowly dried up in octahedra. I have repeatedly of late boiled sul- 

 phuric acid in the laboratory, so that there can be no lack of sodium 

 sulphate in the dust. 



2. Drew a strong current of air over drops of sodium sulphate in a 

 glass tube : inactive. 



3. Drops of sodium sulphate put upon the leaves of many plants in 

 my garden. They slowly evaporated, giving the 7-atom salt. The 

 leaves were covered with, dust, as the garden opens on to a road, and the 

 weather has been hot and dry ; we are not far from Bristol, so we might 

 expect to find sulphates. 



4. Carried sodium sulphate to an upper room ; drops on the wash- 

 hand stand, on the window-sill inside and out, on the iron bars outside : 

 all inactive. Washed my hands and spread a drop with the finger on 

 the window-sill, inside : inactive. Three drops crystallized on the 

 mantlepiece, and one on the window-sill. Several drops on the window- 

 frame evaporated as 7-atom salt. 



5. Potash alum on a window-sill outside gave a modified film. 



6. Sodium acetate put upon the cork of a large bottle which had stood 

 for two years untouched in my laboratory. The drops were quite thick 

 with dust, but remained liquid for more than 24 hours. 



7. Other drops of the same put on the floor of the laboratory, on the 



