Mr. J. Gr. Grrcnfell on Supersaturated Saline Solutions. 305 



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 minimum, 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 out- 

 wards. 



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

 calcium chloride : no film was formed, but the drops crystallized 

 very slowly in 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-liquor 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 

 Gernez 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 suppose, and, in fact, they bring that quantity down 

 within 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, producing a strong current of air : most of them did 



Phil Mag. S. 5. Vol. 3. No. 18. April 1877. X 



