410 Transactions, — Cheviistri/t 



a useful contribution to those who have taken this question as an especial 

 subject for research. 

 These results are : — 



1st. That crystals of the common hydrated sulphate of copper, epsom 

 salts, nitrate of baryta, borax, and glauber salts, after being sluiced with 

 water, and while still wet all over, do not act as nuclei in a super- saturated 

 solution of sodic acetate Avhen applied thereto. 



2nd. That on the other hand a crystal of sodic acetate similarly treated 

 (as above) does act as a nucleus with such solutions. 



3rd. That cliloride of sodium does not when sluiced and mixed while 

 • wet with a super-saturated solution of alum, act as a nucleus thereto, but 

 that a crystal of alum similarly treated does. 



4th. That the formation of solid sulphate of baryta formed in any of 

 the super- saturated solutions of a sulphate (by adding baric-chloride thereto) 

 is not attended or followed upon by the crystallization of such solution. 



5th. That the electro-deposition of copper in a super-saturated solution 

 of either sodic acetate or alum (charged feebly with cupric sulphate) is also 

 unattended by crystallization in either case. 



These results show that only those salts* which are of the same kind as 

 that of the super-saturated solution, are nuclear thereto, and they also tend 

 to show that solids having no affinity for water have not any nuclear effect 

 in such solutions, not even when applied thereto as freshly prepared, per- 

 haps I may correctly say in a nascent state. 



It will be seen, therefore, that so far as these results are indicative of 

 anything at all, their general character is such as to sustain De Gernezf 

 in his assumption that " sudden crystallization of a super-saturated solution 

 is in all cases induced only by contact with a crystal of the same salt" 

 (allowing, of course, that he excludes from his cases those of the oils and 

 alcohols, also salts, having notable affinities for water) ; and so far they are 

 antagonistic to the results of Jeannel, as stated in the same work, viz., that 

 any solid substance applied to such a solution of sodic-acetate causes crystal- 

 lization, and so tends to upset the theory by which he seeks to explam the 

 phenomena in question. 



A due consideration of all which has been now advanced on this subject 

 will, I think, have the effect of inclining you strongly to the followmg 

 general deductions which I draw therefrom : — 



1. That the only substance which acts directly upon a super-satm-ated 

 solution as a nucleus, is a salt of the same kind as that of such solution, 



* Only those salts are indicated here which have feeble affinities for water ; it is 



always understood, I believe, that those having strong affinities for it are left out of the 



question. 



t Watts' Dictionary of Chemistry, Vol. V., p. 350. 



