68 ME. CHAELES TOMLINSON ON SUPEESATUEATED SALINE SOLUTIONS. 
but when crystallization does take place it will be found that a small fly, a speck of soot, &c. 
has entered the flask and acted as a nucleus. The finger, if cleaned in caustic alkali 
or alcohol, or by drawing it through the flame of a spirit-lamp, may be held in a super- 
saturated saline solution for some time without inducing crystallization ; but if the 
finger while in this condition be drawn along the side of the glass so as to produce a 
smear, such smear is immediately active, the skin supplying the matter of the active 
film. 
(6) Is it true that a fully hydrated salt, chemically clean, is not a nucleus to its super- 
saturated solution ? — In my first paper on Supersaturated Saline Solutions 15 it was 
shown that a crystal is not necessarily a nucleus to a saturated saline solution of its own 
kind. For example, a supersaturated solution of Glauber’s salt, by long keeping, parts 
with a portion of its water by evaporation through the cotton-wool plug, and crystals 
are formed on the glass above the solution ; on washing these down they do not act as 
nuclei : so also a cold supersaturated solution of magnesic sulphate in vacuo over sul- 
phuric acid forms, also by evaporation, crystalline crusts on the surface, and these crusts 
are not nuclear. Cases of other salts have been urged where such a crystal becomes a 
nucleus, and the old fact has been restated that a salt of a lower degree of hydration is 
not a nucleus to a supersaturated solution of a salt of a higher degree of hydration. 
What is wanted is to show that the sodic sulphate and the magnesic sulphate, formed 
as above described, are not the normal salts, but salts of a lower degree of hydration ; 
and this has not yet been done. 
15 Philosophical Transactions, for 1868, p. 665. 
