MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 65 
Addenda. 
[18th February, 1871.] Since the reading of the foregoing paper and the publica- 
tion of the Abstract thereof in the Proceedings (vol. xviii. p. 533), some points connected 
with Section I. seemed to require further elucidation, which I am permitted to offer in 
this place by way of addenda. 
(1) On the definition of a nucleus. — In the case of a supersaturated saline solution, a 
nucleus is defined as “ a body that has a stronger attraction for the salt of the solution 
than for the liquid which holds it in solution.” This is true in the great majority of 
cases ; but in certain instances, such as those of absolute alcohol and washed ether, the 
nucleus has an attraction for the aqueous rather than the saline molecules of the solu- 
tion. In order therefore to make the definition include such cases, it may be necessary 
to add to the definition, as given above, the following words : — “ or a stronger attrac- 
tion for the liquid than for the salt.” In the case of supersaturated gaseous solutions, 
such as soda-water, Seltzer-water, champagne, &c., it has been shown 11 that a glass rod, 
or other body, covered with a film of foreign matter is active in liberating gas, while 
the same body in a catharized state is inactive. Similar phenomena are exhibited in 
the case of liquids at or near the boiling-point, which I regard as supersaturated solu- 
tions of their own vapour 13 . A catharized body is inactive ; but if such body be drawn 
through the hand or in any other way covered with a film, it immediately produces a 
burst of vapour when placed in the vaporous solution. Hence in saline, gaseous, and 
vaporous supersaturated solutions the action of nuclei is the same, and may fairly come 
under the same definition, subject only, in the case of saline solutions, to the modifi- 
cation above given, and, in the case of vaporous solutions, to the additional fact that 
porous bodies, whether catharized or not, are energetic nuclei. Gathering up these 
various details, a nucleus may be defined as a body that has a preferential attraction for 
the salt or the gas or the vapour of a supersaturated solution, or for the liquid which 
holds any one of the three in solution, separation of the constituents of the solution 
being in either case the result ; only in the case of vaporous solutions, porous bodies act 
as permanent nuclei. 
(2) On the pure substances capable of causing immediate crystallization. — Conclusions 
cannot, obviously, be drawn from liquids that have contracted dust or products of oxida- 
tion &c., since such liquids are generally active as nuclei, whether in a lenticular or a 
filmy form. Fixed oils require to be refined or filtered, volatile oils to be distilled, 
and solid fats to be fused and filtered or passed through flame, alcohol or naphtha 
to be distilled, ether to be washed, and so on. Whatever the body that forms films, it 
must be treated in some way or other that shall ensure the deposit of a pure film on 
the supersaturated saline solution, and not a film containing a small speck of foreign 
matter that is likely to act as a nucleus. Moreover, in trying the action of a film of 
any one substance, the experiment should not he limited to one flask of solution, but to 
11 Phil. Mag. July 1867. 12 Proceedings of the Royal Society, vol. xvii. p. 240. 
MDCCCLXXI. K 
