664 MR. CHARLES TOMLINSON ON SUPERSATURATED SALINE SOLUTIONS. 
The sodic arseniate may be taken as another example. Two parts of this salt to three 
of water were boiled at 223°, and filtered into a clean bottle with a thermometer passing 
through the plug. As the solution cooled it became viscid ; but at 35° there was no 
sign of crystallization. The thermometer was taken out of the flask for an instant, and 
then reinserted. The effect was striking. The thermometer had caught up some che- 
mically unclean particles from the air, and these acted as nuclei. The whole of the 
immersed portion of the thermometer became at once covered with crystalline tufts 
which propagated their action, and the temperature rose to 54°. 
Cases like these may be multiplied ; for it must not be supposed that I am dealing 
with exceptional instances with respect to these motes and particles. The air of every 
house is full of them (as a sunbeam often reveals) ; they come in contact with solutions 
and start crystallization, or they enter by the necks of flasks and bottles, and render 
their inner surfaces “ active.” 
The cotton-wool used in plugging the vessels is not chemically clean ; it induces crys- 
tallization if the solution touch it for an instant, as it may sometimes do on inclining 
or shaking the flask for the purpose of watching the effects of cooling. We can also 
induce crystallization in any part of the vessel by making that part chemically unclean. 
For example, a large tube was washed out with a caustic alkaline solution, rinsed first 
with water, then with spirit, and again with water, when a boiling solution of sodic 
phosphate was poured into it, and in another experiment sodic sulphate (5 salt to 
1 water) ; but before plugging it, the upper part of the inside of the tube was touched 
with a finger slightly greased. When cold the solution was brought into contact with 
different parts of the tube that were clean, without producing crystallization ; but the 
moment the solution came upon the edge of the finger-mark, crystallization set in, and 
the solution became solid. 
I believe this experiment will explain the statement so often made as to the necessity 
of keeping supersaturated solutions quite still ; for if stirred or agitated they suddenly 
deposit the excess of salt. In such cases, no care having been taken to produce che- 
mically clean surfaces, the solution itself does something in that direction to the extent 
that it occupies in the vessel ; but agitation brings a portion of the solution against the 
upper and more “ active” part of the sides, or the rod used for stirring is equally 
“ active,” and either may perform the part of a nucleus accordingly. 
The salt used for making a solution is likely to be unclean. During its formation in 
an open evaporating-dish, and its subsequent handling, packing, &c., it contracts vari- 
ous impurities, among which may often be found hairs and other greasy substances, 
which do not always lose their property of becoming nuclei during the boiling of the 
solution. They may be removed by filtration; but as they often contaminate the 
vessel in which the salt is boiled, it is desirable to filter the hot solution into a 
second clean flask, in which it may be boiled up again. In the case of the acetates, 
a few drops of acetic acid should be added to the second flask to make up for any 
loss during the boiling. But in such case the temperature may pass beyond the 
