718 
ON THE FUNCTIONS OF NUCLEI. 
clean surfaces and the solutions, may be seen in this flask. It contains a highly 
supersaturated solution of sodic sulphate resting on a considerable mass of the 
modified 7-atom salt. This being chemically clean, does not act as a nucleus to 
the rest of the solution. I can shake the flask violently, so as to break up and 
rattle about these crystals at the bottom, and the solution does not solidify. 
But now look at this:—I take out the cotton wool, and instantly lines of 
crystals closely packed together, diverge from some point on the surface, and 
the whole saline contents of the flask form one solid mass. 
This differential kind of action which belongs to nuclei may also be exhibited 
with reference to liquids at or near the boiling-point. My definition of such a 
liquid is that it is a supersaturated solution of its own vapour. It is a solution 
of vapour constituted like the solution of carbonic acid in water, or of sodic 
sulphate or of alum in water. 
Such being the case, a body introduced into a liquid at or near the boiling- 
point acts, or does not act, as a nucleus in separating vapour according as it is 
unclean or clean. If clean, the solution adheres to it perfectly and there is no 
separation of vapour; if unclean, the vapour adheres to the surface as strongly 
as before, but the liquid adheres less strongly or not at all, and hence there is a 
separation of vapour on the surface of the nucleus. 
Now you are all acquainted with a very unpleasant phenomenon that often 
accompanies the processes of boiling and distillation: I mean the bumping of the 
vessel, or jumping ebullition (soubresauts in the language of French science). In 
such cases the liquid makes the interior of the vessel clean, and the solution of 
vapour adheres to it with great force ; the temperature rises above the boiling- 
point until the liquid, not being able to dissolve any more vapour, a portion of the 
latter escapes with a burst along the line of least resistance or towards the opeuing 
of the vessel. This upward action is accompanied by an equal reaction in a con¬ 
trary direction, which tends to force the retort or flask further into the ring 
of the retort stand, or down upon any other support, and it is the rebound from 
this that causes the soubresaut. Now for the purpose of preventing or miti¬ 
gating this inconvenient and even dangerous action, it is the practice to intro¬ 
duce into the vessel certain angular or rough bodies, such as sand, powdered 
glass, bits of tobacco-pipe, platinum, copper, etc. Such bodies have been called 
‘•promoters of vaporization.” How they act has not been explained, but it has 
often been noticed that they cease to act in liberating vapour after prolonged 
contact with the boiling liquid. 
I need scarcely say that such bodies cease to act as nuclei in liberating vapour 
from solution as soon as they become chemically clean. 
Now here is water boiling in a clean flask with some difficulty and a disposi¬ 
tion towards bumping. I put into the flask a pinch of sand from the sand bath, 
and the boiling immediately becomes soft and regular, and there is no tendency 
whatever to bumping. 
But suppose we make the sand chemically clean by heating it in contact with 
strong sulphuric acid, rinsing with water, washing with alkali, and again rinsing. 
Such sand, in very small quantity, in a chemically clean flask, containing clean 
distilled water, will increase the bumping to a remarkable extent. Here is such 
a flask standing over a spirit lamp. You see that it bounds up from the sup¬ 
port with explosive force, and one wonders the flask is not shivered to pieces. 
The bits of glass, etc., put into a retort containing alcohol, ether, etc., soon be¬ 
have in this way, for the liquid makes them, as well as the interior of the vessel, 
chemically clean, and in such case we multiply the adhesion surfaces, when 
what is wanted is an extension of the vapour-giving surfaces. 
it is clearly wrong to employ as nuclei bodies that soon cease to act as such, 
supposing it were possible to find permanent nuclei. Porous bodies, such as 
charcoal, coke, pumice-stone, etc., perforin this useful function. The same 
