TRANSACTIONS OF THE SECTIONS. 45 
one yolume of hydrogen ; the gases, therefore, do not seem to he taken up accord- 
ing to their atomic weights. By attention to this, he hoped that some light would 
be thrown on the physical atomic constitution of bodies. , 
In a practical point of view, he hoped to gain by the inquiry some knowledge 
of the phenomena of spontaneous combustion to which several substances are sub- 
jected. 
Experiments on the absorption of mixed gases had been made, but were left for 
future description, and also those on the extrusion of one gas by another. 
Experiments on salts were not sufficiently telling, and a better mode of making 
them had to be found; but it was clear to him that the charcoal took up most 
readily those oxides the metals of which were less inclined to oxidize. The com- 
binations being weaker, the bases were removed from the acid, a struggle against 
chemical action existing. In other words, an action with little chemical character 
opposed itself to the purely chemical, and by aid of mass gained something, a phe- 
nomenon which is frequent whenever chemical action is weak, and one which in- 
terferes much with exactness in analysis. 
On the Action of Nuclei in inducing Crystallization. 
By Cuarres Tomirnson, F.R.S. 
It had been noticed during the last three-quarters of a century that solids acted 
as nuclei in liberating gases from their solutions (soda-water, champagne, &c.), 
or in inducing crystallization in saline solutions, only under certain conditions. If 
they had been previously exposed to the air, they were “active ;” if kept in water, 
and dried out of contact with the air, or if passed through flame, or boiled up with 
the saline solution, they become “inactive” as nuclei. Hence it was supposed 
that there was some mysterious property in the air which converted “ inactive ” 
into “active ” nuclei. 
The author explains the action of nuclei with reference to differences in the force 
of adhesion acting on chemically clean or chemically unclean surfaces. If chemi- 
cally clean, the solution, whether of gas in water or of salt in water, will adhere to 
such surfaces as a whole, and there will be no separation either of gas or of salt. 
But if by exposure to the air, or by handling, &c., a nucleus, such as a glass rod, be 
made chemically unclean, the force of adhesion will be different. The gas or the 
salt of the solution will adhere to the unclean surface; the water of the solution 
will not do so, or, at any rate, but feebly ; hence there will be a separation of the 
gas or of the salt, and the nucleus will be “active” when, in fact, it is simply 
unclean. 
When supersaturated solutions are kept in clean tubes and protected from the 
air by haying the mouths plugged with cotton-wool, many of them may be kept 
during a long period without any separation of the salt. They may even be re- 
duced to low temperatures, approaching zero (Fahr.), without change of state. 
During a rising barometer air enters the tubes, the cotton-wool filtering it from 
the motes and dust which act as nuclei, the air itself not being a nucleus. During 
a falling barometer, on the contrary, air escapes from the tubes, and drags away 
with it some of the aqueous molecules of the solution. The effect of this action, 
then repeated, is to depress the liquid-surface and leave a ring of salt just above it. 
This salt being chemically clean does not act as a nucleus to the rest of the solu- 
tion; and the latter being supersaturated does not dissolve it. We may even 
lower clean crystals of the salt (the magnesic sulphate is well adapted to the expe- 
riment) into a cold highly supersaturated solution, without any action on their 
part as nuclei in inducing crystallization. The author described two experiments 
of this kind with magnesic sulphate; and in answer to an objection that in nursing 
a crystal of alum, for example, we must be dealing with chemically clean surfaces, 
he showed that in such a case none of the conditions of chemical purity were ob- 
served; the evaporating-dish and the solution exposed to the air were chemically 
unclean, as was also the hair by which the crystal was suspended, while the crystal 
itself was frequently handled, and abnormal growths chipped off with the thumb- 
nail; the result of all this being the production of an opaque octahedron from 
the deposit of a multitude of minute crystals upon chemically unclean facets, 
