well ascertained case, I
mentioned only ionisation by the X rays in the first instance, I ought
not to give the impression that the phenomenon is confined to these
rays. It is, on the contrary, very general, and ionisation is just as
well produced by the cathode rays, by the radiations emitted by
radio-active bodies, by the ultra-violet rays, by heating to a high
temperature, by certain chemical actions, and finally by the impact of
the ions already existing in neutral molecules.

Of late years these new questions have been the object of a multitude
of researches, and if it has not always been possible to avoid some
confusion, yet certain general conclusions may be drawn. The
ionisation by flames, in particular, is fairly well known. For it to
be produced spontaneously, it would appear that there must exist
simultaneously a rather high temperature and a chemical action in the
gas. According to M. Moreau, the ionisation is very marked when the
flame contains the vapour of the salt of an alkali or of an alkaline
earth, but much less so when it contains that of other salts.
Arrhenius, Mr C.T.R. Wilson, and M. Moreau, have studied all the
circumstances of the phenomenon; and it seems indeed that there is a
somewhat close analogy between what first occurs in the saline vapours
and that which is noted in liquid electrolytes. There should be
produced, as soon as a certain temperature is reached, a dissociation
of the saline molecule; and, as M. Moreau has shown in a series of
very well conducted researches, the ions formed at about 100 deg.C. seem
constituted by an electrified centre of the size of a gas molecule,
surrounded by some ten layers of other molecules. We are thus dealing
with rather large ions, but according to Mr Wilson, this condensation
phenomenon does not affect the number of ions produced by
dissociation. In proportion as the temperature rises, the molecules
condensed round the nucleus disappear, and, as in all other
circumstances, the negative ion tends to become an electron, while the
positive ion continues the size of an atom.

In other cases, ions are found still larger than those of saline
vapours, as, for example, those produced by phosphorus. It has long
been known that air in the neighbourhood of phosphorus becomes a
conductor, and the fact, pointed out as far back as 1885 by Matteucci,
has been well studied by various experimenters, by MM. Elster and
Geitel in 1890, for instance. On the other hand,