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, in 1893 Mr Barus
established that the approach of a stick of phosphorus brings about
the condensation of water vapour, and we really have before us,
therefore, in this instance, an ionisation.
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