EMISSION OF POSITIVE IONS BY HOT METALS 223

emissions were greatest at first and decayed with time, there
was no evidence of a close correspondence between them. If
the effect of heating in gases is due to the opening up of the
metal by their solution or diffusion, (5) would be expected, as
the gases carbon monoxide and hydrogen are notable for their
power of diffusing into metals. Their chemical activity may
also be a factor, as the positive emission from some salts has
been found to be increased in the presence of reducing gases.
The experiment of Garrett (6) has only an indirect bearing on
the present question as it refers to a salt and not to a metal;
but, in any event, it has not been confirmed as a fact by
more recent and veiy careful experiments by Davisson.1 The
writer's experiments have afforded no evidence of the occur-
rence of hydrogen ions in the initial positive discharge from
hot metals.

At first sight Thomson's experiments (7) appear to offer
an immediate contradiction to the position now being main-
tained. His values of M are certainly quite different from
those found by the writer, and there does not appear to be any
likelihood that the dififerences can be attributed to errors in the
measurements under comparison. There is, however, a very
important difference in the.conditions under which the experi-
ments were made. Thomson used wires which had been
heated for a long time in a vacuum before testing; so that
presumably all, or almost all, the initial ionization would have
been given off. Under these conditions there is no reason to
expect that the values obtained would be those belonging to
the carriers of the large initial emission.

If this is the explanation of the difference it follows that
Thomson's measurements refer either to the positive emission
due to the residual gases referred to under (4) or else to a per-
manent emission characteristic of the metal, and not, since the
values of M are different, to remaining traces of the initial
emission. Thomson's first experiments with platinum were
made in an atmosphere of oxygen at 0*007 mm. pressure, and
the majority of the ions were found to have a value of M near

1« Phil Mag.," Vol. XXIII, p. 121 (1912). both