| 2.64 | 1.21 |
| CH3Cl | 4.7 | 1.09 |
| Ni(CO)4 | 5.1 | .867 |
+---------+------------------------+-----------------+

The rate of leak of electricity through gas contained in a closed vessel
depends to some extent on the material of which the walls of the vessel
are made; thus it is greater, other circumstances being the same, when
the vessel is made of lead than when it is made of aluminium. It also
varies, as Campbell and Wood (_Phil. Mag._ [6], 13, p. 265) have shown,
with the time of the day, having a well-marked minimum at about 3
o'clock in the morning: it also varies from month to month. Rutherford
(_Phys. Rev._, 1903, 16, p. 183), Cooke (_Phil. Mag._, 1903 [6], 6, p.
403) and M'Clennan and Burton (_Phys. Rev._, 1903, 16, p. 184) have
shown that the leak in a closed vessel can be reduced by about 30% by
surrounding the vessel with sheets of thick lead, but that the reduction
is not increased beyond this amount, however thick the lead sheets may
be. This result indicates that part of the leak is due to a very
penetrating kind of radiation, which can get through the thin walls of
the vessel but is stopped by the thick lead. A large part of the leak we
are describing is due to the presence of radioactive substances such as
radium and thorium in the earth's crust and in the walls of the vessel,
and to the gaseous radioactive emanations which diffuse from them into
the atmosphere. This explains the very interesting effect discovered by
J. Elster and H. Geitel (_Phys. Zeit._, 1901, 2, p. 560), that the rate
of leak in caves and cellars when the air is stagnant and only renewed
slowly is much greater than in the open air. In some cases the
difference is very marked; thus they found that in the cave called the
Baumannshohle in the Harz mountains the electricity escaped at seven
times the rate it did in the air outside. In caves and cellars the
radioactive emanations from the walls can accumulate and are not blown
away as in the open air.

The electrical conductivity of gases in the normal state is, as we have
seen, exceedingly small, so small that the investigation of its
properties is a matter of considerable difficulty; there are, however,
many ways by which the electrical conductivity of a gas can be increased
so greatly that the investigation becomes comparatively easy. Among such
methods are raising the temperature of