0, 2, p. 116) and C. T. R. Wilson
(_Proc. Camb. Phil._ Soc., 1900, 11, p. 32). These observers established
that (1) the rate of escape of electricity in a closed vessel is much
smaller than in the open, and the larger the vessel the greater is the
rate of leak; and (2) the rate of leak does not increase in proportion
to the differences of potential between the charged body and the walls
of the vessel: the rate soon reaches a limit beyond which it does not
increase, however much the potential difference may be increased,
provided, of course, that this is not great enough to cause sparks to
pass from the charged body. On the assumption that the maximum leak is
proportional to the volume, Wilson's experiments, which were made in
vessels less than 1 litre in volume, showed that in dust-free air at
atmospheric pressure the maximum quantity of electricity which can
escape in one second from a charged body in a closed volume of V cubic
centimetres is about 10^-8V electrostatic units. E. Rutherford and S. T.
Allan (_Phys. Zeit._, 1902, 3, p. 225), working in Montreal, obtained
results in close agreement with this. Working between pressures of from
43 to 743 millimetres of mercury, Wilson showed that the maximum rate of
leak is very approximately proportional to the pressure; it is thus
exceedingly small when the pressure is low--a result illustrated in a
striking way by an experiment of Sir W. Crookes (_Proc. Roy. Soc._,
1879, 28, p. 347) in which a pair of gold leaves retained an electric
charge for several months in a very high vacuum. Subsequent experiments
have shown that it is only in very small vessels that the rate of leak
is proportional to the volume and to the pressure; in large vessels the
rate of leak per unit volume is considerably smaller than in small ones.
In small vessels the maximum rate of leak in different gases, is, with
the exception of hydrogen, approximately proportional to the density of
the gas. Wilson's results on this point are shown in the following table
(Proc. Roy. Soc., 1901, 60, p. 277):--

+---------+------------------------+-----------------+
| Gas. | Relative Rate of Leak. | _Rate of Leak._ |
| | | Sp. Gr. |
+---------+------------------------+-----------------+
| Air | 1.00 | 1 |
| H2 | .184 | 2.7 |
| CO2 | 1.69 | 1.10 |
| SO2