dy charged with
electricity slowly lost its charge even when insulated with the greatest
care, and though long ago some physicists believed that part of the leak
of electricity took place through the air, the general view seems to
have been that it was due to almost unavoidable defects in the
insulation or to dust in the air, which after striking the charged body
was repelled from it and went off with some of the charge. C. A.
Coulomb, who made some very careful experiments which were published in
1785 (_Mem. de l'Acad. des Sciences_, 1785, p. 612), came to the
conclusion that after allowing for the leakage along the threads which
supported the charged body there was a balance over, which he attributed
to leakage through the air. His view was that when the molecules of air
come into contact with a charged body some of the electricity goes on to
the molecules, which are then repelled from the body carrying their
charge with them. We shall see later that this explanation is not
tenable. C. Matteucci (_Ann. chim. phys._, 1850, 28, p. 390) in 1850
also came to the conclusion that the electricity from a charged body
passes through the air; he was the first to prove that the rate at
which electricity escapes is less when the pressure of the gas is low
than when it is high. He found that the rate was the same whether the
charged body was surrounded by air, carbonic acid or hydrogen.
Subsequent investigations have shown that the rate in hydrogen is in
general much less than in air. Thus in 1872 E. G. Warburg (_Pogg. Ann._,
1872, 145, p. 578) found that the leak through hydrogen was only about
one-half of that through air: he confirmed Matteucci's observations on
the effect of pressure on the rate of leak, and also found that it was
the same whether the gas was dry or damp. He was inclined to attribute
the leak to dust in the air, a view which was strengthened by an
experiment of J. W. Hittorf's (_Wied. Ann._, 1879, 7, p. 595), in which
a small carefully insulated electroscope, placed in a small vessel
filled with carefully filtered gas, retained its charge for several
days; we know now that this was due to the smallness of the vessel and
not to the absence of dust, as it has been proved that the rate of leak
in small vessels is less than in large ones.

Great light was thrown on this subject by some experiments on the rates
of leak from charged bodies in closed vessels made almost simultaneously
by H. Geitel (_Phys. Zeit._, 190