nt to the
electrode where there is no ionization. A plate of metal will be as
effectual as one made of a non-conductor, and thus we get the
remarkable result that by interposing a plate of an excellent
conductor like copper or silver between the electrode, we can entirely
stop the current. This experiment can easily be tried by using a hot
plate as the electrode at which the ionization takes place: then if
the other electrode is cold the current which passes when the hot
plate is cathode can be entirely stopped by interposing a cold metal
plate between the electrodes.

_Methods of counting the Number of Ions._--The detection of the ions and
the estimation of their number in a given volume is much facilitated by
the property they possess of promoting the condensation of water-drops
in dust-free air supersaturated with water vapour. If such air contains
no ions, then it requires about an eightfold supersaturation before any
water-drops are formed; if, however, ions are present C. T. R. Wilson
(_Phil. Trans._ 189, p. 265) has shown that a sixfold supersaturation is
sufficient to cause the water vapour to condense round the ions and to
fall down as raindrops. The absence of the drops when no ions are
present is due to the curvature of the drop combined with the surface
tension causing, as Lord Kelvin showed, the evaporation from a small
drop to be exceeding rapid, so that even if a drop of water were formed
the evaporation would be so great in its early stages that it would
rapidly evaporate and disappear. It has been shown, however (J. J.
Thomson, _Application of Dynamics to Physics and Chemistry_, p. 164;
_Conduction of Electricity through Gases_, 2nd ed. p. 179), that if a
drop of water is charged with electricity the effect of the charge is to
diminish the evaporation; if the drop is below a certain size the effect
the charge has in promoting condensation more than counterbalances the
effect of the surface tension in promoting evaporation. Thus the
electric charge protects the drop in the most critical period of its
growth. The effect is easily shown experimentally by taking a bulb
connected with a piston arranged so as to move with great rapidity. When
the piston moves so as to increase the volume of the air contained in
the bulb the air is cooled by expansion, and if it was saturated with
water vapour before it is supersaturated after the expansion. By
altering the throw of the piston the amount of s