upersaturation can be
adjusted within very wide limits. Let it be adjusted so that the
expansion produces about a sixfold supersaturation; then if the gas is
not exposed to any ionizing agents very few drops (and these probably
due to the small amount of ionization which we have seen is always
present in gases) are formed. If, however, the bulb is exposed to strong
Rontgen rays expansion produces a dense cloud which gradually falls down
and disappears. If the gas in the bulb at the time of its exposure to
the Rontgen rays is subject to a strong electric field hardly any cloud
is formed when the gas is suddenly expanded. The electric field removes
the charged ions from the gas as soon as they are formed so that the
number of ions present is greatly reduced. This experiment furnishes a
very direct proof that the drops of water which form the cloud are only
formed round the ions.

This method gives us an exceedingly delicate test for the presence of
ions, for there is no difficulty in detecting ten or so raindrops per
cubic centimetre; we are thus able to detect the presence of this number
of ions. This result illustrates the enormous difference between the
delicacy of the methods of detecting ions and those for detecting
uncharged molecules; we have seen that we can easily detect ten ions per
cubic centimetre, but there is no known method, spectroscopic or
chemical, which would enable us to detect a billion (10^12) times this
number of uncharged molecules. The formation of the water-drops round
the charged ions gives us a means of counting the number of ions present
in a cubic centimetre of gas; we cool the gas by sudden expansion until
the supersaturation produced by the cooling is sufficient to cause a
cloud to be formed round the ions, and the problem of finding the number
of ions per cubic centimetre of gas is thus reduced to that of finding
the number of drops per cubic centimetre in the cloud. Unless the drops
are very few and far between we cannot do this by direct counting; we
can, however, arrive at the result in the following way. From the amount
of expansion of the gas we can calculate the lowering produced in its
temperature and hence the total quantity of water precipitated. The
water is precipitated as drops, and if all the drops are the same size
the number per cubic centimetre will be equal to the volume of water
deposited per cubic centimetre, divided by the volume of one of the
drops. Hence we can calcul