GASE.S BY THE ACTION OF RONTGEN RAYS AND OTHER AGENTS- 
451 
One way of accounting for the fact that an electric field has no eftect on the rain¬ 
like condensation under consideration would be to suppose that the nuclei are 
produced at the moment of expansion. They might, for example, be caused by the 
motion of the piston or plunger thi'ough the M^ater. The fact, however, that the 
condensation is just as easily observed when the cloud-vessel is connected with the 
rest of the apparatus by a bent tube of considerable length, as for example in the 
aj^paratus shown in fig. 2, shows that this is not the source of the nuclei. 
The question whether the nuclei which exist in small numbers in moist air are 
charged or not must, I think, be left an open one for the present. It is manifestly a 
matter of considerable meteorological interest. 
The view which was taken in previous papers concerning the dense fogs which are 
obtained with expansions exceeding the second limit v-ijvx = 1‘38, was that the degree 
of supersaturation is then great enough to cause condensation to take place indepen¬ 
dently of all nuclei other than the molecules of gas or vapour themselves. According 
to this view no effect is to be expected on applying an electric field when expansions 
so great as this are used. In fact, the same apparatus being used as before, no 
difierence could be detected in the appearance of these fogs, whether they were pro¬ 
duced in the absence of any electric field, or with a difierence of potential of 
225 volts, between the tin and water surfaces, these being 1 centim. apart. 
§11. Ox Ions and Condensation. 
Tlie experiments described in this paper furnish strong evidence that the passage 
of electricity through gases is effected by carriers of the same nature, whether the 
conduction is the result of exposure of the gas to Ilontgen rays or Uranium rays, or 
the action of ultra-violet light on a negatively electrified zinc plate, or consists in the 
escape of electricity from a pointed platinum wire. It is not only in their efficiency 
as condensation nuclei that the carriers from the first three of the aljove-mentioned 
sources agree, for IIutherfoed has shown that their velocity in an electric field of 
the same strength is almost identical. 
Further, these carriers are by no means of the nature of dust particles, for unlike 
the latter, which require only an exceedingly slight supersaturation in order that 
condensation may take place on them, they do not act as centres of condensation 
unless the vapour is about 4'2 times as dense as that in equilibrium over a flat surface 
of water at the same temperature {vide ‘Phil. Trans.,’ loc. cit.). In the paper just 
referred to the number 8’6 X 10~® was given as an a^iproximate value of the radius 
in centims. of water-drops equivalent in their action to these nuclei. The nuclei are 
therefore not much larger than molecules ; the fact that dense condensation takes 
place with a supersaturation only twice as great when, as far as can be judged, no 
nuclei are present other than the molecules of vapour and gas, is further evidence 
that the nuclei with which we are here concerned are not very large compared with 
3 M 2 
