264 Mr Kleeman, The Unstable Nature of the Ion in a Gas. 
which it is derived may therefore be explained by the ion cluster 
continually changing in complexity, the effect of which might be— 
to produce approximately the same average velocity in each case. 
The ions made in a gas are initially elementary ions, and a 
certain time must therefore elapse after their production before — 
all the possible kinds of clusters are formed and they are in equi- 
librium with one another. Therefore if the ionic velocity of freshly | 
made ions is measured over a distance of such magnitude that all 
the possible clusters have not time to form, the velocity will be 
greater than that obtained over much larger distances. Moreover, — 
the velocity will depend on the magnitude of the electric field 
applied. By means of this result the increase of the ionic velocity — 
greater than inversely proportional to the pressure of the gas at 
low pressures was explained. 
Haperiments on the Ionisation by Collision with Positive Ions. 
In the experiments on ionisation by collision the initial ionisa- 
tion usually takes place in the powerful electric field in which 
new ions are produced by collision. The elementary ions are then ~ 
seized upon by the electric field before they have time to attach 
themselves to neutral molecules, and given a velocity sufficiently 
large that ions by collision are produced, after which their chance 
of attaching themselves to neutral molecules is small. Therefore 
if the initial ions are made in a weak electric field so that they — 
have a chance of forming clusters, and they are then drawn into 
the powerful field, we should expect that results would be obtained 
which differ in many respects from those obtained in the former 
case. The writer has carried out a set of experiments of this 
nature*, Fig. 1 indicates the experimental arrangement used. 
A is a plate connected to an electrometer, and B a wire gauze at 
a distance ‘5 cm. from the plate. The chamber C, which was in 
electrical connection with the gauze, was connected with a battery 
of cells. Ions were made in the part a@ of the chamber and drawn 
through the gauze by the weak field existing between gauze and 
chamber due to some of the lines of force which end on the plate 
threading through the gauze on to the chamber wall. Negative 
or positive ions were drawn through the gauze accordingly as the 
chamber was raised to a negative or positive potential. It was 
found that the admixture of a gas to a gas of a different kind 
produced an effect which could only be explained by the nature 
of the positive ion depending on the nature of the atom or mole- 
cule from which it is produced. This result is of importance 
* Proc. Camb, Phil. Soc. vol. xvi. pt. 7, p. 621. Erte 
Note. In the figures of this paper containing curves read 200 volts per division 
instead of 40, the small divisions not having come out in the photographs. 
