205 
These results relate to two gases only : but so far as tliey go 
they show: — (i.) That the range varies inversely as the pres- 
sure, which result might have been anticipated ; (ii.) that the 
total number of ions set free in a gas is independent of the 
pressure, but is different in different gases. The total ionisa- 
tion is greater in ethyl chloride than in air. This is a con- 
trary result to that which we obtained during our experiments 
on absorption. We were unaware at that time of the enor- 
mous force required to saturate the complex gas- 
Finally, ■ the following experiments may be briefly de- 
scribed : — 
We have tried the effect of reversing the field on the rela- 
tion between current and 23otential, and found a result which 
w^as practically negative. We have found a similar result 
when the a particles were not shot straight across the ionisa- 
tion chamber in the direction of the lines of force, but in a 
slanting direction. These experiments were made in the en- 
deavour to find whether there was any relation between the 
direction in which electrons were projected and the direction 
of the applied field. We have also tried to alter the range 
in air by using different j^otential gradients, with the idea 
that it might be possible to obtain ions from an atom traversed 
by a slower a particle, if only enough electric force were 
applied. But the result was the same, no matter whether the 
force was 20 volts to the cm. or 2,000 : and a variation of '2 
mm. could hardly have escaped detection. 
In a paper which we had tlie honour to lay before this 
Society on June 6, 1905, we described the results of some in- 
vestigations into the correct form of the ionisation curve. 
Assuming that the a particle had lost almost all it-s energy 
when it ceased to ionise, we showed that it spent its energy 
at a rate which was inversely proportional to the square root 
of its speed. This assumption appeared to us at the time to 
be reasonable, but Rutherford has shown since then (" Philo- 
sophical Magazine," July, 1905), that the a particle of RaC 
still retains 40% of its initial energy when it ceases to ionise 
the gas through which it passes. In consequence, the conclu- 
sion which we drew from our experimental results needs modi- 
fication. Recalculation shows that the a particle spends 
energy at a rate which is inversely proportional to the square 
of its speed. This is interesting, since this is the rate at 
which any particle moving wath great speed gives up energy 
to a particle, relatively at rest, which it passes by, it being 
supposed that a force acts between the tw-o which is a function 
of their mutual distance (Report of the Australasian Associ- 
ation for the Advancement of Science. Dunedin, 1904. p. 64). 
Rutherford's remarkable discovery does not affect any other 
of our conclusions. 
