200 
to the ionisation curves in different gases, we found the enr- 
rents to be unexpectedly small in the case of some gases. We 
suggested that possibly some of the ions made bv the a 
particles did not get away from their parent atoms. We pro- 
posed to make a special investigation of the point, and it was 
with this purpose that the work described in this paper was 
undertaken. 
It now appears that our suggestion was justified, but it 
is also clear that we should have obtained larger currents if 
we had used a higher potential gradient : 500 volts per cm. 
was insufficient. 
Consider the curves in Fig. 3. In A is shown the rela- 
tion between current and potential gradient up to 3,000 volts 
per cm. for ethyl chloride at 56 cm. pressure, the 
saturation current per sq.cm. being about 3 x 10"^^ 
B shows the same relation in the case of air 
at atmospheric pressure, the saturation current being 
rather smaller- Comparison of these two shows how 
much more difficult it is to obtain the full current in the 
more complex gas. Again, C shows the results of experiments 
in which the depth of the ionisation chamber was varied. The 
crosses refer to a 2 mm. chamber, the dots in circles to a 4 
mm- chamber. The currents were of the 10""^^ order. The 
two sets of observations lie on practically the same curve. 
This shows that general recombination is not resj^onsible for 
the lack of saturation, and that the cause is probably similar 
to that whose effects in the case of air have been described 
above. Curves D and E refer to experiments in which the 
chamber was maintained at the same depth, 2 mm., but the 
currents were altered by varying the distance of the radium. 
In the former curve the saturation current is about 10~'^, in 
the latter six times as much. In the case of the results shown 
in C, D, and E the gas contained a certain proportion of air. 
These results all go to show that the form of the curve 
for ethyl chloride is almost independent, as in the case of air, 
of strength of current and depth of ionisation chamber, when 
the ionisation is small. But also, as in the case of air, it 
depends greatly on the density of the gas- F represents the 
results of experiments at a pressure of 36 cm., and is to be 
compared with A. All the conditions, except as regards pres- 
sure, were the same for the two curves. 
We have also carried out experiments, similar to some 
of those just described, for a mixture of carbon tetrachloride 
and air, and obtained similar results. Although there was 
only 5% (by pressure) of the denser gas in the mixture, yet 
the current at a potential gradient of 330 volts per cm. was 
only 82% of the saturation value, whilst in air under similar 
conditions it was 93'/o. 
