203 
of saturation, from tlioii- pai-cnt atoms, (jr that botii tiiese 
effects take place- With tiie object of helping to a decision 
on this point we have begun a set of exi)eriinents, of which 
those now described are tlie first examples. 
The method used is to measure the co-ordinates of 
some standard j^oint on the ionisation curve of the gas investi- 
gated, under different pressures. The point chosen is that 
where the side of the RaC curve is struck by the top of tlie 
curve which belongs to the a particles of next velocity to 
those of RaC. This point in air, at 760 mm. and 20 C-, is 
at a height of nearly 4'83 cm. It is a convenient point to 
choose, for the following reasons : — Being on a part of the 
RaC curve, where no great change in the ionisation takes 
place for a considerable alteration in range, the measure- 
ments there are usually pretty consonant with each other, 
even though they are taken quickly, and if several be taken 
on the RaC curve they check each otner. Tiie ordinate of this 
point can also be determined with great precision by measur- 
ing two or three points along the top slope of the curve of RaA 
(or emanation, whichever it tinally proves to be). Thus, a 
few readings can be quickly taken in succession which deter- 
mine the point accurately, and very little leakage of aii into 
the apparatus takes place while the experiments go on- This 
is a desirable thing, because our apparatus leaks slowly when 
the j^ressure within is much reduced, on account of the large 
number of connecting tubes and mechanical arrangements. 
We find that this method is very satisfactory. We may men- 
tion also that to save time it is not well, in the case of gases 
like ethyl chloride, which are at first in the liquid form, to 
admit any of the liquid into the apparatus, as it takes so long 
to evaporate completely. It is better to let the liquid evapo- 
rate in another chamber, which can be quite small, and then 
to take over gas only. 
Fig. 4 shows the results of some experiments with C.2H5CI. 
The curves shown are portions of the ionisation curves in this 
gas at different pressures. In all cases the apparatus was 
exhausted of air to about 10 mm. pressure, then partly filled 
with gas, re-exhausted, and filled again to the desired pres- 
sure. The observations were made at once, those in the 
neighbourhood of the standard point being made first, so that 
the gas might be as pure as possible whilst the important 
readings were being taken- A potential of 900 volts was used 
for the three greater pressures, and of 300 for the low pres- 
sure. The chamber was 3 mm. wide, and therefore these 
potential gradients were, respectively, 3,000 and 1,000 volts 
per cm. 
The results for ethvl chloride and for air are contained 
