82. Mr. McClung on the Effect of Temperature on the 
measuring the rate of leak between two metal electrodes, or 
sets of electrodes, when the gas between them has been 
ionized. For the purposes of the present experiments this 
had to be done at various temperatures. The gas to be 
experimented upon had therefore to be inclosed in a vessel 
which could be heated to a fairly high temperature. 
When investigating air the experiments may be made in 
either of two ways. The first way is to inclose the air in a 
vessel which is not air-tight, and which will allow the air to 
expand freely when heated, so that the measurements may be 
taken in air at a constant pressure but in which the density 
changes as the temperature is varied. The second way is to 
inclose the air in an air-tight vessel so that the volume and 
density of the air experimented upon are kept constant at the 
various temperatures. In the present investigation both of 
these methods have been employed. A description of each, 
with the results obtained, will be given in turn. 
Experiments on Air at Constant Pressure. 
As already mentioned these experiments arose out of the 
investigation on the rate of recombination of ions, and the 
apparatus used was originally designed for the experiments 
on the recombination. As it was suitable for the present 
experiments it was therefore employed, although it was a little 
more elaborate than would have been really necessary for the 
present instance. In fact, in the course of investigating the 
rate of recombination one of the quantities measured was 
the amount of ionization, and therefore some of the results 
given in this paper were obtained concurrently with those on 
recombination. 
A full description of this apparatus has already been given 
in the paper on recombination of ions, and diagrams showing 
its general arrangement and construction also appeared in 
that paper. It will therefore not be necessary to give a full 
description of the apparatus here, but a less detailed diagram 
may be given for reference to show the general arrangement 
of the apparatus. This is shown in fig. 1. 
The Rontgen-ray -bulb and induction-coil were as usual 
inclosed in a lead-covered box, and the rays emerged through 
a well-defined circular orifice, A, in the lead, and passed into 
the brass cylinder BC, where they ionized the air. The bulb 
used was one with an automatic vacuum regulator attached. 
The brass cylinder BC was surrounded by a sheet-iron 
cylinder, as shown in the diagram, so that there was a uni- 
form air-space of about 10 cms. between the two cylinders. 
The inclosed air was heated by means of a long Bunsen 
