2 Messrs. Richardson, Nicol, and Parnell on the 
temperature of the metal over as wide a range as possible. 
The method used was to measure the decrease of pressure in 
a vessel containing hydrogen, into one part of which a 
platinum tube was sealed. The tube was heated electrically 
to a constant temperature. The pressure of the hydrogen at 
the other side of the walls of the tabe was maintained at zero, 
either by leaving the tube exposed to the air and allowing 
the hydrogen to burn, or by connecting with a mercury pump. 
Of the practical difficulties connected with measurements 
of this kind, a large number are described by Winkelmann 
in the papers cited. A most troublesome source of error, 
which we ultimately managed to get rid of, is the gradual 
alteration in the state of the metal surface due to long 
continued heating. Instead of being smooth and featureless 
as when originally drawn, a platinum tube which has been 
heated for some time to a high temperature, at any rate in an 
atmosphere of hydrogen, gradually develops a crystalline 
structure, becomes full of cracks and excrescences, and, as a 
rule, ultimately breaks. Naturally this recrystallization of 
the metal is accompanied by a change in its permeability to 
hydrogen. Winkelmann found that in his experiments 
continued heating invariably increased the rate of diffusion 
of hydrogen through the tubes. After much trouble from 
this cause, especially through the tubes breaking, we found 
that the effect could be almost entirely avoided by taking 
care that the alternate heating and cooling produced no 
strain on the platinum tube. 
In measuring the rate of diffusion of hydrogen through a 
platinum tube there are obviously two alternative modes of 
procedure. Wemay either let the hydrogen in from outside, 
or out from inside the tube. The latter method is more 
conducive to simplicity in the general design of the apparatus, 
since the escaped hydrogen may simply be aliowed to burn 
and there is no need to surround the hot tube with an air- 
tight containing vessel. It has, however, one very obvious 
disadvantage. Since the hot platinum only lets hydrogen 
through, all the traces of impurity originally present 
gradually get collected in the small tube, so that one runs a 
considerable risk of getting variations due to the hydrogen 
becoming impure. For this reason we thought it would be 
interesting to make preliminary experiments, at any rate, 
using the other method. Since only rough measurements 
were made with it, we shall not describe the apparatus used 
in this part of the investigation, but shall content ourselves 
with stating that the rate of diffusion was found to vary with 
the pressure very nearly as the square root, and to increase 
very quickly with the temperature. So far as they go, these 
