669 
here the argon is dried by immersing this spiral in alcohol and. 
cooling it down to its melting point by means of liquid air. Through 
k, and k,, the gas then reaches the compression tube A, within the 
compression cylinder A, *). In this compression tube the mercury is 
forced by means of compressed air from the steel cylinder B; by 
this the required amount of argon is compressed into the dilatometer 
on closing the tap 4, and opening #, and #,. This arrangement for 
compressing pure gases has already been fully described in previous 
Communications *); moreover, its mode of operation is easily seen 
from the accompanying diagram. 
Through the tap C,, it was possible to establish communication 
between the air compressor and our accurate closed hydrogen mano- 
meter’), and by this means we were able during the actual measu- 
rements to obtain a few further determinations of the vapour pressure, 
which will be published shortly. 
The cryostat Cr was the same as that used in the investigation 
of oxygen, the sole modification being the introduction of a different 
type of stirrer, Ag, provided with valves. *). 
As the appendix of the dilatometer formerly used was found to 
be too narrow, another dilatometer Dil, very accurately calibrated, 
was employed with an appendix sufficiently wide to allow of the 
suitable measurement of the small volume of the liquid coexisting 
with the vapour. 
A GAEDE vacuum pump was used, and we found it of the greatest 
utility, particularly during the actual measurements, in ensuring the 
continued absence of leaks. 
Two platinum resistance thermometers were introduced into the 
cryostat to serve forthe regulation und measurement of the temperature. 
The argon used for the present measurements was taken from 
the same supply as that employed in the previous investigations 
already quoted. The impurity in this argon is certainly less than 0.1 °/, ®). 
§ 3. Experiments. We may now give a short description of the 
sequence of Operations involved in the measurements : 
1) At the cylinder A is connected the glass manometer Ps for high pressures 
and of small volume, especially constructed for the use of such cylinders as reser- 
voirs for the rare gases. 
*) Proc, April 1901 Comm. No. 69, Proc. March 1907 Comm. 97a. 
3) April 1902 Comm. no. 78, Proc. March 1907 Comm. no. 97a. 
4) Proc. June 1911. Comm. no. 123. 
5) For a detailed description of the preparation and of the analysis of this 
argon see G. A. GROMMELIN, Thesis for the Doctorate, Leiden, 1910, 
