STUDY OF OXYGEN AT LOW PRESSURES. 81 
iodine being liberated in large enough quantities to colour the 
solution, but it was considered to be of some advantage to use it 
as an additional verification. Oxygen was prepared and purified 
in the usual manner, and stored in a gas holder. 
On leaving the gas holder, the gas passed through (1) a system 
of purifying tubes containing (a) nitrate of silver, (6) solid potash, 
(c) sulphuric acid, (d) phosphorus pentoxide, (2) a wash-bottle 
containing a small quantity of the potassium iodide solution, (3) 
an ozoniser by which the oxygen could, when required, be 
ozonised without altering any of the apparatus. 
Two diagonal glass taps, in series, allowed the purified gas to 
pass into the exhausted part of the apparatus. This consisted of 
a glass tube about 0-2 cm. in diameter and 0:30 em. long, to which 
was fixed a mercury pressure gauge of the U type, 0°1 cm. in 
diameter. In order to prevent a possible loss of active oxygen 
through the action of the mercury in the gauge, the latter was 
connected to the exhausted space by a capillary connection. 
The exhausted tube was connected through a small wash-bottle 
with a Fleuss pump, the wash-bottle containing a small quantity 
of the sensitive solution. A similar wash-bottle, containing the 
Same solution, was arranged to stand close to the bottle through 
which the gas was passed in order to enable colour comparisons 
to be made. All the apparatus was, practically speaking, either 
fused together or had joints protected by paraffin and mercury, 
the use of india-rubber being of course inadmissible. The Fleuss 
pump was worked by an electric motor, and the taps were adjusted 
until a steady stream of oxygen could be passed through the 
Spparatus at a pressure of about 0°25 mm. of mercury. 
The apparatus, after having been made entirely air-tight, was 
filled with oxygen and exhausted several times; a steady stream 
of oxygen, at atmospheric pressure, was then run through it for 
an hour in order to get rid of traces of air. It was then 
exhausted and kept at a constant pressure of nearly 0°25 mm., 
(never less than 0-1, nor greater than 0:4 mm.) with the oxygen 
ubbles coming through at the rate of twenty per minute. a 
F—June 2, 1897, : 
