d anxious friend, who had
been told these little creatures were very sensitive to the ill
effects of a vitiated atmosphere. They flourished in our midst and
proved amusing companions.

It is essential before a U-boat submerges to drive out the exhausted
air through powerful ventilating machines, and to suck in the purest
air obtainable; but often in war time one is obliged to dive with the
emanations of cooking, machine oil, and the breath of the crew still
permeating the atmosphere, for it is of the utmost importance to the
success of a submarine attack that the enemy should not detect our
presence; therefore, it is impossible at such short notice to clear
the air within the boat. These conditions, however, are bearable,
although one must be constantly on the watch to supply in time fresh
ventilation.

Notwithstanding certain assertions in the press of alleged discoveries
to supply new sources of air, the actual amount remains unchanged from
the moment of submersion, and there is no possibility, either through
ventilators or any other device so far known in U-boat construction,
to draw in fresh air under water; this air, however, can be purified
from the carbonic acid gas exhalations by releasing the necessary
proportion of oxygen. If the carbonic acid gas increases in excess
proportion then it produces well-known symptoms, in a different
degree, in different individuals, such as extreme fatigue and violent
headaches. Under such conditions the crew would be unable to perform
the strenuous maneuver demanded of it, and the carbonic acid must be
withdrawn and oxygen admitted.

The ventilation system of the entire submarine is connected with
certain chemicals, through which the air circulates, whose property is
to absorb and retain the carbonic acid. Preparations of potassium are
usually employed for this purpose. Simultaneously, cylinders of
oxygen, under fairly high pressure, spray oxygen into the ventilation
system, which is released in a measure proportionate to the number of
the crew; there is a meter in the distributing section of the oxygen
tubes, which is set to act automatically at a certain ratio per man.
The ordinary atmosphere is bearable for a long time and this costly
method of cleansing the air is used only as a last resort; the moment
at which it must be employed is closely calculated to correspond, not
only with the atmospheric conditions at the time of submersion, but
also to the cubic quantity of air