kinds of shell containing gases of
different densities, one of heavy gas fired as a curtain to the rear
to permit reinforcement of the trenches and another of lighter gas to
demolish the trenches and destroy the firing line. As a general rule
these gases are employed when the fire trenches of the opposing forces
are close together though the shell containers may be used at long
ranges. All of these gases being heavier than air lie close to the
ground and flow over and down into the trenches.

=1195. Protection against gases.= The best protection against these
gases, is a mask of some kind. The commonest form employed is a
flexible mask that conforms to the head, is fitted with glass for
seeing through, and has an arrangement of tubes and valves which
require the wearer to inhale through his nose and exhale through his
mouth.

These masks have an absorbent composed of hyposulphite of sodium or of
72 per cent of the nitrous thiosulphate and 28 per cent of bicarbonate
of soda. This absorbent placed so that air must be breathed through
it, neutralizes the acids in the gases. Soldiers are provided with
these masks, sometimes with two of them, and are required to have them
renewed every three months.

Trench sprays may be used to spray neutralizing liquid in the trenches
to kill the gases.

The _favorable conditions_ for the employment of gases are wind
blowing toward the enemy's trenches and warm weather. _Unfavorable
conditions_ are rain, cold, and adverse winds.

In some localities weather vanes placed in the direction of the
enemy's trenches and arranged so that they may be watched at night
give an indication of favorable winds and enable the defender to
prepare for a gas attack.

Before the masks were provided bonfires were prepared of oil soaked
materials which; when ignited, produced an intense heat and the
resulting column of air diverted the gas clouds.

=1196. Liquid fire.= By use of hand or motor driven pumps, and a light
grade of petroleum, columns of liquid fire may be squirted into the
opposing trenches. If the oil should fail to remain lighted it may be
fired by bursting hand grenades or throwing fire balls into the
trenches. This means of attack is employed when opposing trenches are
close together.

As a defense measure ditches may be dug in front of the trenches and
filled with a porous material which is then soaked with oil. Heavy
oils, being hard to ignite, are not dangerous to the defense, and