Poisonous gases, under which head come carbon monoxide and cyanogen.
Gases which affect the throat and bronchial tubes, such as chlorine and bromine. The latter class is most commonly employed.
The methods usually employed for liberating these gases are to have a plant some distance in rear of the trenches where the gas is stored under pressure and carried to the trenches through pipes, where it can be liberated towards the enemy's trenches when there is a favorable wind to carry it along; or, the gas may be carried in cylinders or other containers and liberated at the desired points. Hand grenades or bombs are also employed which, upon bursting, liberate the gas or in some cases scatter acids or caustic soda. Some of these bombs contain a chemical which when liberated affects the eyes, causing impaired vision. The Germans employ several 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.