(a) Because of the weather conditions. In very dry air it may be almost transparent and slightly greenish in color, while in damp weather it forms a white cloud.

(b) Because it may be mixed with smoke of any color.

A cloud attack can only take place when there is a steady but not too strong wind blowing from the enemy's lines towards our own. A wind between 4 and 8 miles an hour is the most likely condition. An 8-mile wind will carry the gas cloud twice as quickly as a man walks rapidly.

Gas attacks may occur at any time of the day, but are most likely to be made during the night or in the early morning.

Gentle rain is without appreciable effect on a gas attack, but strong rain washes down the gas. Fogs have hardly any effect and may, in fact, be taken advantage of to make an attack unexpectedly. Water courses and ponds are no obstruction to a gas cloud.

The gas used by the enemy is generally a mixture of chlorine and phosgene, both of which are strongly asphyxiating. The gases are heavier than air, and therefore, tend to flow along the ground and into trenches, shelters, craters and hollows. The gas cloud may flow round slight eminences, thus leaving patches of country which remain free from gas.

Chlorine and phosgene strongly attack the mucous membranes of the respiratory organs, causing bad coughing. In strong concentrations of gas, or by longer exposure to low concentrations, the lungs are injured and breathing becomes more and more difficult and eventually impossible, so that the unprotected man dies of suffocation. Death is sometimes caused by two or three breaths of the gas. Even when very dilute, chlorine can be recognized by its peculiar smell, which is like chloride of lime, but stronger and more irritating.

Both chlorine and phosgene also exert a strongly corrosive action on metals, so that the metal parts of arms must be carefully protected by greasing them.

The speed with which the gas cloud approaches depends entirely on the wind velocity. Gas attacks have been made with wind velocities varying from 3 to 20 miles per hour, _i.e._, from 1-1/2 to 10 yards per second. In a 9-mile wind, the gas would reach trenches 100 yards distant in 20 seconds.

Gas attacks have been made on fronts varying from 1 to 5 miles; their effects at points up to 8 miles behind the front trenches have been sufficiently severe to make it necessary to wear helmets.

(2) GAS PROJECTILES:

The use of these is not entirely dependent on the direction of the wind. In gas projectiles such as sh.e.l.ls, hand grenades, and trench mortar bombs, a part or the whole of the explosive charge is replaced by a liquid which is converted into gas by the explosion. The explosive force and noise of detonation of these projectiles is less than that of the ordinary kind, and a large number of them are usually discharged into a comparatively small s.p.a.ce. After the explosion, the irritant chemicals form a small gas cloud, though some may sink to the ground and remain active for a considerable time.

For using gas sh.e.l.ls, the best condition is calm, or a wind of low velocity.

Gas projectiles can be used in all types of country. Woods, bushes, corn fields and clumps of buildings may hold the gas active for a considerable time.

Two kinds of sh.e.l.l gases are used by the enemy, viz., lachrymators, which mainly affect the eyes, and poison gases, which may affect the eyes and are just as deadly as the gases used in the form of clouds.

(1) TEAR, OR LACHRYMATORY Sh.e.l.lS:

These sh.e.l.ls on explosion drive the liquid chemical which they contain into the air as a mist. They cause the eyes to water strongly and thereby gradually put men out of action.

Their actual smell may be slight. Large concentrations of lachrymators begin to affect the lungs and cause sickness, coughing and general irritation.

(2) POISON Sh.e.l.lS:

Besides the comparatively harmless lachrymators the enemy also uses projectiles which contain a gas, the action of which is very similar to that of phosgene. Because of their slight detonation, these sh.e.l.ls are liable to be mistaken for blinds, but they emit large quant.i.ties of a gas which attacks the lungs strongly and is very dangerous, and even in slight cases may cause serious after effects.

(3) SMOKE:

The enemy may make use of smoke, either in the form of a cloud or emitted from sh.e.l.ls and bombs. Smoke may be used with gas or between gas clouds; it may also be used alone to distract attention from a real discharge of gas, to cover the advance of infantry, or merely as a false gas attack.

(4) MINE AND EXPLOSION GASES:

The poisonous gases which occur in mines, and which are formed in large quant.i.ties when high explosive goes off in an enclosed s.p.a.ce, _e.g._, from a direct hit in a shelter, or on the explosion of a charge in a mine, are not protected against by the ordinary anti-gas appliances. The chief of these gases is carbon monoxide. Protection against such gases will not be considered in these notes.

Officers are held responsible that all the anti-gas appliances for protecting their men are maintained in perfect condition, and that all ranks under their command are thoroughly trained in the use of these appliances and in all other measures which may affect their safety against gas.

Summary of Protective Measures:

(a) Provision to each man of individual protective devices.

(b) Arrangement for the inspection of those appliances and training in their use and instruction in all other measures of gas defense.

(c) Provision of protected and gas-proof shelters.

(d) Weather observations to determine periods when the conditions are favorable to a hostile gas attack.

(e) Arrangement of signals and messages; for immediate warning of a gas attack.

(f) Provision of appliances for clearing gas from trenches and shelters.

C. Protection of Shelters:

(1) METHODS OF PROTECTION:

Protection of dugouts, cellars, buildings, etc., is given if all entrances are closed by well-fitting doors or by blankets sprayed with hypo. solution. Practically no gas pa.s.ses through a wet blanket, and the protection depends on getting a good joint at the sides and bottom of a doorway, so as to stop all draughts. This can be effected by letting the blanket rest on battens, fixed with a slight slope, against the door frame. The blanket should overlap the outer sides and a fold should lie on the ground at the bottom. A pole is fastened to the blanket, which allows the latter to be rolled up on the frame and causes it to fall evenly.

Wherever possible, particularly where there is likely to be movement in and out of the shelter, two blankets fitted in this way but sloping in opposite directions should be provided. There should be an interval of at least three feet between the two frames, and the larger this vestibule is made the more efficient is the dugout.

When not in use, the blankets should be rolled up and held so that they can be readily released, and should be sprayed occasionally with water or a little Vermorel sprayer solution.

If the blankets became stiff from a deposit of chemicals, they should be sprayed with water.

All ranks must be taught how to use gas-proof dugouts, _e.g._, how to enter a protected doorway quickly, replacing the blanket immediately, and carrying in as little outside air as possible.

The protection afforded by these means is just as complete against lachrymatory gases as it is against cloud gas and poisonous sh.e.l.l gases.

(2) SHELTERS WHICH SHOULD BE PROTECTED:

The following should always be protected:

Medical aid posts and advanced dressing stations; Company, Battalion and Brigade Headquarters; signal shelters and any other place where work has to be carried out during a gas attack.

In addition to the above, it is desirable to protect all dugouts, cellars and buildings within the sh.e.l.l area, particularly those of artillery personnel. It should be noted, however, that the protection of dugouts for troops in the front line of trenches is usually inadvisable on account of the delay involved in getting men out in time of attack. It is desirable to protect stretcher bearers' dugouts with a view to putting casualties in them.

D. Protection of Weapons and Equipment:

Arms and ammunition and the metal parts of special equipment (_e.g._, telephone instruments) must be carefully protected against gas by greasing them or keeping them completely covered. Otherwise, particularly in damp weather, they may rust or corrode so badly as to refuse to act. A mineral oil must be used for this purpose. The following in particular should be protected:

(1) SMALL ARMS AND S.A.A.

Machine guns and rifles must be kept carefully cleaned and well oiled.

The effects of corrosion of ammunition are of even more importance than the direct effects of gas upon machine guns and rifles.

Ammunition boxes must be kept closed. Vickers belts should be kept in their boxes until actually required for use. The wooden belt boxes are fairly gas-tight, but the metal belt boxes should be made gas-tight by inserting strips of flannelette in the joint between the lid and the box.

Lewis magazines should be kept in some form of box, the joints of which are made as gas-tight as possible with flannelette.

A recess should be made, high up in the parapet if possible, for storing ammunition and guns. A blanket curtain, moistened with water or sprayer solution, will greatly a.s.sist in keeping the gas out.

(2) HAND AND RIFLE GRENADES: