The use of poisonous and asphyxiating gases, which was first adopted by the Germans in the Ypres salient in April of 1915, is now becoming an accepted fact in the present war. It is to a certain extent in one shape or another, before one every day of his life in or near the trenches. Every one should therefore be well acquainted with the various ways in which gases are used in an attack, as well as precautionary methods to be taken in counteracting its effects while on the defense.

In an attack there are only two methods which can be used—emanation and shells and grenades. The emanation method can only be employed under very favorable circumstances and in a few cases where rather a long chance was taken, it reacted very badly on the enemy. The first thing to make a gas attack successful must be a favorable breeze of about five miles an hour, as if the wind blows any faster it does not give the gas a chance to settle down into the trenches. The object of this gas is to create a poisonous and irritant atmosphere, and this is accomplished either by a gas forced through tubes in the direction of the enemy, or a liquefied gas stored in cylinders under very high pressure. To be successful, as before mentioned, the wind must be a steady breeze of not much over 5 miles per hour, no rain, and the element of surprise must figure very largely. The gas used must be heavier than air and not allow of being held back by any protective measures taken by the enemy. If the wind is too strong, it is obvious that any gas employed will be carried too quickly over the enemy’s trenches, so that it cannot settle to any degree which will allow of its obtaining the desired effect. If the wind is too light, it will be carried up into the air by local eddies, or may even be blown back.

For these reasons it is impossible to fix a definite hour for gas attacks, as everything depends on the wind.

Arsenic and phosphorus compounds are used in the tube method, and their presence can be detected at once by the smell of garlic. Should such gases get into your own trenches, chloride of lime scattered freely about will disperse them.

The gases used in liquid form from cylinders are a mixture of chlorine and other matter annoying to the ordinary infantry officer and soldier. If successful in surprising the enemy, their trenches should be cleared at once, but if the element of surprise is not there and time is given for defensive measures to be taken, the effect is lost. In an assault following a gas attack, men should always wear smoke helmets for at least 30 minutes after the gas dissemination has ceased, and the assaulting party must have the strictest orders not to remove their helmets until the officer in charge has given the command.

In the shell and grenade method of dissemination, shells and bombs are used containing liquid gas, or a substance which gives off irritant fumes.

It is easy to tell a gas shell when it lights as it comes down, the same as a “dud” shell; that is, one which does not explode, the outer casing of the shell simply collapsing. The liquid soaks into the ground, and men should be warned against standing over this ground and inhaling any of the fumes, which are very slight and rather hard to notice but very powerful and with very quick action. When a man thinks he has inhaled any of this gas he should at once be made to lie down, not undergo any exercise whatever, and as soon as possible have him carried out on a stretcher to the dressing station.

Tear shells, which are used in great profusion during an attack, are for the moment blinding in their effect, causing smarting of the eyes and a great amount of watering. This effect is only for a minute, and the men must be impressed with the fact that if they continue moving forward instead of sitting down and rubbing their eyes, it will pass off almost at once. These shells are also greatly used against the artillery during heavy bombardments. Adequate protection is furnished in the shape of goggles to fit over the eyes, as the gas has no other effect whatever.

As in other branches of military art, the best means of learning defense is to have a thorough knowledge of attack. Thus, direction of wind must always be noted, and if favorable for an enemy attack, special observers must be placed to give warning and surprise guarded against in every way. Sentries are specially placed in the trench, and often in listening posts, to get early warnings of an impending attack. If a sentry at a listening post discovers that a gas attack is being made, he at once warns the sentry at the end of his cord or wire, giving a pre-arranged signal. This sentry passes the alarm on a Strombon horn, which is something similar to a Klaxton, and will automatically give a warning which can be heard for 3 or 4 miles, and which lasts about a minute.

When a horn is not in use, generally shell cases are hung in the trenches. These are beat on by the sentry who is warned and taken up all along the line. It is then the duty of that sentry not for the instant to put on his gas helmet, but to proceed along the front line waking all the occupants of dugouts, etc., who may be sleeping in the area guarded by him. Every man without exception stands to in his trench with his helmet on and will not reËnter dugouts until first given permission by officer or N.C.O.

These attacks are generally carried out, when possible, just before dawn or during the middle of the night, and the only warning given before the actual gas reaches a trench, is a slight hissing sound which is made and can be easily heard as the gas escapes from the cylinder. Great care must be taken when the wind is favorable for an attack that this sound be listened for.

Any man wounded during a gas attack must not be placed in a dugout or on the bottom of a trench, and even if considerable shell fire be going on it is far better that he be laid out in the open on the top of the ground, where he will have a far better chance than lying in the bottom of the trench or in a dugout. After gas has passed through a trench system, and before the officer thinks that it is safe to remove the helmets, the trenches must be sprayed with a machine known as Vermeral Sprayer. A man with this sprayer on his back and wearing his helmet, slowly traverses the trench working the spray. This small tank on his back is charged with nothing more or less than “hypo” (sodium hyposulphite), about 6 pounds of which is dissolved in a bucket of water and a handful of ordinary washing soda added.

Garden syringes and buckets may be used if sprayers are not available, but their effect is not so quick.

When the officer thinks that the trench has been sprayed sufficiently and all gas has gone, he may then allow the men to take off their helmets, but not to reËnter their dugouts until they have been thoroughly cleaned.

This is sometimes done by fanning the gas out, sometimes by building a fire and smoking it out, and by the use of the sprayer. Great care must be taken that no one enters until every last vestige of gas is gone, and it is generally well that the medical officer should inspect infected dugouts before allowing the men to return.