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.