This method has for its object to create a poisonous or irritant atmosphere. This is accomplished by means of the arsenic and phosphorous gas being forced through tubes in the direction of the enemy or by means of liquefied chlorine, bromide, phosgene and sulphuretted hydrogen gas stored in cylinders under high pressure. To be successful the gas attack must be attended by the following conditions: 1. The weather must be comparatively calm with a wind blowing in the direction of the enemy at about 5 miles an hour. If the wind is too strong the gas will be carried over the enemy's trenches so rapidly that it will not settle in them. If the wind be too light the gas will be carried up into the air and disseminate or may even be blown back into our own trenches, in which case chloride of lime scattered about freely will disperse them. 2. There must be no rain, for that would quickly disseminate the gas and negative the effect. 3. The attack must come as a surprise. If the elements of surprise are missing and the enemy has time to take protective measures, the effect is lost. If the surprise is complete, the enemy trenches should be emptied very quickly. 4. The gas used must be heavier than the air, so that it will sift into the enemy's trenches as it passes them. It is impracticable to decide upon any definite hour for launching the gas attack. Everything depends upon the direction and velocity of the wind. If an hour has been tentatively designated and the wind changes, the attack will have to be postponed. When an assault follows the gas attack the men should wear the smoke helmets for at least 30 minutes after the dissemination has ceased; in fact they must not be removed until the order to do so is given by the officer commanding the attack. You will appreciate that the enemy's machine gunners may have better protection than the men in the bays of the trenches. SHELL AND GRENADE METHOD In this method the gas dissemination is effected by means of shells or bombs being fired into the enemy's trenches containing the desired substances which are released and give off irritant fumes on explosion. The grenades used weigh about 1 pound. They are similar in appearance to the ordinary tin can grenade. Their effect in a trench will continue for 20 to 30 minutes. In the attack a large number should be concentrated in a particular area to produce a large volume of gas. They are thrown by hand, trench mortar or catapult. DEFENSE