Modern artillery shell have distinctive colors for high explosive, for shrapnel, for incendiary materials, and for gases. A grayish color has been adopted as the general color of the paint on all gas shells, bombs and cylinders. In addition a system of colored bands has been adopted. These bands are white to indicate poisonous non-persistent substances, and red—persistent. Yellow is used to indicate smoke. With any given combination of red and white and yellow bands, the artillery-man at the front can tell, at a glance, whether the gas is non-persistent or whether it is persistent, and also whether or not it contains smoke. There will be secondary markings on each shell which, to the trained Chemical Warfare Service officer, will indicate the particular gas or gases in the shell. These markings however, will be inconspicuous and no attempt will be made to give the information to the soldier or even to the average officer firing gas. These secondary markings are for the purpose of enabling the Chemical Warfare Service officers in charge to use certain gases for particular uses in those comparatively rare cases when sufficient gas is on hand and sufficient time available to enable such a choice to be made. CHAPTER XXIV DEFENSE AGAINST GAS (From the Field Point of View) The best defense against any implement of war is a vigorous offense with the same implement. This is a military axiom that cannot be too often, or too greatly emphasized, though like other axioms it cannot be applied too literally. It needs a proper interpretation—the interpretation varying with time and circumstances. Thus in gas warfare, a vigorous offense with gas is the best defense against gas. This does not mean that the enemy’s gas can be ignored. Indeed, it is more important to make use of all defensive measures against gas than it is against any other form of attack. Gas being heavier than air, rolls along the ground, filling dugouts, trenches, woods and valleys—just the places that are safest from bullets and high explosives. There it remains for hours after it has blown away in the open, and, since the very air itself is poisoned, it is necessary not only that protection be general but that it be continuous during the whole time the gas is present. EARLIEST PROTECTIVE APPLIANCES