If we succeed in getting a poisonous gas that has no odor it will be highly desirable to fire it so that it will not be visible. In that case no smoke will be used. Carbon monoxide is such a gas, but there are several important reasons why it has not been used in war. (See page 190). These considerations indicate the general requirements for a successful poisonous gas. If non-persistent it must be quickly volatilized, or must be capable of being driven off by heat or by other means, which can be readily and safely produced in the field. It must be highly poisonous, producing deaths in high concentrations, and more or less serious injuries when taken into the system in quantities as small as one-tenth of that necessary to produce death. If it has a slightly delayed action with no intervening discomfort, it is still better than one that produces immediate discomfort and more or less immediate action. It must be readily compressed into a liquid and remain so at ordinary temperatures, with the pressure not much above 25 or 30 pounds per square inch. As a persistent gas it must be effective in extremely low concentrations, in addition to having the other qualities mentioned above. These general characteristics concerning gases apply whether used by Chemical Warfare troops, the Artillery, the Air Service, the Navy, or the Infantry. In speaking of these substances being used by the Infantry, it is understood that an ample number of Chemical Warfare officers will be present to insure that the gases may not be turned loose when weather conditions are such that the gas might drift back and become a menace to our own troops. This is absolutely essential since no troops who have as varied duties to perform as the Infantry, can be sufficiently trained in the technical side of chemical warfare to know when to put it off on a large scale with safety and efficiency. ARTILLERY USE OF GAS The Artillery of the future will probably fire more gas than any other one branch of the Army. There are two reasons for this—first, the large number of guns now accompanying every Army, and second, the long range of many of these guns. As before indicated, the gases are adaptable to various uses, and hence to guns differing both in caliber and range. The gas will be fired by practically all guns—from the 75 mm. to the very largest in use. It is even possible that if guns smaller than the 75 mm. become generally useful that certain gases will be fired by them.