Before taking up in some detail the methods of projecting gas upon the enemy, it is very desirable to understand the fundamentals of chemical warfare, in so far as they pertain to poisonous gases. Following the first use of pure chlorine all the principal nations engaged in the war began investigations into a wide range of substances in the hope of finding others more poisonous, more easily produced, and more readily projected upon the enemy. These investigations led to the use of a large number of gases which seriously complicated manufacture, supply, and the actual use of the gases in the field. Gradually a more rational conception of chemical warfare led both the Allies and the enemy to restrict the numbers of gases to a comparative few, and still later to divide all gases into three groups. Thus the German divided his into three groups known as (1) Green Cross, the highly poisonous non-persistent gases, (2) Blue Cross, or diphenylchloroarsine, popularly known as sneezing gas, and (3) Yellow Cross, highly persistent gases, such as mustard gas. In the American Chemical Warfare Service we have finally divided all gases into two primary groups. These groups are known as “Non-persistent” and “Persistent.” The “Non-persistent” gases are those quickly volatilizing upon exposure to the air, and hence those that are carried away at once by air currents, or that in a dead calm will be completely dissipated into the surrounding air in a few hours. If sufficient high explosive be used to pulverize solids, they may be used in the same way, and to a large extent certain highly persistent liquid gases may have their persistency greatly reduced by using a large amount of high explosive, which divides the liquid into a fine spray. The “Persistent” group constitutes those gases that are very slowly volatilized upon exposure to the atmosphere. The principal ones of these now used or proposed are mustard gas and bromobenzylcyanide. For purposes of economy, and hence efficiency, certain gases, both persistent and non-persistent, are placed in a third group known as the “Irritant Group.” These gases are effective in extremely low concentrations against the lungs and other air passages, or the eyes. Diphenylchloroarsine, and some other solids when divided into minute particles by high explosive or heat, irritate the nose, throat and lungs to such an extent in a concentration of one part in ten millions of air as to be unbearable in a few minutes. The tear gases are equally powerful in their effects on the eyes. The irritating gases are used to force the wearing of the mask, which in turn reduces the physical vigor and efficiency of the troops. This reduction in efficiency, even with the best masks, is probably 25 per cent for short periods, and much more if prolonged wearing of the mask is forced. EFFICIENCY OF IRRITANT GASES