One pound of the irritant gases is equal to 500 to 1,000 pounds of other gases when forcing the wearing of the mask alone is desired. The great economy resulting from their use is thus apparent. Due to the rapid evaporation of the non-persistent gases they are used generally only in dense clouds, whether those clouds be produced from cylinders or from bombs. These gases are used only for producing immediate casualties, as the necessary amount of gas to force the enemy to constantly wear his mask by the use of non-persistent gases alone could not possibly be taken to the front. Mustard gas, which is highly persistent and also attacks the lungs, eyes and skin of the body, may and will be used to force the wearing of the mask. It has one disadvantage when it is desired to force immediately the wearing of the mask, and that is its delayed action and the fact that it acts so slowly, and is usually encountered in such slight concentrations that several hours’ exposure are necessary to produce a severe casualty. For these reasons the enemy may often take chances in the heat of battle with mustard gas, and while himself becoming a casualty, inflict quite heavy casualties upon opposing troops by continuing to operate his guns or rifles without masks. A powerful tear gas on the other hand forces the immediate wearing of the mask. MATERIAL OF CHEMICAL WARFARE USED BY C. W. S. TROOPS Chemical warfare troops, in making gas attacks, use cylinders for the cloud or wave attack, and the Livens’ projector and the 4-inch Stokes’ mortar for attacks with heavy concentrations of gas projected by bombs with ranges up to a mile. This distance will in the future probably be increased to 1½ or 1¾ miles. The original cylinders used in wave attacks were heavy, cumbersome and very laborious to install, and notwithstanding the wave attack was known to be the deadliest form of gas attack used in the war, fell into disrepute after the use of gas became general in artillery shells and in special bombs.