It is believed that the first gas shell contained lachrymators or tear gases. Although the use of these shell continued up to and even after the introduction of mustard gas, they gradually fell off in number—the true poison gas shell taking their place. Towards the end of 1915 Auld states that the Germans were using chloromethyl chloroformate (palite) in shell. In 1916, during the battle of the Somme, palite was replaced by superpalite (trichloromethyl chloroformate, or diphosgene) which is more toxic than palite, and about as toxic as phosgene. It has the advantage over phosgene of being much more persistent. In spite of the fact that American chemists were not able to manufacture superpalite on a large scale, or at least so successfully that it would compete in price with other war gases, the Germans used large quantities of it, alone and mixed with chloropicrin, in shell of every caliber up to and including the 15 cm. Howitzer. [Illustration: FIG. 3.—Stokes’ Mortar.] CHLOROPICRIN The next gas to be introduced was chloropicrin, trichloronitromethane or “vomiting gas.” It has been stated that a mixture of chloropicrin (25 per cent) and chlorine (75 per cent) has been used in cloud attacks, but the high boiling point of chloropicrin (112° C.) makes its considerable use for this purpose very unlikely. The gas is moderately toxic and somewhat lachrymatory, but it was mainly used because of its peculiar property of causing vomiting when inhaled. Its value was further increased at first because it was particularly difficult to prepare a charcoal which would absorb it. Its peculiar properties are apt to cause it to be used for a long time. SNEEZING GAS During the summer of 1917 two new and very important gases were introduced, and, as before, by the Germans. One of these was diphenylchloroarsine, “sneezing gas” or “Blue Cross.” This is a white solid which was placed in a bottle and embedded in TNT in the shell. Upon explosion of the shell the solid was atomized into very fine particles. Since the ordinary mask does not remove smoke or mists, the sneezing gas penetrates the mask and causes violent sneezing. The purpose, of course, is to compel the removal of the mask in an atmosphere of lethal gas. (The firing regulations prescribed its use with phosgene or other lethal shell.) The latest type masks protect against this dust, but as it is extraordinarily powerful, its use will continue. MUSTARD GAS The second gas was dichloroethyl sulfide, mustard gas, Yellow Cross or Yperite. Mustard gas, as it is commonly designated, is probably the most important single poisonous substance used in gas warfare. It was first used by the Germans at Ypres, July 12, 1917. The amount of this gas used is illustrated by the fact that at Nieuport more than 50,000 shell were fired in one night, some of which contained nearly three gallons of the liquid.