The first cloud attack, in which pure chlorine was used, was very effective, but only because the troops attacked with it were entirely unprotected. Later, in spite of the varied methods of attack, the results were less and less promising, due to the increased protection of the men and also to the gas discipline which was gradually being developed. During this time the Allies had started their gas attacks (Sept., 1915), and it soon became evident that, if Germany was to keep her supremacy in gas warfare, new gases or new tactics would have to be introduced. The second poison gas was used in December, 1915, when about 20-25 per cent of phosgene was mixed with the chlorine. Here again the Germans made use of an industry already established. Phosgene is used commercially in the preparation of certain dyestuffs, especially methyl violet, and was manufactured before and during the war by the Bayer Company and the Badische Anilin und Soda Fabrik. Phosgene can not be used alone in gas cylinders because of its high boiling point (8° C.). While this is considerably below ordinary temperatures, especially during the summer months, the rate of evaporation is so slow that a cloud attack could never be made with it alone. However, when a mixture of 25 per cent phosgene and 75 per cent chlorine, or 50 per cent phosgene and 50 per cent chlorine is used in warm weather there is no difficulty in carrying out gas attacks from cylinders. At the same time the percentage of phosgene in the mixture is sufficiently high to secure the advantages which it possesses. These advantages are at least three: (_a_) Phosgene is more toxic than chlorine. It requires 2.5 milligrams per liter of chlorine to kill a dog on an exposure of 30 minutes, but 0.3 milligram of phosgene will have the same effect. This of course means that a cloud of phosgene containing one-eighth (by weight) of the concentration of a chlorine cloud will have the same lethal properties. (_b_) Phosgene is much less reactive than chlorine, so that the matter of protection becomes more difficult. Fortunately, word was received by the British of the intended first use of phosgene against them and consequently they were able to add hexamethylenetetramine to the impregnating solution used in the cloth masks. (_c_) The third, and a very important, factor in the use of phosgene is the so-called delayed effect. In low concentrations, men may breathe phosgene for some time with apparently no ill effects. Ten or twelve hours later, or perhaps earlier if they attempt any work, the men become casualties. Pure phosgene has been used in projector attacks (described in Chapter II). The substance has also been used in large quantities in shell; the Germans also used shell containing mixtures with superpalite (trichloromethyl chloroformate) or sneezing gas (diphenylchloroarsine). MANUFACTURE