Probably the chief reason that has caused the boche to hold back with his cloud attacks has been his conclusion that they were unprofitable against well-disciplined, highly trained and thoroughly protected troops. With a limited amount of gas available he naturally chose the method that would give him the best results. For the cloud attack his cheapest target was the Russians, who were incompletely equipped with gas masks of a modern kind and who for a long time were badly disciplined in anti-gas measures. Against such troops the gas cloud is just the thing, and the Germans have estimated that ten to fifteen per cent of all troops exposed to a successful gas cloud would become casualties. This was probably true on the Russian Front, but was certainly not true in the West. Then the gas cloud has almost reached its apparent limit of development. There is a limit to the number of gases that can be used from cylinders, and there is a limit to the number of cylinders that can be discharged at one time. Besides this the gas cloud is largely dependent on infantry labour for carrying and installation, and it is mighty difficult to bring off a complete surprise owing to the time it takes to prepare an attack. On top of all this the whole procedure is wrong as regards efficiency, for it puts up the highest concentration of gas where the boche does not want it--just in front of his own trenches instead of in ours. For all these reasons the boche during the past year has specialised on the development of his gas shells. Of course he may come back with the cloud again, and we do not relax our vigilance or it certainly would reappear. But unless he discovers something new in the cloud line, and if we keep up a high standard of training, he will not do much damage, though for that matter the same thing is true about gas shells and trench mortar bombs. CHAPTER VII The rising importance of the gas shell--The variety of gases practicable with the shell--The deadly Green Cross Shell--Risks of transporting “duds” for chemical analysis--Reduced Allied casualties--German blunders in shelling tactics--Importance of universal discipline. One of the most interesting things about the development of gas warfare has been the way in which the gas shell, from being the least important method of poisoning the air, has become the chief gas weapon in the German armoury. The reasons for this extraordinary development, though various, are not far to seek. They lie chiefly in the fact that unlike the gas cloud we have not even yet approached the limit of the number or size of the gas projectiles that can be used. Nor, which is even more important, is there any limit to the variety of the poisons that can be used in gas shell.