On account of its persistence it has been generally referred to as a defensive gas and for that purpose it is incomparable. The use of sufficient quantities of mustard gas will almost certainly stop the occupation of areas by the enemy and probably even stop his crossing them. It also enables strong points which it is not desired to attack to be completely neutralized,—that is, made so unhabitable that the area must be evacuated. A use that was proposed toward the end of the war, and that will undoubtedly be made of the gas in the future, is to have it planted in drums in the ground and exploded when an enemy is attempting to advance. This would be a highly economic way to distribute great quantities of the material at the moment and in the place most heeded. It has even been proposed, and this would seem entirely feasible, to sprinkle certain of these areas with mustard gas by means of sprinklers attached to drums or even tanks mounted on trucks. Just before the Armistice the German made another development in the use of mustard gas. Instead of the ordinary amount of explosive, which only fairly opened up the shell and allowed the liquid to escape, he filled nearly 30 per cent of the total space of the shell with high explosive. This completely broke up the shell and distributed the greater part of the liquid mustard gas in the form of a fine spray. This spray, when breathed, proved extremely deadly, as might be expected from the fact that when in the form of minute particles one can draw into the lungs in a single breath one hundred times or more the amount that he would get of pure gas. Since mustard gas has such a delay action and is effective in such small concentrations it can be used very effectively in small calibre guns, as the 75 mm. or 3-inch. Furthermore, since it lasts for two or three days at the very least, a small number of guns can keep a very large area neutralized with the gas. With phosgene and similar non-persistent gases that volatilize almost completely upon the burst of the shell it is necessary to build up a high concentration immediately. The exact opposite is true of mustard gas. Mustard gas can be fired very slowly with the certain knowledge that all shells fired at one moment will be effective when the next is fired, though twelve hours or more may intervene between the first and last firing. Thus, while with phosgene a large number of guns are needed for a gas attack, with mustard gas the number can be reduced to one-tenth or even less. Mustard gas may be in the future and has been in the past used safely in hand grenades because of its very low vapor tension, whereby the pressure at ordinary temperatures is exceedingly low. This has an important bearing on cylinders and other containers for shipping mustard gas, that is, they need be only strong enough to be safe against handling and not to withstand the high pressure encountered with phosgene or chlorine cylinders.