=Firing Non-Persistent Gases.= Of the non-persistent gases phosgene is the type and the one most used at present. Furthermore, so far as can now be foreseen, it will continue to be the non-persistent gas most used. It volatilizes very quickly upon the bursting of the shell. Accordingly, in order that the shell fired at the beginning of a gas “shoot,” as they are generally referred to in the field, shall still be effective when the last shell are fired, it is necessary that the whole number be fired within two to three minutes. The temperature and velocity of the wind both affect this. If it be in a dead calm, the time may be considerably extended; if in a considerable wind, it must be shortened. Another important consideration requiring the rapid firing of non-persistent gases is the fact that nearly all masks thoroughly protect against phosgene and similar gases. It is accordingly necessary to take the enemy unawares and gas him before he can adjust his mask; otherwise, practically no harm will result. From the considerations previously mentioned, these “gas shoots” are usually made at night when, as before stated, carelessness, sleepiness and the resulting confusion of battle conditions always insure more casualties than firing gas in the daytime. =Firing Persistent Gases.= The persistent gases will be fired by all caliber guns, but to a less extent by the 155 mm. than by the other calibers. Persistent gases must be sufficiently effective in low concentrations to act more or less alone. If it be desirable to fill an atmosphere over a given area with mustard gas, the firing may extend for two or three, or even five or six hours and all shell still act together. The same is true of bromobenzylcyanide. This, then, permits the minimum number of guns to be used in firing these persistent gases. Inasmuch as they persist and force the wearing of the mask, they are available for use in long-range, large-caliber guns for interdiction firing on cross-roads, in villages, and on woods that afford hiding places, as well as on other similar concentration points. =Firing Irritant Gases.= The irritant gases will be fired by the various caliber guns, in the same manner as the persistent and non-persistent gases. We will have non-persistent irritant gases and persistent irritant gases. They are, however, considered as a group because they are used for harassing purposes, due to their efficiency in forcing the wearing of the mask. Before the signing of the Armistice, the General Staff, A. E. F., had authorized, beginning January 1, 1919, the filling of 25 per cent of all shell with Chemical Warfare materials. The interpretation there given to shell was that it included both shrapnel and high explosive.