=Efficiency of Artillery Gas Shell.= It is well to remember in the beginning that all artillery shell so far designed and used, contain only about 10 per cent gas, i.e., 10 per cent of the total weight of shell and gas. It is hoped that gas shell may later be so designed that a somewhat greater proportion of the total weight of the shell will be gas than is now true. This is very desirable from the point of efficiency. As stated above the bombs used by Chemical Warfare troops contain nearly 50 per cent of their total weight in gas, and hence are nearly five times as efficient as artillery shell within the limit of range of these bombs. This fact alone is enough to warrant the use of gas troops to their full maximum capacity in order that the artillery may not fire gas at the ranges covered. GUNS FIRING PERSISTENT AND NON-PERSISTENT GASES Considering the firing of non-persistent and persistent gases, it may be said generally that non-persistent gases will be fired only by the medium caliber guns which are available in large numbers. In fact, the firing of non-persistent gases will be confined mainly to the 6-inch or 155 mm. Howitzer and gun. As our Army was organized in France, and as it is organized at present, the number of 155 mm. guns will be greater than all others put together, except the 75 mm. In order that a non-persistent gas may be most effective a high concentration must be built up very quickly. This necessitates the use of the largest caliber shell that are available in large numbers. Of course, a certain percentage of the gas shell of other calibers may consist of non-persistent gases in order to help out the 155 mm. gun. This is in accordance with the present program for loading gas shell and applies particularly to the 8-inch and 240 mm. Howitzer. =Few Ideally Persistent or Non-Persistent Gases.= Naturally there will be very few gases that are ideally non-persistent or ideally persistent. The groups will merge into one another. Those on the border line will be arbitrarily assigned to one group or the other. It might be said definitely, however, that a gas which will linger more than six or possibly eight hours under any conditions, except great cold, will not be considered non-persistent. For reasons of efficiency and economy persistent gases will not be chosen unless they will persist under ordinary conditions for two or three days or more. Accordingly, a gas which would persist for one day only would have to be extraordinarily useful to lead to its adoption.