For some time after the introduction of gas warfare, the gases used were of the so-called non-persistent type. Under these conditions it was necessary to wear the mask for only relatively short periods, after which the cloud dissipated. With the increasing use of gas and the introduction of the more persistent gases, particularly mustard gas, it not only became necessary to wear the mask for long periods of time but also to do relatively heavy physical work, such as serving artillery, when wearing the mask. [Illustration: FIG. 66.—Ammonia Mask, Showing Relative Size of Canister.] Under these conditions, it became evident that the wearing of the mask caused a very great reduction in the military efficiency of the soldier. The reasons for this reduction in efficiency have been made the subject of extensive research by a group of the foremost physiologists and psychologists of the country. As a result of their work, the causes contributing to this reduction in efficiency may be grouped about the following main factors: (1) The physical discomfort of the mask arising from causes such as pressure on the head and face, due to improperly fitting facepieces and harness, the noseclip, and the mouthpiece. (2) Abnormal conditions of vision, due to poor optical qualities in eye pieces and restrictions of vision, both as to total field and binocular field. (3) Abnormal conditions of respiration, among them being (_a_) the unnatural channels of respiration caused by wearing the box respirator, (_b_) increase in dead air space in respiratory circuit, and (_c_) the increase in resistance to both inhalation and exhalation, the last two mentioned being present to a greater or less degree in all types of mask. Of these general subdivisions the various phases of the first two are so evident that no further discussion will be given. The effects of the changed conditions of respiration are, however, less obvious, and it may be of interest to present in a general way the results of the research along this line, particularly as regards the harmful effects of increasing the resistance and dead air space in the respiratory tract above the normal.