It is essential that a mask, or respirator, to use its correct name, should remove all traces of gas or smoke from the air before it reaches the eyes, nose, or mouth of the wearer. The principal features of the mask of the “Box Respirator” type, as used by the American forces throughout the war were: (_a_) A canister of metal containing both neutralizing and absorptive chemicals and a smoke filter. The air to be breathed passes in through an inlet check valve and through chemicals and smoke filter. (_b_) A flexible rubber-hose through which the purified air passes from the canister to the face-piece. (_c_) A face-piece, effectively covering the eyes, cheeks, lower forehead, nose, mouth, and chin, provided with eye-pieces permitting vision and a harness to hold the face-piece in place. (_d_) An exhalation valve which affords easy discharge of exhaled air and at the same time instantly closes upon inspiration. (_e_) A knapsack slung from the neck or shoulder, in which the mask and canister are carried. In the box respirator type, the inhaled air, passing through the canister and hose, went directly into the mouth through a rubber mouth-piece, which in this manner offered protection to the lungs in the event of the face-piece being damaged or not fitting. The mask was also provided with a spring and rubber clamp which closed the nostrils and compelled the wearer to breathe entirely through the mouth. While the box respirator was in process of manufacture, much thought and effort was devoted to developing a mask which would combine with its safety and good vision a greater measure of comfort, it being particularly desired to eliminate the nose-clip and the mouth-piece, which are the box respirator’s most uncomfortable features. The starting-point in these attempts was the French Tissot mask, several modifications of which were put into production. The best mask of this type was designed, curiously enough, by a New York corset manufacturer, Major Waldemar Kops, whose name was given to his invention, which is known as the K.T. or Kops-Tissot mask. One hundred and eighty-nine thousand of the K.T. masks, which were radically different and far more comfortable than the box respirator type, had been manufactured when the Armistice was signed. The total number of masks produced by the Gas Defense Division was more than three and a half million.