The PH helmet was unsatisfactory because of the following reasons: (1) It was warm and stuffy in summer; (2) It deteriorated upon exposure to air; (3) It was incapable of further development; (4) It had a peculiar odor and, when wet, frequently burned the foreheads of the men; (5) It offered practically no protection against lachrymators. [Illustration: FIG. 43.—Method of Wearing the P. H. Helmet.] [Illustration: FIG. 44.—Early Type of Standard (British) Box Respirator (S. B. R.)] =Box Respirator.= The increasing concentration of gas from cylinder attacks and the introduction of shell, with such gases as chloropicrin and superpalite, led, early in 1916, to very definite and constructive efforts on the part of the British to increase the protection offered by the mask. The result was a “polyvalent” respirator of the canister type (the Standard Box Respirator). This mask was probably the result of experience with oxygen apparatus in mine rescue work. The lines on which this canister was modeled involved the use of a canister filled with highly sensitive absorbent charcoal mixed with or alternating in layers with oxidizing granules of alkaline permanganate. It was the result of innumerable experiments, partly conducted in France but mostly in England under the direction of the late Lieut. Col. Harrison, who was almost entirely responsible for the wonderful production of this respirator. The respirator (Figure 44) consisted of the canister mentioned above, which is attached by a flexible tube to a facepiece or mask. The facepiece is made of rubberized fabric and fits the face so that there is little or no leakage. This is secured by means of tape and elastic bands which fit over the head. The nose is closed by means of clips, which are wire springs with rubbered jaws covered with gauze (Fig. 45). Breathing is done through a mouthpiece of rubber; the teeth close on the rubber tabs and the rubber flange lies between the teeth and the lips. The expired air finds exit through a rubber flutter valve in an angle tube just outside the mask. This arrangement furnishes a double line of protection; if the face piece is punctured or torn, gas-containing air cannot be breathed as long as the noseclip and mouthpiece are in position. The early English canister was packed with 675 cc. of 8-14 mesh war gas mixture, 40 per cent of which was wood charcoal and 60 per cent reddish brown soda-lime granules. The metal dome at the bottom of the can was covered with a thin film of cotton. At two-thirds of the distance to the top was placed a paper filter and a heavy wire screen which differs from our heavy screen in that it is more loosely woven. The mixture was covered with a cotton filter pad and a wire screen, over which was placed the wire spring.