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powdered charcoal, a layer of charcoal granules, and a layer of pumice mixed with urotropine. In April, 1918, a layer of charcoal and zinr oxide was substituted for the layer of earth granules.

To protect against irritant smokes, a paper disk filter in a perforated metal container, which was fitted over the canister, was later issued.

This mask had the advantage of compactness, but as there was no outlet valve the wearer had to continually breathe a certain amount of his own expired air. Also the entire weight of the German mask and canister was carried by the head and produced fatigue of the neck muscles after a short period of wear.

The French develo|>ed three masks, the M2, the Tissot and the A.R.S. {Appareil Respiratoire Spécial).

The M2 Mask (see Fig. 113) was in the form of a snout covering the face. It was made of 32 layers of muslin impregnated with neutralizing chemicals. Celluloid eyepieces were provided for vision. There was no outlet valve. Air was inhaled and exhaled through the fabric.

The facepiece of the Tissot Mask (see Fig. 114) was made of pure rubber and was connected by a tube to a canister of absorbent chemicals carried on the back. The mask is noteworthy as being the first to provide for drawing the incoming air across the eyepieces to prevent them from dimming. It was used extensively by artillerymen and special observers in both the French and American armies. It was clumsy, however, and difficult to adjust and was hence unsuitable for front-line troops.

The French now turned to the German iyi>e of snout canister mask, developing the A.R.S. Mask, experimentation with which began in Scj>-tember, 1917. This mask was an improvement on the German in thai it incorporated the Tissot principle of preventing dimming of the eye-

Fic. 112.—Early German gas mask pieecs by drawing the dry inspired air across them and it also included fcn ovrtYfcl valve which the German type lacked. The French snout canister, however, gave somewhat less protection than the German. .Moreover, the French did not furnish each soldier with an extra canister to carry with his mask as did the Germans.

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