The canister is markedly different from all other canisters described in this chapter in that a highly hygroscopic chemical absorbent is used. An approximate determination showed about 70 per cent sodium hydroxide. The use of caustic soda in the canister is made possible by the intermixing of steel wool with the granules of caustic. A layer of absorbent having the appearance of vegetable charcoal is placed at the top of the canister. The canister has the shape of a rectangular prism 8 × 6½ × 2½ inches; and, owing to the use of steel wool, is large in proportion to the weight of absorbent contained. Valves are supplied which prevent exhalation through the canister. When not in use the opening in the bottom of the canister is plugged with a rubber stopper to protect the absorbents from moisture. The canister is carried against the body and is connected to the facepiece with a flexible rubber-fabric tube. =A. R. S. Mask (Appareil Respiratoire Special).= One of the latest types of French mask is the so-called A. R. S. mask, which is based upon, or at least resembles closely, the German mask. This is a frame mask made from well rubberized balloon material, provided on the inside with a lining of oiled or waxed linen and fitted with a drum which is screwed on. The mask is provided with eyepieces of cellophane, fastened by metal rings into rubber goggles, which are sewed in the mask. A metal mouth-ring is tied in the mask with tape. This ring is placed somewhat higher than in the German mask, in this way reducing the harmful space under the mask. An inlet and outlet valve is placed in the mouth-ring; the first is of mica while the other, which is in direct communication with the interior of the mask, is of rubber. On the inside of the mask, in front of the valves, a baffle is sewed in, whereby the inhaled air is forced to pass in front of the eyepieces to prevent dimming and, at the same time, condensed vapor is prevented from entering the valves. [Illustration: FIG. 49.—French A. R. S. Mask.] The mask or head straps are arranged in the same way as on the latest M-2 mask, i.e., one elastic band is placed across the top of the head and the other across the back; the two are joined by an elastic. Below these two straps is an adjustable elastic neck band. The drum is made of metal similar in shape to the German drum and fits in the mouth-ring by means of a thread. It is made tight by a rubber ring as in the German mask. The thread differs from that on the German mask, making an interchange of canisters impossible. The canister or drum includes a bottom screen, springs and wire screens between the layers. It is closed by a perforated bottom. There are three layers. On the top is a thin layer of absorbent cotton. Beneath this is a central layer of charcoal, which is a little finer than the German charcoal. The lower layer consists of soda-lime, mixed with charcoal and zinc oxide and moistened with glycerine. GERMAN MASK