This mask fits the face tightly and as a consequence the inhaled air can be obtained only by drawing it through the pores of the impregnated fabric. There is no outlet valve. The exhaled air makes its escape through the fabric. The eyepieces are made of a special non-dimming celluloid. The mask is protected from rain by a flap of weather proof fabric, which also protects the absorbent chemicals from deterioration. At the beginning of the war the United States experimented considerably with the French mask. Several modifications of the impregnating solutions were suggested, as well as methods of application. One of these was to separate the components of the complexene solution and impregnate two separate layers of cloth; this would make a three-layer mask. In view of the phosgene which was in use at that time, the following arrangement was suggested: 20 layers of hexamethylenetetramine, 10 layers of nickel sulfate-sodium carbonate, 10 layers of greasene. This arrangement was more effective than the original French mask and offered the following protection when tested against the following gases (concentration 1 to 1,000, rate 30 liters per minute): Phosgene 65 minutes Hydrocyanic acid 60 minutes Chlorine 60 minutes [Illustration: FIG. 47.—Interior View of M-2 Mask.] [Illustration: FIG. 48.—French Artillery Mask, Tissot Type.] =Tissot Mask.= The French deserve great credit for their development of the Tissot type mask. This was first issued to artillerymen, stretcher bearers, and certain other special classes of soldiers to furnish them with protection and yet enable them to work with greater efficiency because of the decrease in resistance to breathing. The mask (Fig. 48) resembles the British box respirator in that it consists of a canister and rubber facepiece, but differs in that the mouthpiece and noseclip are lacking. The inhaled air enters the mask from two tubes which open directly under the eyepieces and allow the air to sweep across them. This removes, by evaporation, the condensed moisture of the breath from the eyepieces, which otherwise would obstruct the vision. The circulation of the fresh air in the mask also removes and dilutes lachrymatory gases which may filter through the mask. The exhaled air escapes through a simple outlet valve. This type of mask is advantageous because: (1) The facepiece is tight and comfortable. (2) The eyepieces do not become dimmed. (3) There is no difficulty in speaking. (4) Salivation is eliminated because of the absence of the mouthpiece. (5) It is generally more comfortable than the box respirator. This mask, however, was made of thin rubber of great flexibility which, while affording a perfect fit, did not possess sufficient durability to recommend it as the sole defense of the wearer.