The use of this mask ensures that all the air breathed must enter the lungs through the canister. This air passage is entirely independent of leaks in the facepiece, due either to a poor fit about the face or to actual leakage (from a cut or tear) of the fabric itself. The facepiece is readily cleared of poison gases which may leak in. This is accomplished by taking a full inspiration, releasing the noseclip, and exhaling through the nose, which forces the air out around the edges of the facepiece. On the other hand, this type of mask possesses a number of very obvious disadvantages, particularly from a military point of view: The extreme discomfort of the facepiece. This discomfort arises from a number of causes certain of which are inherent in this type of mask, among them being: (_a_) the noseclip, (_b_) the mouthpiece, and (_c_) the lack of ventilation within the facepiece chamber. Aside from the actual physical discomfort of the noseclip and mouthpiece, which becomes intense after long periods of wearing, this combination forces upon the wearer an unnatural method of respiration to which it is not only difficult to become accustomed, but which also causes extreme dryness of the throat. The mouthpiece greatly increases salivation and as swallowing is rather more difficult with the nose closed, this adds another extremely objectionable feature. [Illustration: FIG. 45.—Interior of S. B. R., Showing Cotton Wrapped Noseclips.] [Illustration: FIG. 46.—French M-2 Mask.] The lack of ventilation in the facepiece chamber entraps the heat radiating from the face and retains the moisture which is constantly evaporating from the skin. This moisture condenses on the eyepieces, and even if cleared away by the use of a so-called anti-dimming paste, usually makes vision nearly impossible. FRENCH MASKS =M-2 Mask.= The early protection of the French Army was obtained from a mask of the type M-2 (Fig. 46). This mask consists of a number of layers of muslin impregnated with various absorbent chemicals. A typical mask was made up of 20 layers of cheesecloth impregnated with _Greasene_ and 20 layers impregnated with _Complexene_. These solutions were made up as follows: Complexene: 39.0 lbs. Hexamethylenetetramine 37.5 lbs. Glycerine 27.5 lbs. Nickel sulfate (NiSO₄.7 H₂O) 11.8 lbs. Sodium carbonate (Na₂CO₃) Water Greasene: 107.0 lbs. Castor oil 81.0 lbs. Alcohol (95%) 10.7 lbs. Glycerine (90%) 3.1 lbs. Sodium hydroxide (NaOH)