The life of this canister was the same irrespective of whether its use was continuous or intermittent. The higher the temperature the longer the life because Hopcalite is less sensitive to water vapors at higher temperatures. Since, if the effluent air was sufficiently dried, the Hopcalite should function indefinitely against any concentration of carbon monoxide, the life of the canister is limited solely by the life of the drier. Therefore the net gain in weight is a sure criterion of its condition. After many tests it was determined that any canister which had gained more than 35 grams above its original weight should be withdrawn. The canisters, at the time of breakdown, showed a gain in weight varying between 42 and 71 grams, with a average of 54 grams. It is really, therefore, the actual humidity of the air in which the canister is used that determines its life. [Illustration: FIG. 41.—Navy Head Mask and Canister.] CHAPTER XII DEVELOPMENT OF THE GAS MASK While in ordinary warfare the best defense against any implement of war is a vigorous offense with the same weapon, Chemical Warfare presents a new point of view. Here it is very important to make use of all defensive measures against attack. Because of the nature of the materials used, it has been found possible to furnish, not only general protection, but also continuous protection during the time the gas is present. The first consideration in the protection of troops against a gas attack is the provision of an efficient individual protective appliance for each soldier. The gas attack of April 22, 1915 found the Allies entirely unprepared and unprotected against poisonous gas. While a few of the men had the presence of mind to protect themselves by covering their faces with wet cloths, the majority of them became casualties. Immediately steps were taken to improvise protective devices among which were gags, made with rags soaked in water or washing soda solution, handkerchiefs filled with moist earth, etc. One suggestion was to use bottles with the bottom knocked off and filled with moist earth. The breath was to be taken in through the bottle and let out through the nose; but as bottles were scarce and few of them survived the attempt to get the bottom broken off, the idea was of no value. The first masks were made by the women of England in response to the appeal by Lord Kitchener; they consisted of cotton wool wrapped in muslin or veiling and were to be kept moist with water, soda solution or hypo. ENGLISH MASKS =The Black Veiling Respirator.= The first form of the English mask is known as the Black Veiling respirator and consisted of cotton waste enclosed in a length of black veiling. The waste was soaked in a solution of: Sodium thiosulphate 10 lbs. Washing soda 2.5 lbs. Glycerine 2 lbs. Water 2 gals.