It was mentioned above that hexamethylenetetramine (urotropine) was used in the early pads (black veil and similar masks) and flannel helmets. This was found to be satisfactory against chlorine and phosgene, in the concentrations usually found during a cylinder attack. The mixture used consisted of urotropine, sodium thiosulfate (“hypo”), sodium carbonate and glycerine. The glycerine tended to keep the pads moist, while the other chemicals acted as protective agents against the mixture of phosgene and chlorine. The introduction of the Standard Box Respirator with its charcoal-soda lime filling increased very materially the protection against phosgene. In this filling, the charcoal both absorbs the phosgene and catalyzes the reaction with the moisture of the air with which the phosgene is mixed, to form hydrochloric acid and carbon dioxide. Soda-lime absorbs phosgene but does not catalyze its decomposition. This shows the advantage of the mixture, since the hydrochloric acid, which is formed through the action of the charcoal, is absorbed by the soda-lime. Experiments seem to indicate that it does not matter which material is placed in the bottom of the canister, but that an intimate mixture is the best arrangement. Using a concentration of 5,000 parts per million (20.2 mg. per liter) a type _H_ canister (see page 217) will give complete protection for about 40 minutes; when the air-gas mixture passes at the rate of 16 liters per minute the efficiency or life of a canister increases with a decrease in temperature, as is seen in the following table (the concentration was 5,000 parts per million, the rate of flow 16 liters per minute) Temperature Efficiency ° C. (Time in minutes) -10 223 0 172 10 146 20 130 30 125 40 99 From these figures it is seen that at -10° C. the life is about 50 per cent greater than at summer temperature. As would be expected the life of a canister is shortened by increasing the concentration of phosgene in the phosgene air mixture. This is illustrated by the following figures: Concentration Life p.p.m. (Time in minutes) 5,000 177 10,000 112 15,000 72 20,000 58 25,000 25 (25,000 p.p.m. is equal to 101.1 mg. per liter.) There is rather a definite relation between the concentration of the gas and the life of a canister at any given rate of flow. Many of these relations have been expressed by formulas of which the following is typical. At 32 liters per minute flow, =C⁰˙⁹ × Tb = 101,840=, in which =C= is the concentration and =T= the time. SHELL FILLING