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 combining with known substances and thus of being neutralized by the presence of these substances in the respirator of the gas mask. But others were less active in a purely chemical sense. Poisons differ widely in the relation between their chemical and physiological potency. Some of our most intense poisons are so inactive chemically that it is almost impossible to find a chemical test for them. They are too inert chemically to give any marked reaction although physiologically their effect may be to produce instant death. Toxic gases or fumes which are thus incapable of being chemically arrested by means of their readiness to enter into chemical combination must be stopped mechanically in the gas mask by absorption or by the filtration of the air that carries them. But it is evident that no process of this type can be relied on as a protection for a long time if the concentration of the toxic substance in the air be maintained. All defensive means therefore have no further object than the protection of the soldier until he can escape from the 47