Charcoal is not a satisfactory all-round absorbent because it has too little capacity for certain highly volatile acid gases, such as phosgene and hydrocyanic acid, and because oxidizing agents are needed for certain gases. To overcome these deficiencies the use of an alkali oxidizing agent in combination with the charcoal has been found advisable. The material actually used for this purpose has been granules of soda-lime containing sodium permanganate. Its principal function may be said to be to act as a reservoir of large capacity for the permanent fixation of the more volatile acid and oxidizable gases. The development of a satisfactory soda-lime was a difficult problem. The principal requirements follow: Its _activity_ is not of vital importance, as the charcoal is able to take up gas with extreme rapidity and then later give it off more slowly to the soda-lime. _Absorptive capacity_ is of the greatest importance, since the soda-lime is relied upon to hold in chemical combination a very large amount of toxic gas. Both _chemical stability_ and _mechanical strength_ are difficult to attain. The latter had never been solved until the war made some solution absolutely imperative. COMPOSITION OF REGULAR ARMY SODA-LIME The exact composition of the army soda-lime has undergone considerable modification from time to time as it has been found desirable to change the raw materials or the method of manufacture. A rough average formula which will serve to bring out the interrelation between the different constituents is as follows: COMPOSITION OF WET MIX Per Cent Hydrated lime 45 Cement 14 Kieselguhr 6 Sodium hydroxide 1 Water 33 (approx.) AFTER DRYING Moisture content 8 (approx.) AFTER SPRAYING Moisture content 13 (approx.) Sodium permanganate content 3 (approx.) Within limits, the method of manufacture is more important than the composition or other variables, and has been the subject of a great deal of research work even on apparently minor details. The process finally adopted consists essentially in making a plastic mass of lime, cement, kieselguhr, caustic soda, and water, spreading in slabs on wire-bottomed trays, allowing to set for 2 or 3 days under carefully controlled conditions, drying, grinding, and screening to 8-14 mesh, and finally spraying with a strong solution of sodium permanganate with a specially designed spray nozzle. The spraying process is a recent development, most of the soda-lime having been made by putting the sodium permanganate into the original wet mix. Many difficulties had to be overcome in developing the spraying process, but it eventually gave a better final product, and resulted in a large saving of permanganate which was formerly lost during drying, in fines, etc. FUNCTION OF DIFFERENT COMPONENTS