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decided limitations upon the number of materials or substances which may be used as components.. The material used for the mechanical filter must be sufficiently dense to hold out the extremely minute solid or liquid particles of which the irritant gases and smokes are constituted. These particles, it may be said, are so small that they cannot be seen with an ordinary high-powered microscope while even with the ultramicroscope they are only visible as points of reflected light. On the other hand, the filter material must not be so dense as to impede unduly the flow of air through it. The chemical or chemicals used for the removal of gases must be highly porous in order to provide within small space a relatively enormous absorbent surface. They must not react with each other or corrode their metal container. Their effectiveness must not be appreciably lowered by exposure to air of high humidity. They must remove the gas very rapidly since any given portion of inspired air is in contact with the canister filling for but a fractional part of a second. They must have the capacity to dispose of large amounts of gas since the canister cannot be frequently replaced. They must be fairly cheap and available in great quantity. In turn, thev. as well as the filter, must not cause high breathing resistance.

The only single substance which approximately fulfills ail the requires lucnts of a chemical filling for gas-mask canisters is activated charcoal in the form of small granules. Generally speaking the best charcoals for this pur|x>sc are made from very dense raw materials. The most satisfactory material found during the World War tor canister charcoal was cucoanut shell. Various nut shells, fruit stones, and other substances, however, were also used. Sincc the war, improved methods of manufacture have made possible the use of more readily available materials. Charcoal is a highly porous substance consisting principally of carbon which is made by the carbonization of organic matter. As such, it is called primary charcoal. By subjecting primary charcoal to a certain process of heat and steam, called activation, the property of adsorption of gases, which primary charcoal jkjsscsscs, is greatly increased.

When gas-laden air is passed through activated charcoal, the molecules of gas are attracted and held physically on the surface of the pores in the

54b charcoal granules, the purified air passing on through. This process of removal of the gas is called adsorption. It may roughly be compared to i he action of a magnet in at tracting and holding iron fillings on its surface. Activated charcoal which will adsorb half its own weight of toxic gas ha* l>ecn made on a large scale, while charcoals have been made in the labora-tory which will adsorb more than their own weight of gas. The principal deficiency of charcoal as a canister filling is that it does not hold tena-

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Fio. 117.—Gut^-mnsk canister (scctionjdixcdh riouslv certain highly volatile acid gases, notably phosgene, but gradually releases them to the passing air current. This deficiency is compensated lor by the use of another substance mixed with the charcoal, viz., soda lime.

Soda lime is a mixture consisting of hydrated lime, cement, kieselguhr, sodium hydroxide, and water in various proportions according to the lormula used; there are several. Gases, which the charcoal does not hold firmly by adsorption and which are gradually given off by it, are caught by the soda lime, with which they enter into chemical combination. After continued exposure to certain gases, such as phosgene, a gradual transfer of the gas to the soda lime takes place, thus leaving the charcoal free to pick up more gas. It may therefore be said that the principal iunction of the soda lime is to act as a reservoir of large capacity for the |M'rmancnt fixation of the more volatile acid and oxidizablc gases, while the charcoal furnishes the required degree of activity for all gases as well storage capacity for less volatile ones.

Another reason for the combination al>sorhent is that, while a rise in cidicf temperature or humidity causes a decrease in the adsorptive capacity of charcoal» such conditions increase the reactivity of the soda lime

The canister of the present military mask contains a mixture of soda lime and specially prepared charcoal as well as a highly efficient mechanical filter. It can thus be relied Upon to give full protection against any gas likely to be encountered in the field. The function of the different components of the canister us regards the principal war gases i* set forth below .

(tflu NtutrAliniiitf AjSt-m y Brombenzyl cyunitlc Charcoal

Charcoal

Charcoal Charcoal

Charcoal and filter Charconl-fioda-limc mixture Charcoal-fsoda-lune nnxt ure Charcoal-«xla-limc mixturc

Cldorpierin

Cyanogen chloridc. Mustard gnv.. .

Chfcmtectophviuuic ,

Chlorine

Phosgene

I )i phosgene

Hydrocyanic acid Chareoal-wsla-lime mixture

Diphcnylrlilurarsmc Filter

Di|>liejty|jiminf<'hlorarx[iic, etc Filter

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