=Lime.= The hydrated lime furnishes the backbone of the absorptive properties of the soda-lime. It constitutes over 50 per cent of the finished dry granule and is responsible in a chemical sense for practically all the gas absorption. =Cement.= Cement furnishes a degree of hardness adequate to withstand service conditions. It interferes somewhat with the absorptive properties of the soda-lime and it is an open question whether the gain in hardness produced by its use is valuable enough to compensate for the decreased absorption which results. =Kieselguhr.= The loss in absorptive capacity due to the presence of cement is in part counterbalanced by the simultaneous introduction of a relatively small weight though considerable bulk, of kieselguhr. In some cases, there seems to be a reaction between the lime and the kieselguhr, which results in some increase in hardness. =Sodium Hydroxide.= Sodium hydroxide has two primary functions in the soda-lime granule. In the first place, a small amount serves to give the granule considerable more activity. The second function is to maintain roughly the proper moisture content. This water content (roughly 13-14 per cent after spraying) is very important, in order that the maximum gas absorption may be secured. =Sodium Permanganate.= The function of the sodium permanganate is to oxidize certain gases, such as arsine,[30] and to act as an assurance of protection against possible new gases. The purity of the sodium permanganate solution used was found to be one of the most important factors in making stable soda-lime. It was, therefore, necessary to work out special methods for its manufacture. Two such methods were developed, and successfully put into operation. [Footnote 30: Which, however, was never used on the battlefield.] Careful selection of other material is also necessary, and this phase of the work contributed greatly to the final development of the form of soda-lime. CHAPTER XIV TESTING ABSORBENTS AND GAS MASKS One of the first necessities in the development of absorbents and gas masks was a method of testing them and comparing their deficiencies. While the ultimate test of the value of an absorbent, canister or facepiece is, of course, the actual man test of the complete mask, the time consumed in these tests is so great that more rapid tests were devised for the control of these factors and the man test used as a check of the purely mechanical methods. TESTING OF ABSORBENTS[31] Absorbents should be tested for moisture, hardness, uniformity of sample and efficiency against various gases. [Footnote 31: See Fieldner and others, _J. Ind. Eng. Chem._, =11=, 519 (1919).] _Moisture_ is simply determined by drying for two hours at 150°. The loss in weight is called moisture.