The moisture should, however, be determined by the method devised by Mr Arthur Marshall, F.I.C., of the Royal Gunpowder Works, Waltham Abbey, which is carried out as follows:--The cordite or other explosive is prepared in the manner laid down for the Abel heat test, that is t say, it is ground in a small mill, and that portion is selected which passes through a sieve having holes of the size of No. 8 wire gauge, but not through one with holes No. 14 wire gauge. [Illustration: FIG. 40.--MARSHALL'S APPARATUS FOR MOISTURE IN CORDITE.] The form of apparatus used is shown in Fig. 40. It consists of an aluminium dish A, having the dimensions shown, and the glass cone B weighing not more than 30 grms. Five grms. of the cordite are weighed into the aluminium dish A. This is covered with the cone B, and the whole is accurately weighed, and is then placed upon a metal plate heated by steam from a water bath. It is left upon the bath until all the moisture has been driven off, then it is allowed to cool for about half-an-hour in a desiccator and is weighed. The loss in weight gives accurately the moisture of the sample. For cordite of the original composition, one hour's heating is sufficient to entirely drive off the moisture; for modified cordite containing 65 per cent. of gun-cotton, two hours is enough, provided that there be not more than 1.3 per cent. of moisture present. If the proportion of nitro-glycerine be higher, a longer heating is necessary. The aluminium dish must not be shallower than shown in the figure, for if the distance between the substance and the edge of the glass cone be less than half an inch, some nitro-glycerine will be lost. Again, the sample must not be ground finer than stated, else some of the moisture will be lost in the grinding and sieving operations, and the result will be too low. In order to be able to drive off all the moisture in the times mentioned, it is essential that the glass cone shall not fit too closely on the aluminium dish, consequently the horizontal ledge round the top of the dish should be bent, so as to render it slightly untrue, and leave a clearance of about 0.02 inch in some places. If these few simple precautions be taken, the method will be found to be very accurate. Duplicate determinations do not differ more than 0.01 per cent.[A] [Footnote A: "Determination of Moisture in Nitro-glycerine Explosives," by A. Marshall, _Jour. Soc. Chem. Ind._, Feb. 29, 1904, p. 154.] ~The Vaseline~ (C_{16}H_{34}), or petroleum jelly, used has a flash-point of 400 deg. F. It must not contain more than 0.2 per cent. volatile matter when heated for 12 hours on the water bath, and should have a specific gravity of 0.87 at 100 deg. F., and a melting point of 86 deg. F. It is obtained during the distillation of petroleum, and consists mainly of the portions distilling above 200 deg. C. It boils at about 278 deg. C.