is then placed inside a water oven upon a shelf of coarse wire gauze, the temperature of the oven being kept as near as possible to 120 deg.  F. (49 deg.  C.), the gauze shelves in the oven being kept about 3 inches apart.  The sample is allowed to remain at rest for fifteen minutes in the oven, the door of which is left wide open.  After the lapse of fifteen minutes the tray is removed and exposed to the air of the laboratory (away from acid fumes) for two hours, the sample being at some point within that time rubbed upon the tray with the hand, in order to reduce it to a fine and uniform state of division.  Twenty grains (1.296 grm.) are used for the test. (See Heat Test, page 249.)

If the gun-cotton sample removed from the poacher stands the heat test satisfactorily, the machine is stopped, and the water drained off.  The cotton is allowed some little time to drain, and is then dug out by means of wooden spades, and is then ready for pressing.  The poachers hold about 2,000 lbs. of material, and as this represents the products of many hundred distinct nitrating operations, a very uniform mixture is obtained.  Two per cent. of carbonate of soda is sometimes added, but it is not really necessary if the cotton has been properly washed.

Compressing Gun-Cotton.—­The gun-cotton, in the state in which it is removed from the poacher, contains from 28 to 30 per cent. of water.  In order to remove this, the cotton has to be compressed by hydraulic power.  The dry compressed gun-cotton is packed in boxes containing 2,500 lbs. of dry material.  In order to ascertain how much of the wet cotton must be put into the press, it is necessary to determine the percentage of water.  This may be done by drying 2,000 grains upon a paper tray (previously dried at 100 deg.  C.) in the water oven at 100 deg.  C. for three hours, and re-weighing and calculating the percentage of water.  It is then easy to calculate how much of the wet gun-cotton must be placed in the hopper of the press in order to obtain a block of compressed cotton of the required weight.  Various forms of presses are used, and gun-cotton is sent out either as solid blocks, compressed discs, or in the form of an almost dry powder, in zinc-lined, air-tight cases.  The discs are often soaked in water after compression until they have absorbed 25 per cent. of moisture.

[Illustration:  FIG. 17.—­OLD METHOD. 100 PIECES.]

[Illustration:  FIG. 18.—­NEW METHOD.  ONE SOLID BLOCK.]

At the New Explosives Company’s Stowmarket Works large solid blocks of gun-cotton are pressed up under a new process, whereby blocks of gun-cotton, for use in submarine mines or in torpedo warheads, are produced.  Large charges of compressed gun-cotton have hitherto been built up from a number of suitably shaped charges of small dimensions (Fig. 17), as it has been impossible to compress large charges in a proper manner.  The formation of large-sized blocks of gun-cotton was the invention of Mr A. Hollings.