[Illustration: MAKING MERCURY FULMINATE.] Even these severe means are not enough, for, as the cotton fiber is in the form of hairlike tubes, traces of the acid sufficient to bring about the subsequent decomposition of the gun cotton are retained by capillarity. Therefore, after boiling with a dilute solution of sodium carbonate, the gun cotton is pulped and washed in a beater or rag engine until the fiber is reduced to the fineness of corn meal, and a sample of it will pass the “heat test.” This is a test of the resistance of gun cotton to decomposition, and requires that when the air-dried sample of gun cotton is heated to 65.5° C. in a closed tube in which a moistened strip of potassium iodide and starch paper is suspended, the paper should not become discolored in less than fifteen minutes’ exposure. [Illustration: DETONATOR USED IN THE UNITED STATES NAVY. Contains thirty-five grains of fulminate of mercury.] This pulping of the gun cotton not only enables one to more completely purify it, but it also renders it possible to mold it into convenient forms and to compress it so as to greatly increase its efficiency in use. For this purpose the pulp is suspended in water and pumped to a molding press, where, under a hydraulic pressure of one hundred pounds to the square inch, it is molded into cylinders or prisms about three inches in diameter and five inches and a half high, and these are compressed to two inches in height by a final press exerting a pressure of about sixty-eight hundred pounds to the square inch. As this is regarded as a somewhat hazardous operation, the press is surrounded by a mantlet woven from stout rope to protect the workmen from flying pieces of metal in case of an accident. The operation is analogous to that employed in powder-making, where the gunpowder has been pressed in a great variety of forms and into single grains weighing several pounds apiece. [Illustration: TORPEDO CASES AND BLOCKS OF WOOD DESTROYED BY A NAVAL DETONATOR.] [Illustration: TESTING DETONATORS ON IRON PLATES.] [Illustration: IRON CYLINDER FILLED WITH WATER AND CONTAINING A NAVAL DETONATOR. Before and after firing, shows the work accomplished by thirty-five grains of mercury fulminate.] Even under the enormous pressure of the final press the compressed gun cotton still retains from twelve to sixteen per cent of water, and in this form it is quite safe to store and handle. When dry it is very combustible and burns readily when ignited, but it can be quenched by pouring water upon it. When confined in the chamber of a gun or the bore-hole of a rock, gun cotton will burn like gunpowder when ignited, if dry, and produce an explosion, but, in common with nitroglycerin and other high explosives, gun cotton is best exploded and develops its maximum effect when detonated, a result which is secured by exploding a small quantity of mercury fulminate in contact with the dry material.