use in mining and engineering operations; among others, the Hoosac Tunnel and the works at Hallett's Point in New York harbor undoubtedly owed much to the powers of nitro-glycerine in hastening their completion. The effect of a confined charge upon rock is to pulverize the portion near the blast-hole, the action being so sudden, quick, and intense; it is, therefore, found better not to tamp the hole, a saving both of time and labor, allowing the gases a greater surface upon which to act. Ordinarily the dynamite is contained in paraffined paper cartridges, and is fired with a fulminate-fuse.
Gun-cotton is formed by the action of nitric acid on cotton—a portion of the hydrogen being displaced in the cotton, just as it is in the glycerine by the active constitution of the nitric acid. The essential features of the process are the same as those of the manufacture of nitro-glycerine; that is to say, a mixture of strong nitric and sulphuric acids is made, the cotton exposed to its action, and the excess of acid removed from the cotton by careful washing; the sulphuric acid plays the same part, namely, that of taking up the water formed, and so keeping the nitric acid at its full strength. As in the former case, only perfect purity will insure safety; the presence of acid in the gun-cotton will ultimately cause decomposition and explosion. Many accidents have taken place since the introduction of gun-cotton to public notice some thirty-five years ago; but, as in the case of nitro-glycerine, all of them may be ascribed to imperfect washing, reference being had, of course, to instances of what may be called spontaneous combustion. With the improved methods of to-day, however, and the exercise of ordinary care, gun-cotton can be rendered perfectly stable and safe, far safer for transportation than nitro-glycerine in any of its forms; when wet it can not be readily exploded, and hence it is generally carried in that state, and either dried for use or else exploded by the use of a dry primer of the same material. It possesses, however, the disadvantage of rapidly absorbing moisture, and hence it is extremely difficult to keep primers dry unless they are prepared with great care. In a military point of view this is no great objection, but commercially speaking it is, for the expensive water-proof cases would perhaps make its use financially impracticable.
The details of the usual process of its manufacture are as follows: cotton-waste is picked and cleaned, then dried at a high temperature. After cooling, in quantities of about a pound, it is immersed in the strong acid mixture contained in a trough surrounded by cold water; after a short exposure it is removed, and the acid pressed from it as far as practicable. After another immersion of twenty-four hours, it is placed in a centrifugal strainer, by the rapid revolutions of which nearly all the acid is expelled; it is then washed in a large amount of water, and again placed in the strainer. When the acid-water no longer remains, the gun-cotton is placed in the pulper, an oblong tub
