where the proper care has been given to all the details of the manufacture.
There are other methods of manufacture, differing, however, only in the details, the principle of course being the same.
When pure, nitro-glycerine is not very sensitive to friction, or even to moderate percussion: if a small quantity be placed on an anvil and struck with a hammer, that portion which is touched explodes sharply, but so quickly as to drive away the other particles; if, however, it were even slightly confined, so that none could escape, it would all explode or detonate. It must be fired by a fuse containing fulminate of mercury (the compound used in percussion-caps), not being either readily or certainly fired by gunpowder, the shock of the latter not being sufficiently quick or sharp to detonate the nitro-glycerine. It is highly probable that in this case, as in that of other high explosives, the vibrations set up by the fulminate (which is not stronger than gunpowder) are of just such a character as to find an answering chord, so to speak, in the explosive, so that the desired effect is produced. This would seem to be a correct theory, for it is not always the most powerful explosive which most readily causes the explosion of another body. For instance, although nitro-glycerine is much more powerful than fulminate of mercury, yet seventy grains of it will not explode gun-cotton, while fifteen grains of the weaker fulminate will readily do so. The fuse generally used, then, for firing nitro-glycerine, is composed of from fifteen to twenty-five grains of fulminate, and this quantity is sufficient to detonate a large mass as well as a small one.
If flame be applied to nitro-glycerine it will not explode, but burn with comparative sluggishness. When frozen it is very difficult and uncertain of firing. If the material be perfectly pure, it forms, upon detonation, a volume of gases nearly thirteen hundred times as great as that of the original liquid; these gases are also further expanded, by the heat developed, to a theoretical (though not practical) volume ten thousand times as great as that of the charge. Practically speaking, the forces exerted by gunpowder and nitro-glycerine are in the proportion of one to eight.
The great objection to nitro-glycerine, in its liquid state, is the difficulty of its transportation; it is liable to leak from the packages in which it is contained, and there have been several occasions on which disastrous accidents have taken place owing to this circumstance. The explosion of a large case on board of a steamer in Aspinwall some twelve or fourteen years ago, and, about the same time, of a box in an express-office in New York, caused great precautions to be taken with regard to it, and also very great fear of it on the part of all transportation companies. Fortunately, it has been found that it can be carried from place to place by mixing it with some absorbent substance, which takes up a large quantity of it; it is just as powerful in this state, the presence of the absorbent having no deleterious effect. This