I have previously referred to the necessity of using a heavy charge of fulminate of mercury, in order to secure perfect and instantaneous explosion of a charge of Nitro-Glycerin, without confining the latter; the manipulating this explosive salt (fulminate of mercury) without hazard to the operators (generally girls), was accomplished by precipitating gum mastich from its alcoholic solution, by the addition of water, and mixing in the moist fulminate, and then filling the pasty compound into a stout copper capsule, which is subsequently enclosed in a wooden case, saturated with paraffine. The resistance of the stout copper capsule, immensely adds to the effective force of the exploder, and ensures the most effective explosion of the Nitro-Glycerin, which cannot be obtained by a wooden capsule alone. These details as to the requirements for effectively exploding the nitro-compounds, have been very fully examined and proved, by Abel, Article, Pyroxylin, Watts’ Chem. Dictionary, Vol. 4, p. 776, et seq., and daily use confirms them. My observation of the fatalities that have occurred with over-sensitive priming composition, introduced with a view to compensate for deficient electric force, and thus to permit the use of a weak battery and cheap cotton covered wire varnished over (instead of gutta-percha insulation), in order to substitute a weak current that would be sufficient to fire these over-sensitive exploders for the stronger current required to fire a safe priming, satisfy me that electric blasting had better be discontinued, and tape fuse resumed, unless the work will bear the expense of absolutely safe materials. Better to face the difficulty, construct efficient electric apparatus, convey the electricity along wires of perfect insulation to a safe priming, and by complete and violent explosion of the Nitro-Glycerin, or powder, make such effective blasting as not to throw away the labor of drilling, candles, power, and blasting materials. I believe this the true economy. These details may seem wearisome, but the casualties of blasting can best be diminished by avoiding missed holes, a result only attainable by using materials absolutely reliable; and the reader, if he has ever attempted to harness up as a team those subtile, evasive, terrific forces—electricity and explosives, for the service of his fellowman, will excuse the writer’s earnestness and agree with him that in such a task the rule should be “Aut nunquam tenta aut perfice.” [Illustration] CHAPTER V. The Tri-Nitro-Glycerin Manufactured at the Hoosac Tunnel—How Tri-Nitro-Glycerin is Made—How Stored—How Gutta-Percha is Purified—How the Conducting Wires are Insulated—How the Exploders are Manufactured.