To guard against misfires, the machine or the battery used should be constructed to give a very powerful current. If this precaution be observed, and the number of fuses in circuit be limited to one-half that which the machine is capable of firing with a fair degree of certainty, perfectly satisfactory results may be obtained. The employment of weak machines and batteries leads inevitably to failure. In the minds of those who have hitherto tried electrical blasting in this country, there seems to be no notion of any relation existing between the work to be done and the force employed to do it. The electrical exploder is regarded as a sort of magic box that needs only to be set in action to produce any required result. Whenever failure ensues, the cause is unhesitatingly attributed to the fuses.
CHAPTER II.
EXPLOSIVE AGENTS USED IN BLASTING ROCKS.
Section I.—Phenomena accompanying an Explosion.
Nature of an Explosion.
—The combination of oxygen with other substances for which it has affinity is called generally “oxidation.” The result of this combination is a new substance, and the process of change is accompanied by the liberation of heat. The quantity of heat set free when two substances combine chemically is constant, that is, it is the same under all conditions. If the change takes place within a short space of time, the heat becomes sensible; but if the change proceeds very slowly, the heat cannot be felt. The same quantity, however, is liberated in both cases. Thus, though the quantity of heat set free by a chemical combination is under all conditions the same, the degree or intensity of the heat is determined by the rapidity with which the change is effected.
When oxidation is sufficiently rapid to cause a sensible degree of heat, the process is described as “combustion.” The oxidation of a lump of coke in the furnace, for example, is effected within a short space of time, and, as the quantity of heat liberated by the oxidation of that weight of carbon is great, a high degree results. And it is well known and obvious that as combustion is quickened, or, in other words, as the time of change is shortened, the intensity of the heat is proportionally increased. So in the case of common illuminating gas, the oxidation of the hydrogen is rapidly effected, and, consequently, a high degree of heat ensues.
When oxidation takes place within a space of time so short as to be inappreciable to the senses, the process is described as “explosion.” The combustion of a charge of gunpowder, for example, proceeds with such rapidity that no interval can be perceived to intervene between the commencement and the termination of the process. Oxidation is in this case, therefore, correctly described as an explosion; but the combustion of a train of gunpowder, or of a piece of quick-match, though exceedingly rapid, yet, as it extends over an appreciable space of time, is not to be so described. By analogy, the sudden change of state which takes place when water is “flashed” into steam, is called an explosion. It may be remarked here that the application of this expression to the bursting of a steam boiler is an abuse of language; as well may we speak of an “explosion” of rock.
From a consideration of the facts stated in the foregoing paragraphs, it will be observed that oxidation by explosion gives the maximum intensity of heat.