mula for "making thunder and lightning," are still in use, and in nearly the same proportion now as at that early date. The three solid substances which, when properly and intimately mixed, form gunpowder, are saltpeter, sulphur, and charcoal; the first being three fourths, or a little more, of the mixture, the two latter, in nearly equal but varying proportions, forming the remaining one fourth. The saltpeter contains a large amount of oxygen, which it gives up with considerable readiness upon the application of heat; this unites with the carbon contained in the charcoal, forming a large volume of carbonic acid gas; the potash of the saltpeter unites with the sulphur, while the nitrogen is set free, adding to the volume of the gases evolved. As these gases are given off with great rapidity, they are still further expanded by the action of the heat produced by the change of the solids into the gaseous form, until, under ordinary circumstances, they occupy a volume nearly three hundred times as great as that occupied by the powder itself. One can easily imagine that the expansion of a pound of powder to a size (if we may so say) so much greater than its original bulk would exert a tremendous pressure upon any object in which it was confined. If a small quantity of powder is freely exposed in the air and fired, the resulting explosive effect is small, as the gases produced so rapidly can readily push aside the air; but if it be confined in a large block of steel, in which is a cavity which will just receive it snugly, the resulting pressure is nearly ninety-three thousand pounds to the square inch.
From this illustration it will at once be understood that confinement, in a greater or less degree, is necessary, in order that the greatest amount of work may be accomplished by the explosion. Those explosives which give off their gases with intense rapidity require but little restraint, while the slower ones require more confinement. Among these less quick ones must be reckoned gunpowder, for, although the explosion seems to follow immediately upon the application of heat, yet an appreciable amount of time really elapses, in which the combustion goes on.
The ingredients of which gunpowder is composed are selected with the greatest possible care: the saltpeter is procured chiefly from India, being extracted from the soil by natural processes and then secured by the natives, for marketing. It is also largely prepared by artificial means, it being quite evident that it would not do for any country to depend upon a supply without its own borders in case of war. In this process, heaps of earth are mixed with decomposing organic matter—ashes, old mortar, and materials of like nature—the whole being wet with the liquor from stables sufficiently to keep it in a moist condition; the moistened heaps are worked over from time to time, and the air allowed free access. A chemical reaction takes place, and in time the crude saltpeter appears upon the exterior of the heap, whence it is removed and treated for the extraction of the pure
