Bramah’s hydrostatic press, or a very strong wooden press working with a powerful screw, lever, and windlass, constitutes the description of mechanism by which density is imparted to gunpowder. The incorporated or mill-cake powder is laid on the bed or follower of the press, and separated, at equal distances, by sheets of copper, so that when the operation is over, it comes out in large thin solid cakes, or strata, distinguished by the term press-cake. The mill-cake powder at Waltham Abbey, is submitted to a mean theoretic pressure of 70 to 75 tons per superficial foot.
Gunpowder should be thoroughly dried, but not by too high a degree of heat; that of 140° or 150° of Fahrenheit’s thermometer is sufficient. It appears to be of no consequence whether it be dried by solar heat; by radiation from red-hot iron, as in the gloom stove; or by a temperature raised by means of steam. Her Majesty’s gunpowder is dried by the last two methods. The grain should not be suddenly exposed to the highest degree of heat, but gradually.
The method of trial best adapted to shew the real inherent strength and goodness of gunpowder, appears to be an eight or ten-inch iron or brass mortar, with a truly spherical solid shot, having not more than one-tenth of an inch windage, and fired with a low charge. The eight-inch mortar, fired with two ounces of powder, is one of the established methods of proof at Her Majesty’s works. Gunpowders that range equally in this mode of trial, may be depended on as being equally strong.
Another proof is by four drachms of powder laid in a small neat heap, on a clean, polished, copper plate; which heap is fired at the apex, by a red-hot iron. The explosion should be sharp and quick; not tardy, nor lingering; it should produce a sudden concussion in the air, and the force and power of that concussion ought to be judged of by comparison with that produced by powder of known good quality. No sparks should fly off, nor should beads, or globules of alkaline residuum, be left on the copper. If the copper be left clean, i. e. without gross foulness, and no lights, i. e. sparks, be seen, the ingredients may be considered to have been carefully prepared, and the powder to have been well manipulated, particularly if pressed and glazed; but if the contrary be the result, there has been a want of skill or of carefulness manifested in the manufacture.
“Gunpowder,” says Captain Bishop “explodes exactly at the 600° of heat by Fahrenheit’s thermometer; when gunpowder is exposed to 500° it alters its nature altogether; not only the whole of the moisture is driven off, but the saltpetre and sulphur are actually reduced to fusion, both of which liquefy under the above degree. The powder on cooling, is found to have changed its colour from a gray to a deep black; the grain has become extremely indurated, and by exposure even to very moist air, it then suffers no alteration by imbibing moisture.”
The mill for grinding the gunpowder cake may be understood from the following representation: ([fig. 531.]) p, is the water wheel, which may drive several pairs of stones; q, q, two vertical bevel wheels, fixed upon the axis of the great wheel; r, r, two horizontal bevel wheels working in q, q, and turning the shafts s, s; t, t, two horizontal spur wheels fixed to the upper part of the vertical shafts, and driving the large wheels u, u. To the shafts of these latter wheels are fixed the runners v, v, which traverse upon the bed stone w, w; x, x, are the curbs surrounding the bed stone to prevent the powder from falling off; o is the scraper. Mill A represents a view, and mill B a section of the bed stone and curb.
GYPSUM, Sulphate of Lime, [Alabaster], or Paris Plaster. This substance is found in three geological positions in the crust of the earth; among transition rocks; in the red marl formation; and above the chalk, in the tertiary beds.
1. The alpine gypsums are ranged by M. Brochant among the transition class, and are characterized by the presence of anthracite or stone coal; some of them are white and pure, others gray or yellowish, and mixed with mica, talc, steatite, black oxide of iron, pyrites, compact carbonate of lime, sulphur, and common salt. Examples of such localities are found in the gypsum of Val-Canaria at the foot of Saint Gothard, that of Brigg in the upper Valais; of the Grilla in the valley of Chamouni, and of Saint Gervais-les-Bains, near Sallenches in Savoy.