GUNPOWDER.

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The mills may be worked by any description of power, provided the speed be dulyregulated; but the cylinders or runners cannot safely be made to revolve more thaneight times per minute. Formerly, the runners and beds were ordinarily of compactlimestone or marble ; but cast iron is now used with safety, provided the gudgeonswork in gun-metal. The runners or cylinders are 6 feet in diameter, and about 18inches thick, and should weigh about 3 tons each. The circular bed in which theytravel is about 7 feet in diameter.

The thorough incorporation and combination of the elementary parts of the ingre-dients are most essential in good gunpowder. The operation is one of tact, andrequires experience to judge of its sufficiency, the practical indication of which is auniform greyness of appearance, and a liveliness of the composition during the latterpart of the process.

The incorporated material is termed mill-cake, and is then subjected to a pressureof about 75 tons per superficial foot, in Bramah’s hydrostatic press, or by a powerfulmechanical power, by which it is brought into a much harder substance, called press-cake ; after which it is crushed between toothed rollers of different successive gauges,or broken by wooden mallets into small pieces, which are put into parchmentsieves in a frame suspended at the corners, to which a shaking motion is given.Each sieve has in it two pieces of lignum-vit®, which, by the motion given tothe frame, continue to crush the powder until it be of a size to pass through theholes which are pierced in the parchment, of the size required for the gunpowder,into hair-cloth sieves, which retain the grained powder, and allow the dust to passthrough.

The gunpowder is then glazed, i. e. placed for 1J hours in a canvass cylinder or alarge cask, which is made to perform about 40 revolutions per minute, by whichprocess of abrasion of the grains the angular points are broken off, and the grainsacquire roundness as well as smoothness and polish of surface.

The operations of pressing and glazing do not impart strength or propellingforce to the gunpowder, but give an equal degree of density to the grains anda polish to their surface, which render the gunpowder less susceptible of ab-sorbing moisture and more competent to withstand the shaking and friction ofcarriage.

The next operation in the manufacture of gunpowder is the drying it thoroughlyby a degree of heat of not less than 140° or 150° of Fahrenheit, either in a gloomstove, or by a temperature raised by means of steam, so as effectually to drive off allremaining humidity, which the charcoal or any deliquescent impurity that mightaccidentally be still in combination with the saltpetre may have induced.

Note .—The theory of gunpowder is this. The sulphur accelerates deflagrationand supplies heat; the nitre supplies oxygen and nitrogen gases; and the carbon, byits strong affinity for oxygen, promotes the decomposition of the nitre, combiningwith its oxygen so as to produce carbonic acid gas. The sulphur melts at 226°, andunder 280° forms a clear liquid of an amber colour, and below 600° it inflames. Inthe decomposition of gunpowder by explosion, the sulphur combines with the base ofthe potash to form a solid residuum as sulphuret of potassium, whilst 3 equivalentsof carbonic acid and 1 of nitrogen are the gaseous products. The actual volume ofgas is, when cold, 300 times the bulk of the gunpowder; but the explosive force is soaugmented by the elevated temperature that it has been assumed as not less than1000 atmospheres. The above noted qualities of sulphur point to some precautionsin the arrangement and use of gunpowder, as the sulphur may by a very moderatetemperature be made to run, and thus cake the gunpowder, independently of moisture.

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