widely extended, gun-metal and copper have been largely substituted for wood in the structure of the machines, refining-houses have been erected for purifying the saltpetre and sulphur, and retorts for preparing the charcoal. Machinery has been designed and erected for the preparation of the large cannon-powder introduced of late years, and in the mills iron runners, driven by steam, have taken the place of the stone runners, drawn by old horses. A complete code of rules and precautions has been introduced, and every building protected by a system of lightning-conductors. The factory gives employment to about two hundred men, and can produce twenty-four thousand barrels of gunpowder in the year, and the powder is believed to be at once the best and cheapest made by any existing factory.
Before describing the process of its manufacture at Waltham, it may be as well to note a few facts on its composition and action. Gunpowder may be regarded as a solid, which, by ignition, can be very rapidly converted into a large volume of gas at a high temperature. It is this quality which constitutes it an explosive, for the sudden expansion is what we call explosion, though the name is sometimes given to the loud report which accompanies it, caused by the outrush of the gas generating sound-waves in the air. When the explosion occurs in a confined space, the weakest portion of the confining bodies gives way before it. In quarrying, the rocks are rent, as the gas from the blasting-powder forces them apart. In blowing down walls and gates, the mass of earth heaped on one side to form the "tamping" offers a greater resistance than the wood or stone on the other, and the wall or gate gives way. In firing a cannon, the loose shot offers less resistance than the solid coils of the gun, and it is driven out to a distance proportioned to the force of the charge. If there is any defect impairing the strength of the cannon, or if the shot wedges in the bore, the gun bursts; for nothing we know of can resist the force of the gas. Recent experiments prove that this force, exerted in closed vessels unrelieved by expansion, is equal to a pressure of about forty tons on the square inch.
Of the three materials of which gunpowder consists—sulphur, charcoal, and saltpetre—only the last two are, strictly speaking, essential to it. The gas is actually generated from the charcoal and saltpetre, therefore a mixture of these only will explode. On ignition the charcoal decomposes the saltpetre, its combustion being supported by the oxygen of the latter, in combination with which it forms carbonic-acid gas, and this, mixed with the nitrogen from the saltpetre, is the gas which produces the useful effect. But, when gunpowder is thus made with saltpetre and charcoal only, the power developed by the explosion is comparatively trifling, and sulphur has to be added to increase it to such an extent as to make it really efficient. The sulphur acts in two ways to this end. In the first place, it ignites at a lower temperature than either charcoal or saltpetre, and its combustion accelerates
