7 20 THE POPULAR SCIENCE MONTHLY. 



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 pre- 

 paring 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 tempera- 

 ture. It is this quality which constitutes it an explosive, for the sud- 

 den expansion is what we call explosion, though the name is some- 

 times given to the loud report which accompanies it, caused by the 

 outrush of the gas generating sound-waves in the air. When the ex- 

 plosion occurs in a confined space, the weakest portion of the confin- 

 ing 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, essen- 

 tial to it. The gas is actually generated from the charcoal and salt- 

 petre, 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 explo- 

 sion 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 tempera- 

 ture than either charcoal or saltpetre, and its combustion accelerates 



