=Gunpowder.=--Gunpowder is composed of charcoal, sulphur, and saltpeter in proportions varying according to the quality of the powder. For mining purposes the composition employed is 65% saltpeter, 15% sulphur, and 20% charcoal. It is a black granulated powder having a specific gravity of 1.5; the black color is given by the charcoal; and the grains have an angular form, and vary in size from ¹⁄₈ in. to ³⁄₈ in. Good blasting powder should contain no fine grains, which may be detected by pouring some of the powder upon a sheet of white paper. The force developed by the explosion of gunpowder is not accurately known; it depends upon the space in which it is confined. Different authorities estimate the pressure at from 15,000 lbs. per sq. in. in loose blasts to 200,000 lbs. per sq. in. in gunnery. Authorities also differ in opinion as to the character of the gases developed by the explosion of gunpowder, a matter of vital concern to the tunnel engineer, since they are likely to affect the health and comfort of his workmen. It may be assumed in a general way, however, that the oxygen of the saltpeter converts nearly all of the carbon of the charcoal into carbon dioxide, a portion of which combines with the potash of the saltpeter to form carbonate of potash, the remainder continuing in the form of gas. The sulphur is converted into sulphuric acid, and forms a sulphate of potash, which by reaction is decomposed into hyposulphite and sulphide. The nitrogen of the saltpeter is almost entirely evolved in a free state; and the carbon not having been wholly burnt into carbonic acid, there is a proportion of carbonic oxide. =Nitroglycerine.=--Nitroglycerine is one of the modern explosives used as a substitute for gunpowder. It is a fluid produced by mixing glycerine with nitric and sulphuric acids; it freezes at +41° F., and burns very quietly, developing carbonic acid, nitrogen, oxygen, and water. By percussion or by the explosion of some substances, such as capsules of gunpowder or fulminate of mercury, nitroglycerine produces a sudden explosion in which about 1250 volumes of gases are produced. The pressure of these gases has been calculated at 26,000 atmospheres, or 324,000 lbs. per sq. in. Nitroglycerine explodes very easily by percussion in its normal state, but with great difficulty when frozen; hence, in America, at the beginning of its use, it was transported only in a frozen state. When dirty, nitroglycerine undergoes a spontaneous decomposition accompanied by the development of gases and the evolution of heat, which, reaching 388° F., causes it to explode. Notwithstanding the enormous pressures which nitroglycerine develops, it is very seldom used in its liquid state, but is mixed with a granular absorbent earth composed of the shells of diatoms. The fluid undergoes no chemical change by being absorbed, and explodes, freezes, and burns under the same conditions as in the fluid state.