to be useful where a stronger explosive than ordinary gunpowder is desired is known as Abel's picric powder; this is a mixture of picrate of ammonia and saltpeter, prepared in an ordinary powder-mill by the processes just detailed. Picric acid (by means of which the picrate of ammonia is obtained) is derived from the action of strong nitric acid upon carbolic acid. This powder requires confinement to develop its force, is not exploded by friction, and, as it absorbs no moisture by exposure to the air, can be used and stored like gunpowder; for the ordinary uses of gunnery, however, it can never supersede its elder brother.
As has already been said, gunpowder burns, rapidly it is true, but nevertheless the action is a true combustion. We now come to the consideration of a new class of explosives, which do not burn, but resolve themselves into their constituent gases immediately upon the application of the force which suffices to bring about their disintegration. These are not mixtures like gunpowder, but definite chemical compounds in which the carbon, oxygen, and hydrogen are held a little way apart, as it were, by the nitrogen, but ready to rush together at the first opportunity; the explosion of one particle means generally the contemporaneous explosion, or detonation, of the whole mass: with such bodies confinement to any great extent is not necessary.
Pre-eminent among these detonating substances for its use in mining and engineering operations of a like character stands the compound known as nitro-glycerine. This is manufactured in greatly increasing quantities in many places, both in this country and in Europe, and its use for the purposes above mentioned is becoming more and more general. Its transportation in the liquid state being excessively dangerous, it was for a long time but little used; but, it having been ascertained that it can be mixed with other materials and carried with comparative safety, its field of usefulness has become greatly enlarged. It is formed by the action of very strong nitric acid upon glycerine at a low temperature, the resulting product being removed and washed free of its impurities.
Glycerine is a by-product of the stearine-candle manufacture, being separated from the stearic acid contained in animal fat upon the application of superheated steam. It is subsequently redistilled until its impurities are removed, in which condition it is proper to be used in the manufacture of nitro-glycerine. Unfortunately, however, many manufacturers do not restrain themselves to a pure quality of the glycerine, but, using inferior ones (which are, of course, cheaper), make a nitro-glycerine which is of a much more dangerous character than should ever be used.
The process of manufacture is as follows: The nitric acid is mixed with twice the quantity of sulphuric acid, and both then mixed with the glycerine; the nitric acid acts upon the glycerine, leaving a quantity of water free; were the sulphuric acid not present, this water