The granulated matter consists of particles of very dif ferent sizes, and some reduced to dust. These are passed through wire sieves of different sizes, to give the differ ent sized grains in which it is sold. Those which pass through the finest sieve, and which are called dust, are made up into paste, and worked over again.
The proper grains are next to be glazed or polished. This is performed in a very expeditious manner, by put ting the grains into a revolving cylinder, working like a barrel churn. This vessel should not be more than half full at once. The grains, by rubbing one against another, become smooth, and approach a spherical form.
The next operation, which has been attended with the greatest danger, is the drying. This has generally been effected by placing the powder upon shelves on three sides of a small room, on the other side of which is an iron stove; the fire being fed from the back of the wall. When we recollect that the stove itself is frequently hot enough to explode gunpowder, it is surprising that more safe me thods have not been adopted. Steam appears to be the most proper agent to be employed, not only for the sake of avoiding danger, but it is better calculated to dry the pow der more•effectually, without any fear of the powder being injured in its quality. It is recommended that powder he not dried too quick, or with too great a heat, for fear of volatilizing the sulphur. All these precautions are-unne cessary with steam, as sulphur only evaporates at and steam would not raise it so high as 212° without pres sure. The shelves on which the powder is laid to dry might be made of cast iron, and hollow. The inner cavity might have a sloping bottom, so that when the steam came into it from a boiler below, the water, after condensation, might run 'back into the boiler. There would, of course, be no waste of water, which should be rain water. The steam might be formed from a fire, at a sufficient distance to avoid the risk of any accident. This method would not be less desirable in an economical point of view. The powder should be kept on the stove till the time of barrel ling, for which the finest dry weather is always preferred.
A very great improvement has lately been made in pre serving gunpowder, by using barrels of copper instead of wood. These barrels are first made water and air tight ; and then the lid screws on so as to exclude completely all communication with the atmosphere. This has been of
the greatest importance, particularly in the navy.
Having given the direct practical method of forming gunpowder, we shall now give some account of its chemi cal properties, from which alone we can get at any true. theory of its effects.
The ingenious Mr Robins appears to have been the first who has examined gunpowder scientifically. He very properly conceived, that its power consisted in the evolu tion of an abundance of some elastic fluid liberated in an instant, and strongly increased in its force by the heat attend ant on the explosion. Having convinced himself that a per manently elastic fluid was generated during the explosion of gunpowder, his next business was to ascertain what pro portion this bore to the whole weight of the powder. For this purpose, be exhausted the receiver of an air•pump, the capacity of which was equal to 520 cubic inches.• In this he suspended a hot iron capable of firing gunpowder, and a mercurial gauge to ascertain the force of the air gene rated. On letting 27 grains of powder fall on the iron, the mercury indicated an increase of elastic fluid, which raised it two inches ; and upon repeating the experiment, he found this result confirmed. The barometer at this time was 30 inches, so that the quantity of elastic fluid generated was equal to an atmosphere. He guesses the tem perature of the receiver to be such as to increase the vo lume 5th of the whole. He then calculates the weight of this volume of gas, allowing its specific gravity to be the same with common air, to weigh 131 grains, for every ounce of 437.5 grains of gunpowder, which is 134, or near ly of the whole.
From determining the specific gravity of gunpowder, he found that the bulk of the powder to the volume of gas generated, was as 1 to 244.
He next supposes that the heat generated by the explo sion of the powder, would have the same effect in expand ing the generated elastic fluid which a red heat would pro duce. He then ascertained, by an experiment, that air was increased in volume by a red heat 4.1 times nearly. This would make the volume of the powder to that of generated air as 1 to 1000 nearly. The force which would arise from such an increase of volume, Mr Robins finds adequate to the effect of the powder in practice, and in a very ample manner agreed with his theory.