M. Morveau communicated to the Institute some experiments, which may be seen in the Archives des Découvertes, i, p. 269, relative to the time necessary for the inflammation of a given mass of gunpowder, &c. He infers that large grain powder inflames more readily than the fine grain.
Since during the combustion of powder, gaseous bodies more or less considerable are generated, it follows that the full force of fired gunpowder must depend on the maximum of the quantity of those gases; and the powder is more strong as it is susceptible of forming more gas in a given time. Besides the purity and the proper proportion of the materials, the gunpowder, to produce the greatest possible effect, should not only be intimately mixed, but dried perfectly and with care.
It is a fact which is well known, that a musket, fowling piece, &c. are very apt to burst, if the wadding is not rammed down close to the powder. Hence it is obvious, that in loading a screw barrel pistol, care should be taken that the cavity for the powder be entirely filled with it, so as to leave no space between the powder and the ball.
Experience has shown, that if a shell is only half or two-thirds filled with gunpowder, it breaks into a great number of pieces, and on the contrary, if completely filled, it separates only into two or three pieces, which are thrown to a very great distance.
It is also found that the same principle, of leaving a space for air, is applied with success in blasting rocks, and splitting trunks of trees. If the trunk of a tree is charged with gunpowder, and the wadding is rammed down very hard upon the powder, in that case (unless the quantity of powder is great,) the wadding is only driven out, and the tree remains entire; but if, instead of ramming the wad close to the powder, a certain space is left between them, the effects of the powder are then such as to tear the tree asunder.
Addison (Travels through Italy and Swisserland) speaking of the celebrated Grotto Del Cani, which contains carbonic acid gas, and on that account extinguishes flame, and is fatal to animal life, observes, that he laid a train of gunpowder in the channel of a reed, and placed it at the bottom of the grotto, and on inflaming it, that it burnt entirely away, although the carbonic acid gas in the same spot would immediately extinguish a lighted taper, snuff and all; for, he remarks, fire is as soon extinguished in it as in water. If gunpowder did not contain within itself that which was necessary to produce combustion, how are we to account for its combustion in an atmosphere of carbonic acid gas, or in vacuo?
Whether gunpowder be fired in a vacuum or in air, a permanently elastic fluid is generated, the elasticity or pressure of which is, cæteris paribus, directly as its density.
Gregory, (Treatise on Mechanics, &c. ii, p. 56) has given a summary of the results of the experiments of Mr. Robins, which we insert verbatim. "To determine the elasticity and quantity of this fluid (the elastic) produced from the explosion of a given quantity of gunpowder, Mr. Robins premises, that the elasticity increases by heat, and diminishes by cold, in the same manner as that of the air; and that the density of this fluid, and consequently its weight, is the same with an equal bulk of air, having the same elasticity at the same temperature. From these principles, and from the experiments by which they are established (for a detail of which we must refer to the book itself,) he concludes that the fluid produced by the firing of gunpowder, is nearly 3/10ths of the weight of the generating powder itself; and that the volume or bulk of this air or fluid, when expanded to the rarity of common atmospheric air, is about 244 times the bulk of the said generating powder. Count Salace in his Miscel. Phil. Math. Soc. Priv. Taurin, p. 125, makes the proportion as 222 to 1; which he says agrees with the computation of Messrs. Hawkesbe Amontons, and Belidor. Hence it would follow that any quantity of powder fired in any confined space, which it adequately fills, exerts at the instant of its explosion against the sides of the vessel containing it, and the bodies it impels before it, a force at least 244 times greater than the elasticity of common air, or, which is the same thing, than the pressure of the atmosphere; and this without considering the great addition arising from the violent degree of heat, with which it is endued at that time; the quantity of which augmentation is the next head of Robins's inquiry.
He determines that the elasticity of air is augmented in a proportion somewhat greater than that of 4 to 1, when heated to the extremest heat of red-hot iron; and supposing that the flame of fired gunpowder is not of a less degree of heat, increasing the former number a little more than four times, makes nearly 1000; which shows that the elasticity of flame, at the moment of explosion, is about 1000 times stronger than the elasticity of common air, or than the pressure of the atmosphere. But, from the height of the barometer, it is known that the pressure of the atmosphere upon every square inch is on a medium of 143/4ths, and therefore 1000 times this, or 14750 lbs. is the force of pressure of inflamed gunpowder, at the moment of explosion, upon a square inch, which is very nearly equivalent to six tons and a half. This great force, however, diminishes as the fluid dilates itself, and in that proportion; viz. in proportion to the space it occupies, it being only half the strength, when it occupies a double space, one-third the strength, when a triple space, and so on. Mr. Robins further supposed the degree of heat above mentioned to be a kind of medium heat; but that in the case of large quantities of powder the heat will be higher, and in very small quantities lower; and that therefore in the former case the force will be somewhat more, and the latter somewhat less, than 1000 times the force of the atmosphere.
He further found, that the strength of powder is the same in all variations in the density of the atmosphere: but that the moisture of the air has a great effect upon it; for the same quantity which in a dry season would discharge a bullet with the velocity of 1700 feet in one second, will not in damp weather give it a velocity of more than 12 or 1300 feet in a second, or even less, if the powder be bad, or negligently kept. Robins's Tracts vol. i, p. 101, &c. Further, as there is a certain quantity of water, which, when mixed with powder, will prevent its firing at all, it cannot be doubted but every degree of moisture must abate the violence of the explosion; and hence the effects of damp powder are not difficult to account for.