Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/342

322 caused bi Comparatively slight variations in the amount of moisture, the following results may be quoted from the experiments of Noble and Abel. The samples of " pebble " powder used were specially prepared for the Committee on explosives, differing only in the amount of moisture con tained in them. Percentage of Moisture. Muzzle Velocity. Feet per second. Maximum Pressure. Tons per sq. inch. 07 1545 22-02 0-8 1537 21-38 0-9 1530 20-77 1-0 1523 20-18 1-1 1517 19-63 1-2 1512 19-12 1-3 1507 18-63 1-4 1502 18-18 T5 1497 17-76 Keeping gunpowder ia a very damp atmosphere will tend to separate the ingredients by dissolving a portion of the saltpetre, which crystallizes upon the surface of the grains or pieces. FIRED GUNPOWDER. In the explosion of gunpowder, the products of com bustion are very materially affected by the conditions under which it is fired, whether burnt in the open air, exploded Under very great pressure in a tightly closed vessel, or the products allowed to expand in the bore of a gun. It may, however, be stated generally that the oxygen of the salt petre converts nearly all the carbon of the charcoal into carbonic acid (CO 2 ), a portion of which combines with the potash of the nitre to form carbonate of potash (K 2 O.C0 2 ), the remainder existing in the state of gas. The sulphur is for the most part converted into sulphuric acid (S0 3 ), and forms sulphate of potash, a large proportion of which, pro bably by secondary reactions, becomes hyposulphite and sulphide. The nitrogen of the saltpetre is almost entirely evolved in the free state, and the carbon not having been wholly burnt into carbonic acid, there is always a proportion of carbonic oxide (CO) present. Th.3 decomposition is so complicated, and varies so con- .sideribly under different conditions of experiment, that it is impossible to represent the transformation by any single equation, but the following expression may give some idea of the primary reaction : There is a very large proportion of residue, which, on cooling, assumes the solid form. The experiments of Noble and Abel prove that it is liquid very shortly after the explosion ; indeed, it is probable that at the moment of maximum temperature, the ultimately solid products are more or leas in a state of vapour, being deposited in a very finely divided state as the temperature falls. echani- When a charge of gunpowder is exploded in the chamber of a gun, a large quantity of gaseous matter is evolved in fects. a highly condensed state; its tension, or expansive power, is, moreover, greatly increased by the heat generated during the transformation. The pressure being equal in all direc tions, the work done upon the projectile is due to the expansion of the permanent gases in the bore of the gun, which force is also considerably sustained or the reduction of temperature due to the expansion in great measure com pensated for by the heat stored up in the ultimately solid residue. Any calculation which does not take this latter point into consideration will give far too low an estimate of the actual force of fired gunpowder (see EXPLOSIVES). With our present state of knowledge, it may be stated iu round numbers that the gases evolved by gunpowder, if it entirely Jills the close vessel in which it is exploded, will occupy at a temperature of C. and 7 GO mm. barometric pressure, about 280 times the volume of the original powder, and will give a pressure of about 6000 atmospheres, or 40 tons per square inch. In view of the very different results which have been arrived at by various eminent authorities who have experi mented upon fired gunpowder, the following brief account of their researches may be useful : Robins, the father of scientific gunnery, in 1743 read before the Royal Society a paper describing experiments which showed that gunpowder, when fired, generated permanent gases, which, at the ordinary temperature and atmospheric pressure, occupied a volume 244 times greater than that of the unexploded powder. He further considered that the heat evolved was such that the tension of tho permanent gases would be increased fourfold, and hence deduced the maximum pressure to be about 1000 atmospheres. In 1778 Dr Hutton communicated in the same manner an account of his cele brated researches on the combustion of powder ; he deduced the maximum pressure to be about twice that given by Robins, or a little over 2000 atmospheres, differing from the latter chiefly as to the tem perature, for he considered that the volume of gases generated would occupy, under ordinary conditions, 250 times that of the powder. He also deduced formulse for giving the pressure of the gas and velocity of the projectile at any point of the bore, but no allowance is made for the loss of heat in proportion to the work done, that fun damental principle of thermodynamics being then unknown ; the error thus occasioned is, however, in part compensated for by the heat stored up in the non -gaseous products. In 1797 Oouut Rum- ford sent to the Royal Society his experimental determinations of the pressures given by fired gunpowder, which are remarkable as being the first attempt to do this by direct observation. The closed vessel he used was, however, very small, being able to hold but 28 grains when filled, and he only succeeded in measuring the results up to a charge of 18 grains. His plan of operation was to ascertain by repeated trial the least weight which would just confine the pro ducts of explosion, and thence to calculate the pressure ; this he de duced from two series of experiments, which, however, gave some very anomalous results, to be as high as 100,000 atmospheres for gunpowder exploded in its own space. In 1823 Gay-Lussac, the celebrated French chemist, estimated the volume of the permanent gases evolved at 450 times that of the powder fired, but General Piobert assigns very probable reasons for supposing the quantity of gas determined to have been doubled in error. Ga3 -Lussac found the percentage composition of the gaseous products to be 52 6 car bonic acid, 5 of carbonic oxide, and 42"4 nitrogen. In 1857 Bunscn and Schiskoff published (Pogycndorff sAnnalcn, vol. cii. p. 325) the results of their very important experiments ; their determinations were (1) that the permanent gases represented only about 3 2 per cent, of the weight of the charge, and occupied at the standard tem perature and pressure about 193 times the volume of the original powder ; (2) that the heat generated by exploding gunpowder in a close chamber is about 3340 C. ; and (3) that hence the pressure would be about 4374 atmospheres, or 29 tons per square inch. Major Rodman (Boston, 1861) published the results of extensive experiments carried on for the United States Government, in order to ascertain the pressures given by different powders in the bores of heavy guns. He employed a pressure gauge, which bears his name, and consists mainly of a piston to which is attached a knife edge acting upon a piece of soft copper, contained in a small cylinder at right angles to, and communicating with, the bore of the gun ; this apparatus he also used to ascertain the pres sure given by powder exploded in a closed vessel. The chief value of his experiments consists in their having established the fact that the size of the grain should be proportionate to the length and dia meter of the bore of the gun. Von Karolyi (Poggcndorjf s Annalen, April 1863) carried out experiments from which he estimated that, for small-arm powder, tho gases evolved occupied about 226 times the original volume, while cannon powder produced about 200 vol umes. Captain Andrew Noble, F.R.S. , of Elswick. and Professor Abel, C.B., F.R.S., chemist to the British War Department, carried out a most extensive series of experiments on " Fired Gun powder" (Phil. Trans. Royal Soc., 1875), probably the most com plete ever made, and undertaken collaterally with tho Government "Committee on Explosives;" their mean results may be summed up as follows : > (A.) When Fired in a Confined />ae?. (1) The products of com bustion are about 57 per cent, by weight of ultimately solid matter, and 43 per cent, of permanent gases. (2) The latter occupy at C. and 760 mm. barometric pressure about 280 times the volume of the original powder. (3) The tension of the products of combus tion, when the powder entirely fills the space in which it is fired, is about 6400 atmospheres, or 42 tons per square inch. (4) The temperature of explosion is about 2200 C. (5) The chief gaseous products are carbonic acid, nitrogen, and carbonic oxide, with a little sulphydric acid and hydrogen. (6) The solid residue is mainly com posed of potassium carbonate, sulphate, liyposulphate, and sulphide. Resear relativ lired g powtle Robiti: Ilutto Rum- ford. Gay- Lussai Bunse and Schist Rodm: Von Karol; Noble and

Abel.