3. For shape, size and proportion of the grains.—The first is judged by eye, and grains of the size required are obtained by the use of sieves of different sizes.

4. Density.—The density is generally obtained in some form of mercury densimeter, the powder being weighed in air and then under mercury. In some forms of the instrument the air can be pumped out so that the weighing takes place in vacuo.

5. Moisture and absorption of moisture.—The moisture and hygroscopic test consists in weighing a sample, drying at 100° C. for a certain time, weighing again, &c., until constant. The dried weighed sample can then be exposed to an artificial atmosphere of known moisture and temperature, and the gain in weight per hour similarly ascertained by periodic weighings.

6. Firing proof.—The nature of this depends upon the purpose for which the powder is intended. For sporting powders it consists in the “pattern” given by the shot upon a target at a given distance, or, if fired with a bullet, upon the “figure of merit,” or mean radial deviation of a certain number of rounds; also upon the penetrative power. For military purposes the “muzzle” velocity produced by a powder is ascertained by a chronograph which measures the exact time the bullet or other projectile takes to traverse a known distance between two wire screens. By means of “crusher gauges” the exact pressure per square inch upon certain points in the interior of the bore can be found.

In the chemical examination of gunpowder the points to be ascertained are, in addition to moisture, freedom from chlorides or sulphates, and correct proportion of nitre and sulphur to charcoal.

Products of Fired Powder and Changes taking place on Explosion.—With a mixture of the complexity of gunpowder it is quite impossible to say beforehand what will be the relative amounts of products. The desired products are nitrogen and carbon dioxide as gases, and potassium sulphate and carbonate as solids. But the ingredients of the mixture are not in any simple chemical proportion. Burning in contact with air under one atmosphere pressure, and burning in a closed or partially closed vessel under a considerable number of atmospheres pressure, may produce quite different results. The temperature of a reaction always rises with increased pressure. Although the main function of the nitre is to give up oxygen and nitrogen, of the charcoal to produce carbon dioxide and most of the heat, and of the sulphur by vaporizing to accelerate the rate of burning, it is quite impossible to represent the actions taking place on explosion by any simple or single chemical equation. Roughly speaking, the gases from black powder burnt in a closed vessel have a volume at 0° C. and 760 mm. pressure of about 280 times that of the original powder. The temperature produced under one atmosphere is above 2000° C., and under greater pressures considerably higher.

Experiments have been made by Benjamin Robins (1743), Charles Hutton (1778), Count Rumford (1797), Gay-Lussac (1823), R. Bunsen and L. Schiskoff (1857), T. J. Rodman (1861), C. Karolyi (1863), and later many researches by Sir Andrew Noble and Sir F. A. Abel, and by H. Debus and others, all with the idea of getting at the precise mechanism of the explosion. Debus (Ann., 1882, vols. 212, 213; 1891, vol. 265) discussed at great length the results of researches by Bunsen, Karolyi, Noble and Abel, and others on the combustion of powder in closed vessels in such manner that all the products could be collected and examined and the pressures registered. A Waltham Abbey powder, according to an experiment by Noble and Abel, gave when fired in a closed vessel the following quantities of products calculated from one gram of powder:—

Fractions of
a gram.		Fractions of a
molecule or atom.
Potassium carbonate.2615.00189 molecule
Potassium sulphate.1268.00072   ”
Potassium thiosulphate.1666.00087   ”
Potassium sulphide.0252.00017   ”
Sulphur.0012.00004 atom
Carbon dioxide.2678.00608 molecule
Carbon monoxide.0339.00121   ”
Nitrogen.1071.00765 atom
Hydrogen.0008.0008   ”
Hydrogen sulphide.0080.00023 molecule
Potassium thiocyanate.0004
Nitre.0005
Ammonium carbonate.0002

From this, and other results, Debus concluded that Waltham Abbey powder could be represented by the formula 16KNO3 + 21.18C + 6.63S and that on combustion in a closed vessel the end results could be fairly expressed (rounding off fractions) by 16KNO3 + 21C + 5S = 5K2CO3 + K2SO4 + 2K2S2 + 13CO2 + 3CO + 8N2. Some of the sulphur is lost, part combining with the metal of the apparatus and part with hydrogen in the charcoal. The military powders of most nations can be represented by the formula 16KNO3 + 21.2C + 6.6S, proportions which are reasonably near to a theoretical mixture, that is one giving most complete combustion, greatest gas volume and temperature. The combustion of powder consists of two processes: (i.) oxidation, during which potassium carbonate and sulphate, carbon dioxide and nitrogen are mainly formed, and (ii.) a reduction process in which free carbon acts on the potassium sulphate and free sulphur on the potassium carbonate, producing potassium sulphide and carbon monoxide respectively. Most powders contain more carbon and sulphur than necessary, hence the second stage. In this second stage heat is lost. The potassium sulphide is also the most objectionable constituent as regards fouling.

The energy of a powder is given, according to Berthelot, by multiplying the gas volume by the heat (in calories) produced during burning; Debus shows that a powder composed of 16KNO3 to 8C and 8S would have the least, and one of composition 16KNO3 + 24C + 16S the greatest, when completely burnt. The greatest capability with the lowest proportion of carbon and sulphur to nitre would be obtained from the mixture ÷ 16KNO3 + 22C + 8S.