Pure nitric acid will set fire to warm, dry sawdust, and a piece of charcoal or sulphur thrown on the surface of molten nitre takes fire spontaneously and is quickly consumed, giving out a very vivid light. The explanation of this is that the supply of oxygen is abundant; it is also readily available and concentrated in a small space. We can vary the experiment. When a mixture of 75 parts by weight of finely-powdered saltpetre, with 15 of charcoal dust and 10 of ground sulphur, is ignited, it burns very vigorously, and is soon consumed. This mixture is, indeed, home-made gunpowder. Explosives. Gunpowder was discovered in very early times by the Chinese, but for many years the secret of its composition did not get outside the Great Wall. In the fifth century A.D., it was apparently re-discovered at Constantinople, and that city was for a long time defended by the use of what is known in history as Greek Fire, an incendiary mixture very similar to, if not actually the same as, gunpowder. But again the secret of its composition was jealously guarded, and it was not until the thirteenth century that it was discovered, apparently for the third time, and introduced to Western Europe by Roger Bacon. It was used in siege cannon early in the fourteenth century and in field guns at Crécy; but it was apparently not employed for blasting until about 1627, although in 1605, Guy Fawkes and his fellow-conspirators were able to obtain it in large quantity. From the battle of Crécy in 1346 to the beginning of the South African campaign in 1889, gunpowder was the only explosive used in warfare. “Villainous saltpetre” has therefore played a very important part in shaping the course of events in the world’s history. At the present day, gunpowder has become “old-fashioned.” In warfare, it has been superseded by “smokeless” powders of much greater power; while for mining operations, explosives with a much greater shattering effect have long since taken its place. The composition of gunpowder may vary, but on the average it contains 75 parts by weight of saltpetre to 15 of charcoal and 10 of sulphur. It is, therefore, a mixture of two combustible substances, with a large quantity of a third very rich in oxygen. The separate constituents are very finely ground and afterwards thoroughly incorporated. When the mixture is ignited, charcoal and sulphur burn very fiercely in the oxygen supplied by the saltpetre. The secret of the action of gunpowder lies in the extraordinary rapidity with which combustion, started at one point, is propagated through the whole mass. Moreover, the products of combustion are mainly gases, and these occupy several thousand times the volume of the solid from which they are produced. In a confined space, a gas may exert enormous pressure when its normal tendency to expand is resisted.