When the vapour of nitric acid is passed through a red-hot tube, and also when a nitrate is strongly heated, oxygen gas is given off. Analysis shows that the oxygen combined in pure nitric acid amounts to 76 per cent. of its weight, while that in sodium and potassium nitrates is 56 and 50 per cent. respectively. Nitric acid and the nitrates are, therefore, highly oxygenated compounds; moreover, under favourable circumstances, they are rather easily broken up.

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 In warfare, it has been superseded by “smokeless” powders of much greater power; while for mining operations, explosives with a much greater shattering effect have long since taken its place.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.

Propellants. Although combustion is propagated through a quantity of gunpowder with very great rapidity, it is not done instantaneously. The time required is about one-hundredth of a second under ordinary conditions, and this interval, short though it is, is very important. When the object is to throw a projectile, the inertia of the latter has to be overcome, that is, a certain amount of force has to be applied before the heavy body begins to move. In order that the strain on the breech of the gun may be as small as possible, the pressure must be gradually developed and must reach its maximum just as the projectile begins to move.

The time factor in the explosion constitutes the difference between what we now call “propellants” and “high explosive.” Propellants are explosives which develop pressure gradually, and are therefore used to launch the projectile; high explosive develops pressure instantaneously, and is therefore used as the bursting charge inside the shell, bomb, or torpedo, and also in blasting operations.

Cordite, or smokeless powder, is the propellant now most used. It is made by macerating guncotton and nitroglycerine with their common solvent acetone. A pulp is thus made to which 5 per cent. of vaseline is added. The mixture is then forced through a die, and in this way it is formed into threads or rods, which harden as the acetone evaporates. Cordite produces no smoke, because all the products of its combustion are invisible gases.

High Explosive. Nitroglycerine and Guncotton are both explosives of the instantaneous kind. The former is made by forcing glycerine, under pressure in a very fine stream, into a mixture of fuming nitric and concentrated sulphuric acids; the latter by soaking cotton-wool in a similar mixture. Both products are washed with water until quite free from acid, and subsequently dried.

Nitroglycerine is a colourless oil with a burning taste. The oil itself is very dangerous to handle, for it is liable to explode spontaneously even when the utmost care has been taken in its preparation. A mere spot on a filter paper explodes with a deafening report when gently hammered on an anvil; and one drop, when heated on a stout iron plate, blows a hole through the plate. No use could be made of this substance for many years after its discovery because it was so liable to explode during transportation; now, however, it is made safer by mixing with absorbent infusorial earth or kieselguhr. This mixture is known as dynamite. Blasting gelatine, like cordite, is a mixture of nitroglycerine and guncotton.

Trinitrotoluene (T.N.T.) is made from toluene and nitric acid, and is a type of the modern high explosive. It is a yellow crystalline substance which melts at 79°-81·5° C., and is poured into the shell in a molten condition. It is a remarkably stable substance, which burns quickly when heated to 180° C.; it cannot be exploded even by hammering. Explosion is only brought about by that of a subsidiary substance called the detonator. The percentage composition of T.N.T. is as follows—

The oxygen present is only just sufficient to burn the whole of the carbon to carbon monoxide; but since carbon dioxide is also formed, which requires twice as much oxygen for the same weight of carbon, and since the hydrogen and nitrogen may also be oxidized, the combustion of the carbon is not complete; and therefore the explosion of T.N.T. is accompanied by a dense black smoke, consisting of finely divided particles of carbon.

The explosive known as ammonal is a mixture of T.N.T., aluminium powder, and ammonium nitrate; the function of the latter substance is to supply more oxygen to render the combustion of the carbon of T.N.T. complete.

Nitrates and the Food Supply. Chemical analysis shows that compounds of nitrogen enter largely into the composition of the living tissues of all plants and animals; hence, either nitrogen itself or some of its compounds must be assimilated by all living organisms to provide for growth and development, and to repair wastage. Air, since it contains approximately four-fifths of its volume of free nitrogen, is the most obvious source of supply. At every breath, a mixture of oxygen and nitrogen is inhaled by animals, but only part of the oxygen is used. Practically the whole of the nitrogen is returned to the atmosphere unchanged; it serves only to dilute the oxygen. From this it is clear that animals do not build up their nitrogenous constituents from elementary nitrogen.

With plants it is very much the same, for, although they obtain their principal food, namely, carbon, from the carbon dioxide which is present in air, it is only in a few exceptional cases that free nitrogen is assimilated. The exceptions will be considered first, because it was through these that we first began to learn something definite about the great importance of nitrogen in agriculture.

Virgil, who was born in 70 B.C., wrote a poem in praise of agriculture. Almost in the opening lines he deals with the treatment of corn land. He advises that, in alternate years, this should either be left fallow or sown with pulse, vetch, or lupin; but not with flax or oats, because they exhaust the land. From this we learn that rotation of crops was one of the established principles of good husbandry even at the beginning of the Christian era.

It was not until the later years of the nineteenth century that any explanation as to why rotation of crops is beneficial was put forward. Let us first state the facts more precisely. Peas, beans, vetches, clover, and other members of the natural order called Leguminosae, which includes about 7,000 species, produce fruits rich in complex nitrogen compounds without being dependent in any way upon nitrogen compounds in the soil. Moreover, they do not exhaust the land as far as these compounds are concerned; hence wheat and other grain can be grown on the same land the following year.

It is now known that leguminous plants assimilate atmospheric nitrogen with the help of certain bacteria. If anyone will dig up a lupin root, he will observe[2] conspicuous wrinkled swellings or nodules at various points on the roots. These, when examined with a high-power microscope, are found to contain colonies of bacteria. It is these minute vegetable organisms which assimilate nitrogen and pass on nitrogen compounds to the larger plant. Other plants cannot assimilate what we might call raw nitrogen; they require soluble nitrates. These they build up into complex organic nitrogen compounds suitable for the feeding of animals which can assimilate neither free nitrogen nor nitrates.

The Nitrogen Cycle. The supply of nitrates in the soil needs continually to be renewed by the addition of decaying vegetable matter, stable or farmyard manure, or Chili saltpetre. The natural manures contain organic nitrogen compounds which were built up during the life of some animal or plant. They are not immediately available as food for other plants, because they are, as it were, the end products of life, and are not soluble in water. But Nature provides for this. The manures decay, forming humus, and ultimately ammonia, one of the simplest of inorganic nitrogen compounds. Ammonia is then transformed to nitrites by certain bacteria present in the soil, while other bacteria change nitrites into nitrates. Both of these organisms are quite distinct from the root nodule bacteria of the Leguminosae.

The nitrates pass into the plant in solution, and then begins again that wonderful cycle of changes which we have described. This is perhaps made clearer by the following diagram.

It now remains to show why artificial manures also are necessary. Let us consider what happens to a piece of ground which is left uncultivated. Although nothing is taken from it in the way of a crop, yet it very quickly deteriorates, and the soil becomes infertile through the loss of nitrogen compounds. This is explained by the fact that nitrates are soluble in water, and so they get washed away from the top soil. In addition to this, the nitrogen which is returned to the land forms quite an insignificant fraction of that which is taken from it, for we waste a great deal of organic nitrogen. The difference on both these accounts has, therefore, to be made up by the addition of artificial manures containing soluble nitrates.

The natural supply of nitrate is very limited. According to a report of the Chilian Government published in 1909, the nitre beds of that country were expected to last for less than a century at the current rate of consumption. Wheat, above all things, will not grow to perfection on soil which is deficient in nitrate. In 1908, Sir William Crookes called attention to the difficulty which might be experienced in the near future in supplying the people of the world with bread. Statistics showed that wheat was grown on 159,000,000 acres out of a possible 260,000,000. The average yield is 12·7 bushels per acre. By 1931, it is calculated that the population of the world will be 1,746,000,000; and to supply these with bread, wheat would have to be grown on 264,000,000 acres, that is, 4,000,000 acres beyond the total available wheat land.

The remedy which Sir William Crookes suggested in order to avoid famine was to raise the average yield from 12·7 to 20 bushels per acre by the application of an additional 12,000,000 tons of Chili saltpetre per annum. In view of the possible exhaustion of the supply of this substance, this would only mean a postponement of the evil day. The position, however, is now modified to a great extent because undeveloped deposits of sodium nitrate are known to exist in Upper Egypt, and the making of nitric acid from the air, which in 1908 was put forward as a suggestion, is now an accomplished fact.

Nitric Acid from Air. The supply of nitrogen in the air is truly inexhaustible; it amounts to about 7 tons for every square yard of the earth’s surface, which is about 200,000,000 square miles. It is quite evident that anything man may do in the way of taking nitrogen from the air will make no perceptible difference to its composition.

Every time a flash of lightning passes between a cloud and the earth, oxygen and nitrogen combine in the path of the spark, producing oxides of nitrogen. These dissolve in water, and are washed into the earth as a very dilute solution of nitric acid. As long ago as 1785, H. Cavendish imitated this natural phenomenon. A reference to the diagram (Fig. 7) will show how nitric acid can be made from the air on a small scale. The globe contains air under slightly increased pressure. The platinum wires or carbon rods are connected with the terminals of an induction coil, which in its turn is connected to accumulators supplying the current required.

When the coil is put into action, a spark passes across the gap between the ends of the carbon rods. With a larger coil and a more powerful battery, there is an arching flame which can be blown out and re-lighted. This is actually nitrogen burning in oxygen. The result in either case is the same; the air in the globe sooner or later acquires a reddish-brown colour due to oxides of nitrogen, which, when shaken with water, form a very dilute solution of nitric acid.

The same process is now carried out on a large scale. Air is driven by fans through a very powerful electric arc, whereby 1·5 to 2 per cent. is converted into nitric oxide. This combines spontaneously with more oxygen to form nitrogen peroxide, which, when dissolved in water, gives a very dilute solution of nitrous and nitric acids.