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.