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634 remarkable ease. Ammonia is obtained by subjecting the cyanamide to the action of steam.

Calcium carbide, it should be added, is made on a large scale as a source of the gas acetylene (CaC2+OH 2 =C2H 2 +CaO), now so much used as a lighting agent for road-traction purposes and even for domestic lighting away from towns; but chiefly, together with oxygen, in the form of the acetylene blowpipe, in cutting iron plates in the shipbuilding and other trades, in joining iron rails for the electric tram service, etc.

The production of nitrogen for the above-described processes and of oxygen has been greatly promoted by the researches on the liquefaction of gases carried out by Sir James Dewar, at the Royal Institution, Albemarle Street, London, the home of Davy and Faraday. The metallic vacuum vessels invented by this indefatigable student of low -temperature phenomena have made possible also the use of liquefied air, richer in oxygen than air, in various ways in hospitals, for example; also, together with charcoal, as an explosive agent, in mining operations.

The astounding power properly prepared charcoal has, at liquid-air temperatures, of absorbing gases, another discovery made by Sir James Dewar, is proving of the greatest value in operations involving the separation and purification of gases. It is even contemplated that it may be possible to fill airships with the incombustible, rare gas, helium, prepared by taking advantage of this property of charcoal the source of the helium being the natural gas associated with petroleum, in the American oil wells and in certain springs in Canada.

To return to the nitrogen compounds, the outstanding impor- tance of ammonia and nitric acid will be understood when it is realized that cereal crops, including the sugar cane, cannot be grown without nitrogenous fertilizers. At Rothamsted, where wheat has been grown on the same land year after year under the same treatment since 1852, the average yield of grain has been only 12-9 bushels per acre on the permanently unmanured plot; whereas on the plot properly supplied with nitrogenous manures, it has been 31-6 bushels.

Now that both ammonia and nitric acid can be produced, by synthetic means, in any desired quantity, the world need have no anxiety as to the supply of artificial nitrogenous manures. Even if fuel should not be available to supply power, their manu- facture will always be possible where water-power is to hand.

Large quantities of ammonium nitrate were made, during the war, for use in admixture with trinitrotoluene as a high explosive. Sulphate of ammonia and nitrate of soda are both only of partial value as fertilizing agents, as the one necessarily contains excess of acid and the other excess of alkali; these remain after the nitrogenous effect is exhausted; also the constant use of the sulphate involves a steady withdrawal of lime from the soil, ultimately rendering it acid, whilst the tendency of the alkali from the nitrate is to make the soil impervious to water. Of late years, there has been a gradual growth of opinion, therefore, in favour of ammonium nitrate, as this combines in itself the activity of an ammonium salt with that of a nitrate and, being used up entirely in the service of the plant, has not their harmful effect upon the soil. The objection to the use of the nitrate is its tendency to liquefy on exposure to a moist atmosphere and that it sets to a hard mass; moreover, it cannot be transported in bags.

The Germans have foreseen the value of urea, CON 2 H,|, which is free from the disabilities associated with the nitrogenous fertilizers now in use. It is an entirely neutral substance and is undoubtedly an effective fertilizing agent under some conditions but it has yet to be shown that it could be used generally in place of the ammonium salts and nitrates. It can be made merely from ammonia and carbonic acid, so that if its manufacture can be put upon an economic footing and it prove to be suitable at least for most purposes, though it may not supersede ammonium salts, it may largely displace them from use.

Other methods of exploiting nitrogen are being studied which involve the direct absorption of the gas and its conversion to a cyanide; it is well within the bounds of probability that these may ultimately prove equal, if not superior, to the highly mechani- cal methods now coming into vogue: these latter, however, will

have the advantage that they can be carried out with the aid of water-power, unless the fixation methods should also be such as to necessitate the use of electric power.

More natural processes are also in sight. It is now customary, in most civilized countries, not only to waste the excreta of the urban populations but to do so at considerable cost. In the East, in China especially, human excreta are most carefully collected and used on the crops; they are actually a source of revenue to one or more towns. An activated sewage sludge process is coming to the fore which may be of service under European conditions: whether this will do more than conserve nitrogen is a question; if also the waste of phosphate can even be partially prevented, infinite service will be rendered. The chief limiting factor of agricultural production in the near future will clearly be the supply of phosphate and in the next degree of potash; we now know how to bring down nitrogen from the air but the supplies of phosphate and of potash are being drawn upon at exorbitant rates and must ere long be exhausted ; no ways of withdrawing them from the vasty deep, which can be put in practice, are before us. It is found that at least the solid matter in sewage can be recovered in a valuable form by forcing air into the fresh liquid; when this operation has been repeated several times, first forcing in air, then allowing the suspended solid to subside, running off the liquid and adding fresh sewage, the sludge acquires a greatly enhanced bacterial activity and apparently even nitrogen-fixing organisms come into activity: eventually it may contain 6 to 7 % of nitrogen and become equal to farmyard manure in value.

The amount of farmyard manure now available is insufficient, as the number of horses kept is so much less than formerly. Recent inquiry has shown that a complex series of changes is involved in the production of this manure from the straw and animal exuviae of which it is composed and that eventually it may contain a considerable amount of nitrogen beyond that originally present in the raw materials. Organisms are at work which destroy much of the carbonaceous matter but, in the course of the operation, they induce the fixation of a certain amount of atmospheric nitrogen, if supplied with the nitrogenous food they require for their own development. It is therefore conceivable that an economic process may be developed of manufacturing farmyard manure from waste carbonaceous materials with the aid of ammonia. The development of greatest importance in agriculture, however, to which we may look forward, is the direct enrichment of the soil with nitrogen, directly withdrawn from the atmosphere: either by means of organisms functioning in immediate association with leguminous crops; or by organisms within the soil, whose activity is promoted by the judicious use of green manures. No branch of scientific inquiry is of greater importance to mankind than studies to promote such ends.

The soap industry has undergone marked development of late years, owing to the increasing consumption of margarine as a substitute for butter. As the hard fats are required for the manufacture of this material, it has been necessary to make use of the natural fatty oils in soap-making; these differ from the hard fats in that they are glycerides not of saturated but of unsaturated fatty acids. To harden them, i.e. to convert them into glycerides like those contained in the ordinary solid fats, the heated oils are subjected to the action of hydrogen gas in presence of finely divided metallic nickel, which acts as a catalyst. The process is now carried out on a very large scale.

In the metal industry, the developments have been in matters of detail. Aluminium, nickel, tungsten and sodium have been brought greatly to the fore. One of the most notable achieve- ments is the production of a rustless steel, an alloy of iron and chromium, which will bear sharpening when made into knives; apparently the special qualities of the steel are the outcome of a particular structure developed by heat treatment, though why the alloy should be rustless is not clear.

As illustrations of the manner in which the rarer inorganic materials are gradually being imported into industry, reference may be made to the use of vanadium oxide as an oxidizing cata- lyst; of titanium oxide as awhile paint on account of its high