and lime together in the intense heat of the electric furnace, the result of which is a hard greyish substance, that gives off acetylene gas freely directly it is brought into contact with water. The chemical action which takes place is as follows:

Calcium carbide is composed of the metal calcium and carbon-one part of the former to two of the latter-the symbol being CaC. Water is composed of two of hydrogen to one of oxygen-H,O. When these are brought together the hydrogen of the water combining with the carbon of the carbide forms the hydrocarbon, acetylene-C,H2. The oxygen of the water simultaneously combines with the calcium of the carbide, producing lime (calcium oxide) -CaO. The latter, however, owing to the presence of the water or moisture, is actually immediately converted into slaked lime (calcium hydroxide)—Ca(OH). The reactions put into the form of an equation therefore stand—

CaC2+ 2H2O=C2H2+ Ca(OH)2.

During the evolution of the gas a certain amount of heat is developed. 1 lb. of carbide gives off an average of 5 cubic feet of gas.

The acetylene flame has a very high illuminating value, from 10 to 15 times that of coalgas, besides which the light possesses great advantages from its near resemblance to the nature of sunlight, which renders it valuable for colour-matching, &c. A peculiarity of the light is its softness, very different from the dazzling glare of the naked electric arc. Comparatively little heat is produced by the combustion of acetylene, in comparison with the light yielded, and the exhaustion and pollution of the surrounding air is less than with any other form of illuminant, electricity excepted. The flame is remarkably steady. The danger arising from the use of acetylene is that due to its highly explosive character when mixed with air, a danger much more marked than with mixtures of coal-gas and air. A smaller percentage of acetylene than of coal-gas introduced into a volume of air is sufficient to form an explosive mixture, which is capable of being fired by a light, or an incandescent substance, as a cigar; even a spark will suffice (as with other explosives) to cause ignition. Thus it has been

gas

With regard to the practical production of acetylene gas, it would seem that so apparently simple an operation as bringing water and calcium carbide together for the generation of the should involve no difficulties. Yet this matter has occupied the time of many experimentalists, and resulted in the evolution of numerous diverse types of generators. The first problem that arises is whether the water shall be conveyed to the carbide, or the carThis is solved variously bide to the water.

by pursuing both methods, with variations in the precise mode of working. A second important division of types of generators is that into "automatic," and "non-automatic"; in the first the gas is produced automatically, either continuously or intermittently, in such quantities as are demanded, while in the second a definite amount of gas is made from a certain quantity of carbide, and passed into a reservoir, from which it is drawn upon as required for consumption, the generator remaining inactive until another charge of carbide is put in and converted into gas. The automatic generator introduces more complication and chances of mishap, but this is really a question of good design, and careful workmanship in vital parts. The non-automatic type provides

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a store of gas ready to be drawn upon as required to meet sudden and varying demands, but on the other hand the gasholder, of large size frequently, has to be provided, and space given to it, while it may not run for so long a time without attention as the automatic type. There is also a deterioration of the gas and an absorption by the water in the holder as a setoff to the advantages of storing.

With reference to the mode of bringing the two agents together; this leads to two alternative designs of generators, termed "water-tocarbide" and "carbide-to-water" types, both of which are in extensive use. The simplest mechanical method would appear to be that of pouring or dripping the water on to the carbide in such quantities as are required, but this is attended with certain disadvantages. The carbide being acted upon resolves itself into a compact mass, and as lime is produced, this coats the surfaces of the lumps of carbide, and interferes very soon with the proper action of the water, the result being that although the production of gas may cease, there will still be portions of unused carbide in the generator. The trouble and mess of removing these and separating the lime from the carbide, still capable of yielding gas, is sufficient reason for the condemnation of this manner of generating on anything like a reasonable scale. When the carbide is lumped together thus, great heat is also generated, which is very objectionable because of its effects upon the illuminating power of the acetylene; oily or tarry substances are produced by the excessive heating, which coat the carbide and interfere with its proper decomposition, as well as choking and making a mess in the working parts of the plant.

It

Applications where this manner of generating has been sufficiently successful, however, are those of cycle and motor car and similar portable lamps-generators on a small scale. It must be remembered, however, that but a small quantity of carbide is used for each charge, and the rapid passage of the vehicle through the air has a considerable cooling effect, which is not available nor adequate enough for large generators.

Another point is that in these dripping types, the supply of gas does not cease immediately

the water supply is cut off, so disposing of one of the expected advantages of the easily controlled liquid. The amount of water intermingled with the carbide is sufficient to continue the yield of for some time after the water is shut off. gas The objections arising from the presence of a mass of carbide (with its residue of lime) are, however, minimised by dividing up the charge into several portions, or cages, so that lumping is much lessened, and the undesirable heating and choking of action is considerably reduced.

Fig. 6.-Automatic Water-to-Carbide Acetylene Gas Generator. Thorn & Hoddle Acetylene Co., Ltd.

The design of an automatic device to control the supply of water is not difficult, since the quantity of gas produced may itself be made to regulate the water. This is done in the simplest manner by leading the gas into a holder or bell, which is thereby caused to rise, and so operate a cock which shuts off the water from the carbide. As the gas is then used, the bell again sinks, and so opens the cock, allowing more water to decompose the carbide. It may be mentioned here that the same device is also applicable to those other generators in which either the

carbide is lowered into the water, or the latter is caused to rise up around the carbide, as in Fig. 5. In the first case the act of the bell lifting raises the carbide container out of the water, and vice versa; in the second the descent of the bell allows the water to rise up around the carbide, or checks it as the holder fills up. The bell itself may be a portion of the actual generator, as in Fig. 6, or as in the types where a separate gasholder is provided, Fig. 5, this will perform the automatic control of the generation.

The opposite condition of feeding-that of carbide to water-is done in the simplest manner by dropping lumps of carbide into a water container, the gas being generated very coolly and completely in this way. In the small hand-fed designs, carbide is thrown through a spout in the generator until enough gas has been evolved to fill the gasholder, which may be of sufficient capacity to give a supply for several hours' use. The simplicity of this mode of production is a feature which renders it of value for small installations. The method of feeding, however, becomes objectionable on a large scale, and some automatic device for supplying the carbide must be introduced. Many types of mechanism have been tried, some of which have proved erratic and unreliable, in a few cases being dangerous. Certain designs have, however, been sufficiently successful, an automatic ejector shooting the carbide into the water at intervals, as determined and controlled by the yield of gas. This method of letting carbide into a large excess of water has some advantages from the chemical standpoint. Heat is kept down, and the gas as it passes through the body of water is cooled and washed. The residue or sludge falls naturally to the bottom of the generator, whence it may be further led into a subsidiary receptacle for convenience of removal.

With an automatic feed to the carbide it is desirable that the latter should be in lumps of at least approximately even sizes in order to secure some degree of regularity in generation.

Another system is that of lowering a cage of carbide into water, and withdrawing it after a quantity of gas has been generated. This is not in very much favour, because it has the disadvantage that the mass of wet carbide being left isolated from the water gets heated unduly.

In this respect the practice of dropping the carbide bodily into the water is a much better plan. The remaining method, that of causing the water to rise up in excess around the carbide, is in extensive use. The carbide, being contained in cages or perforated trays, is attacked from below, the water being admitted gradually by a cock, opened at intervals by the gasholder as it becomes depleted, and so descends. Each tray of carbide is thus successively attacked and decomposed, the action being divided up amongst several groups—a manifest advantage. Ease of cleaning out is also one of the recommendations of these tray containers, since the entire set can be lifted out bodily for the removal of the residue.

The acetylene gas produced by any of the various generators is never sufficiently pure to be utilised for illuminating, chiefly on account of the impurities present in the original carbide, The resulting gas may give trouble in several ways unless purified; injurious fumes may be evolved on combustion, and the pipes and burners will gradually get choked by deposits. One of the undesirable bodies present in acetylene is moisture or water vapour, which is not due to the carbide, but to the simple fact of the presence of water in the generator. This is mostly got rid of by first passing the gas through a condenser, having pipes immersed in cold water. This condenser, however, is often omitted, when a washer is employed to follow generation. The washer is intended specifically to remove such impurities from the gas as are soluble in water, but as the latter is cold it will also serve to cool down the gas and rid it of most of its moisture. Some of the impurities present in crude acetylene, as ammonia and sulphuretted hydrogen, are capable of removal by passing the gas through water. For such, however, as are not eliminated by the water, including phosphorus compounds, removal must be effected in another way, by the use of various substances. Bleaching powder or chloride of lime is in common use for this function, with additions which modify it somewhat. Chloride of calcium, chromic acid, and cuprous chloride are also employed, all of these being made up under various commercial names. One, "puratylene," besides chloride of lime, contains calcium chlo

ride and calcium oxide (quicklime). Another, "heratol," is made up with chromic acid absorbed into porous substance, "kieselguhr," for convenience of using in a lump state. It is acidified with hydrochloric or acetic acid, to enable it to deal with the ammonia of the gas. These and other agents are made up into porous masses, so that the gas is compelled to thoroughly permeate them, and thus be well purified. The substances are either spread out on trays in the purifier, or simply placed in the latter in a single compartment.

The action of these, besides abstracting the impurities which are undesirable, is also to lessen the unpleasant odour of crude acetylene, though this is not a feature of special value, because the strong smell of the gas is the surest method of detecting leakages. A purifier is most suitably of circular shape, the fittings comprising a removable gas-tight lid, and the inlet and outlet pipes for the gas.

Although some kinds of purifiers serve also to remove the final traces of moisture from the gas; if they do not act perfectly in this way, a separate drier may have to be employed, in which hygroscopic materials, as quicklime or calcium chloride, are used. Puratylene, which contains calcium chloride, will serve as a drier, either when used in the purifying vessel, or in a separate drying chamber.

From the drier, the final portion of the generating plant,-the gas enters the service pipes. A meter (practically identical with that employed for measuring coal-gas) must be used. to gauge the yield, unless the plant is very small, and it is not desired to know the exact production of gas at all times. A pressure governor is sometimes fitted, before the meter is reached, to ensure a steadiness of flow, in instances where there is a tendency to unevenness of pressure from the gasholder, or the purifier.

The location of the generating plant must be governed by circumstances, but it is best set away from the main building, when the gas is used for a dwelling-house. In the case of a public service, a special building will of course be provided. Certain precautions must be taken in running the plant because of the dangerous nature of the gas. A naked light must not be brought near the generator when it

is in operation, or even for some time after it has ceased working, when no acetylene is presumably present. Explosions are easily caused by the presence (usually unsuspected) of traces of the gas. It follows, therefore, that the generator must not be charged or cleaned out at night, when the help of an artificial light is necessary; if the operation is done then, either an electric lamp must be used, or some form of lantern must be located outside the generating house, projecting its beams through a closed window. Precautions must be observed that no irresponsible person is allowed to meddle with the plant, or even go near it with cigar lit, nor may any form of flame be present.

Another matter which has to be considered in an acetylene installation is the risk of freezing in cold weather of the water contained in the various parts. Congelation of this would result in the stoppage of the production of gas. Suitable means for keeping the house up to a proper temperature, such as by hot-water pipes, must be adopted where necessary. One particular risk which presents itself is that of the water seal of the gasholder freezing, and so locking the rising portion. This must be prevented by an admixture of some substance which will lower the freezing point of the water, such as glycerine, alcohol, calcium chloride, salt, &c.

The materials of which the plant is constructed are in the case of generator and holders, steel, galvanised, to prevent corrosion, while for piping about the plant, good wrought-iron tube is best. It is not permissible, as previously mentioned, to employ copper, for pipes, &c., but alloys of it may be safely used, as brass and gunmetal, for the cocks and valves.

The piping for conveying the gas to the points of consumption is an important item, on which much care must be bestowed in order to ensure satisfactory and safe results. All joints must be exceptionally well made (even more so than those of coal-gas installations) to prevent losses, or dangerous leakages. Stout wrought-iron tubing is the most reliable form to employ. Composition tubing, as used for coal-gas, possesses the disadvantage of liability to damage from various causes. It may be mentioned that the sizes of piping for an acetylene installation may be smaller than those of a corresponding