Acetylene, the Principles of Its Generation and Use Part 21

ACETYLENE MIXTURES FOR RAILWAY-CARRIAGE LIGHTING.--In modern practice, the gases which are most commonly employed for diluents of acetylene, under the conditions now being considered, are cannel-coal gas (in France) and oil-gas (elsewhere). Fowler has made a series of observations on the illuminating value of mixtures of oil-gas and acetylene. 13.41 per cent. of acetylene improved the illuminating power of oil-gas from 43 to 49 candles. Thirty-nine-candle-power oil-gas had its illuminating power raised to about 60 candles by an admixture of 20 per cent. of acetylene, to about 80 candles by 40 per cent. of acetylene, and to about 110 candles by 60 per cent. of acetylene. The difficulty of employing mixtures fairly rich in acetylene, or pure acetylene, for railway- carriage lighting, lies in the poor efficiency of the small burners which yield from such rich gas a light of 15 to 20 candle-power, such as is suitable for the purpose. For the lighting of railway carriages it is seldom deemed necessary to have a flame of more than 20 candle-power, and it is somewhat difficult to obtain such a flame from oil-gas mixtures rich in acetylene, unless the illuminative value of the gas is wasted to a considerable extent. According to Bunte, 15 volumes of coal-gas, 8 volumes of German oil-gas, and 1.5 volumes of acetylene all yield an equal amount of light; from which it follows that 1 volume of acetylene is equivalent to 5.3 volumes of German oil-gas.

A lengthy series of experiments upon the illuminating power of mixtures of oil-gas and acetylene in proportions ranging between 10 and 50 per cent. of the latter, consumed in different burners and at different pressures, has been carried out by Borck, of the German State Railway Department. The figures show that per unit of volume such mixtures may give anything up to 6.75 times the light evolved by pure oil-gas; but that the latent illuminating power of the acetylene is less advantageously developed if too much of it is employed. As 20 per cent.

of acetylene is the highest proportion which may be legally added to oil- gas in this country, Borck's results for that mixture may be studied:

______________________________________________________________________ | | | | | | | | | | | | | | | Propor- | | | | | Consump- | | Consump- | tionate | | Kind of | No. of | Pres- | tion per | Candle- | tion per | Illum- | | Burner. | Burner | sure. | Hour. | Power. | Candle- | inating | | | | mm. | Litres. | | Hour. | Power | | | | | | | Litres. | to Pure | | | | | | | | Oil-Gas.| |___________|________|_______|__________|_________|__________|_________| | | | | | | | | | Bray | 00 | 42 | 82 | 56.2 | 1.15 | 3.38 | | " | 000 | 35 | 54 | 28.3 | 1.91 | 4.92 | | " | 0000 | 35 | 43.3 | 16 | 2.71 | 4.90 | | Oil-gas | | | | | | | | burner | 15 | 24 | 21 | 7.25 | 2.89 | 4.53 | | " " | 30 | 15 | 22 | 10.5 | 2.09 | 3.57 | | " " | 40 | 16 | 33.5 | 20.2 | 1.65 | 3.01 | | " " | 60 | 33 | 73 | 45.2 | 1.62 | 3.37 | | | | The oil-gas from which this mixture was prepared showing: | | | | Bray | 00 | 34 | 73.5 | 16.6 | 4.42 | ... | | " | 000 | 30 | 48 | 6.89 | 6.96 | ... | | " | 0000 | 28 | 39 | 3.26 | 11.6 | ... | | Oil-gas | | | | | | | | burner | 15 | 21 | 19 | 1.6 | 11.8 | ... | | " " | 30 | 14 | 21.5 | 2.94 | 7.31 | ... | | " " | 40 | 15 | 33 | 6.7 | 4.92 | ... | | " " | 60 | 25 | 60 | 13.4 | 4.40 | ... | |___________|________|_______|__________|_________|__________|_________|

It will be seen that the original oil-gas, when compressed to 10 atmospheres, gave a light of 1 candle-hour for an average consumption of 7.66 litres in the Bray burners, and for a consumption of 7.11 litres in the ordinary German oil-gas jets; while the mixture containing 20 per cent. of acetylene evolved the same amount of light for a consumption of 2.02 litres in Bray burners, or of 2.06 litres in the oil-gas jets.

Again, taking No. 40 as the most popular and useful size of burner, 1 volume of acetylene oil-gas may be said to be equal to 3 volumes of simple oil-gas, which is the value a.s.signed to the mixture by the German Government officials, who, at the prices ruling there, hold the mixture to be twice as expensive as plain oil-gas per unit of volume, which means that for a given outlay 50 per cent. more light may be obtained from acetylene oil-gas than from oil-gas alone.

This comparison of cost is not applicable, as it stands, to compressed oil-gas, with and without enrichment by acetylene, in this country, owing to the oils from which oil-gas is made being much cheaper and of better quality here than in Germany, where a heavy duty is imposed on imported petroleum. Oil-gas as made from Scotch and other good quality gas-oil in this country, usually has, after compression, an illuminating duty of about 8 candles per cubic foot, which is about double that of the compressed German oil-gas as examined by Borck.

Hence the following table, containing a summary of results obtained by H.

Fowler with compressed oil-gas, as used on English railways, must be accepted rather than the foregoing, in so far as conditions prevailing in this country are concerned. It likewise refers to a mixture of oil-gas and acetylene containing 20 per cent. of acetylene.

______________________________________________________________________ | | | | | | | | | | | | | Ratio of | | | |Consumption| |Candles per| Illuminating | | Burner. |Pressure.| per Hour. |Candle| Cubic Foot| Power to that | | | Inches. |Cubic Feet.|Power.| per Hour. |of Oil-gas [1] | | | | | | | in the same | | | | | | | Burner. | |_____________|_________|___________|______|___________|_______________| | | | | | | | | Oil-gas . . | 0.7 | 0.98 | 12.5 | 12.72 | 1.65 | | Bray 000 . | 0.7 | 1.17 | 14.4 | 12.30 | 1.57 | | " 0000 . | 0.7 | 0.97 | 10.4 | 10.74 | 1.41 | | " 00000 | 0.7 | 0.78 | 5.6 | 7.16 | 1.08 | | " 000000 | 0.7 | 0.55 | 1.9 | 3.52 | 1.14 | |_____________|_________|___________|______|___________|_______________|

[Footnote 1: Data relating to the relative pecuniary values of acetylene (carburetted or not), coal-gas, paraffin, and electricity as heating or illuminating agents, are frequently presented to British readers after simple recalculation into English equivalents of the figures which obtain in France and Germany. Such a method of procedure is utterly incorrect, as it ignores the higher prices of coal, coal-gas, and especially petroleum products on the Continent of Europe, which arise partly from geographical, but mainly from political causes.]

The mixture was tried also at higher pressures in the same burners, but with less favourable results in regard to the duty realised. The oil-gas was also tried at various pressures, and the most favourable result is taken for computing the ratio in the last column. It is evident from this table that 1 volume of this acetylene-oil-gas mixture is equal at the most to 1.65 volume of the simple oil-gas. Whether the mixture will prove cheaper under particular conditions must depend on the relative prices of gas-oil and calcium carbide at the works where the gas is made and compressed. At the prevailing prices in most parts of Britain, simple oil-gas is slightly cheaper, but an appreciable rise in the price of gas- oil would render the mixture with acetylene the cheaper illuminant. The fact remains, however, that per unit weight or volume of cylinder into which the gas is compressed, acetylene oil-gas evolves a higher candle- power, or the same candle-power for a longer period, than simple, unenriched British oil-gas. Latterly, however, the incandescent mantle has found application for railway-carriage lighting, and poorer compressed gases have thereby been rendered available. Thus coal-gas, to which a small proportion of acetylene has been added, may advantageously displace the richer oil-gas and acetylene mixtures.

Patents have been taken out by Schwander for the preparation of a mixture of acetylene, air, and vaporised petroleum spirit. A current of naturally damp, or artificially moistened, air is led over or through a ma.s.s of calcium carbide, whereby the moisture is replaced by an equivalent quant.i.ty of acetylene; and this mixture of acetylene and air is carburetted by pa.s.sing it through a vessel of petroleum spirit in the manner adopted with air-gas. No details as to the composition, illuminating power, and calorific values of the gas so made have been published. It would clearly tend to be of highly indefinite const.i.tution and might range between what would be virtually inferior carburetted acetylene, and a low-grade air-gas. It is also doubtful whether the combustion of such gas would not be accompanied by too grave risks to render the process useful.

CHAPTER XII

SUNDRY USES

There are sundry uses for acetylene, and to some extent for carbide, which are not included in what has been said in previous chapters of this book; and to them a few words may be devoted.

In orchards and market gardens enormous damage is frequently done to the crops by the ravages of caterpillars of numerous species. These caterpillars cannot be caught by hand, and hitherto it has proved exceedingly difficult to cope with them. However, when they have changed into the perfect state, the corresponding b.u.t.terflies and moths, like most other winged insects, are strongly attracted by a bright light. As acetylene can easily be burnt in a portable apparatus, and as the burners can be supplied with gas at such comparatively high pressure that the flames are capable of withstanding sharp gusts of wind even when not protected by gla.s.s, the brilliant light given by acetylene forms an excellent method of destroying the insects before they have had time to lay their eggs. Two methods of using the light have been tried with astonis.h.i.+ng success: in one a naked flame is supported within some receptacle, such as a barrel with one end knocked out, the interior of which is painted heavily with treacle; in the other the flame is supported over an open dish filled with some cheap heavy oil (or perhaps treacle would do equally well). In the first case the insects are attracted by the light and are caught by the adhesive surfaces; in the second they are attracted and singed, and then drowned in, or caught by, the liquid. Either a well-made, powerful, vehicular lamp with its bull's- eye (if any) removed could be used for this purpose, or a portable generator of any kind might be connected with the burner through a flexible tube. It is necessary that the lights should be lit just before dusk when the weather is fine and the nights dark, and for some twenty evenings in June or July, exactly at the period of the year when the perfect insects are coming into existence. In some of the vineyards of Beaujolais, in France, where great havoc has been wrought by the pyralid, a set of 10-candle-power lamps were put up during July 1901, at distances of 150 yards apart, using generators containing 6 oz. of carbide, and dishes filled with water and petroleum 18 or 20 inches in diameter. In eighteen nights, some twenty lamps being employed, the total catch of insects was 170,000, or an average of 3200 per lamp per night. At French prices, the cost is reported to have been 8 centimes per night, or 32 centimes per hectare (2.5 acres). In Germany, where school children are occasionally paid for destroying noxious moths, two acetylene lamps burning for twelve evenings succeeded in catching twice as many insects as the whole juvenile population of a village during August 1902. A similar process has been recommended for the destruction of the malarial mosquito, and should prove of great service to mankind in infected districts. The superiority of acetylene in respect of brilliancy and portability will at once suggest its employment as the illuminant in the "light" moth-traps which entomologists use for entrapping moths. In these traps, the insects, attracted by the light, flutter down panes of gla.s.s, so inclined that ultimate escape is improbable; while they are protected from injury through contact with the flame by moans of an intervening sheet of gla.s.s.

Methods of spraying with carbide dust have been found useful in treating mildew in vines; while a process of burying small quant.i.ties of carbide at the roots has proved highly efficacious in exterminating phylloxera in the French and Spanish vineyards. It was originally believed that the impurities of the slowly formed acetylene, the phosphine in particular, acted as toxic agents upon the phylloxera; and therefore carbide containing an extra amount of decomposable phosphides was specially manufactured for the vine-growers. But more recently it has been argued, with some show of reason, that the acetylene itself plays a part in the process, the effects produced being said to be too great to be ascribed wholly to the phosphine. It is well known that many hydrocarbon vapours, such as the vapour of benzene or of naphthalene, have a highly toxic action on low organisms, and the destructive effect of acetylene on phylloxera may be akin to this action.

As gaseous acetylene will bear a certain amount of pressure in safety--a pressure falling somewhat short of one effective atmosphere--and as pressure naturally rises in a generating apparatus where calcium carbide reacts with water, it becomes possible to use this pressure as a source of energy for several purposes. The pressure of the gas may, in fact, be employed either to force a stream of liquid through a pipe, or to propel certain mechanism. An apparatus has been constructed in France on the lines of some portable fire-extinguis.h.i.+ng appliances in which the pressure set up by the evolution of acetylene in a closed s.p.a.ce produces a spray of water charged with lime and gas under the pressure obtaining; the liquid being thrown over growing vines or other plants in order to destroy parasitic and other forms of life. The apparatus consists of a metal cylinder fitted with straps so that it can be carried by man or beast. At one end it has an attachment for a flexible pipe, at the other end a perforated basket for carbide introduced and withdrawn through a "man-hole" that can be tightly closed. The cylinder is filled with water to a point just below the bottom of the basket when the basket is uppermost; the carbide charge is then inserted, and the cover fastened down. As long as the cylinder is carried in the same position, no reaction between the carbide and the water occurs, and consequently no pressure arises; but on inverting the vessel, the carbide is wetted, and acetylene is liberated in the interior. On opening the c.o.c.k on the outlet pipe, a stream of liquid issues and may be directed as required. By charging the cylinder in the first place with a solution of copper sulphate, the liquid ejected becomes a solution and suspension of copper and calcium salts and hydroxides, resembling "Bordeaux mixture," and may be employed as such. In addition, it is saturated with acetylene which adds to its value as a germicide.

The effective gas pressure set up in a closed generator has also been employed in Italy to drive a gas-turbine, and so to produce motion. The plant has been designed for use in lighthouses where acetylene is burnt, and where a revolving or flas.h.i.+ng light is required. The gas outlet from a suitably arranged generator communicates with the inlet of a gas- turbine, and the outlet of the turbine is connected to a pipe leading to the acetylene burners. The motion of the turbine is employed to rotate screens, coloured gla.s.ses, or any desired optical arrangements round the flames; or, in other situations, periodically to open and close a c.o.c.k on the gas-main leading to the burners. In the latter case, a pilot flame fed separately is always alight, and serves to ignite the gas issuing from the main burners when the c.o.c.k is opened.

Another use for acetylene, which is only dependent upon a suitably lowered price for carbide to become of some importance, consists in the preparation of a black pigment to replace ordinary lampblack. One method for this purpose has been elaborated by Hubou. Acetylene is prepared from carbide smalls or good carbide, according to price, and the gas is pumped into small steel cylinders to a pressure of 2 atmospheres. An electric spark is then pa.s.sed, and the gas, standing at its limit of safety, immediately dissociates, yielding a quant.i.tative amount of hydrogen and free carbon. The hydrogen is drawn off, collected in holders, and used for any convenient purpose; the carbon is withdrawn from the vessel, and is ready for sale. At present the pigment is much too expensive, at least in British conditions, to be available in the manufacture of black paint; but its price would justify its employment in the preparation of the best grades of printers' ink. One of the authors has examined an average sample and has found it fully equal in every way to blacks, such as those termed "spirit blacks," which fetch a price considerably above their real value. It has a pure black cast of tint, is free from greasy matter, and can therefore easily be ground into water, or into linseed oil without interfering with the drying properties of the latter. Acetylene black has also been tried in calico printing, and has given far better results in tone and strength than other blacks per unit weight of pigment. It may be added that the actual yield of pigment from creosote oils, the commonest raw material for the preparation of lampblack ("vegetable black"), seldom exceeds 20 or 25 per cent., although the oil itself contains some 80 per cent, of carbon. The yield from acetylene is clearly about 90 per cent., or from calcium carbide nearly 37.5 per cent, of the original weight.

An objection urged against the Hubou process is that only small quant.i.ties of the gas can be treated with the spark at one time; if the cylinders are too large, it is stated, tarry by-products are formed. A second method of preparing lampblack (or graphite) from acetylene is that devised by Frank, and depends on utilising the reactions between carbon monoxide or dioxide and acetylene or calcium carbide, which have already been sketched in Chapter VI. When acetylene is employed, the yield is pure carbon, for the only by-product is water vapour; but if the carbide process is adopted, the carbon remains mixed with calcium oxide. Possibly such a material as Frank's carbide process would give, viz., 36 parts by weight of carbon mixed with 56 parts of quicklime or 60 parts of carbon mixed with 112 parts of quicklime, might answer the purpose of a pigment in some black paints where the amount of ash left on ignition is not subject to specification. Naturally, however, the lime might be washed away from the carbon by treatment with hydrochloric acid; but the cost of such a purifying operation would probably render the residual pigment too expensive to be of much service except (conceivably) in the manufacture of certain grades of printers' ink, for which purpose it might compete with the carbon obtainable by the Hubou process already referred to.

Acetylene tetrachloride, or tetrachlorethane, C_2H_2Cl_4, is now produced for sale as a solvent for chlorine, sulphur, phosphorus, and organic substances such as fats. It may be obtained by the direct combination of acetylene and chlorine as explained in Chapter VI., but the liability of the reaction to take place with explosive violence would preclude the direct application of it on a commercial scale. Processes free from such risk have now, however, been devised for the production of tetrachlorethane. One patented by the Salzbergwerk Neu-Sta.s.sfurt consists in pa.s.sing acetylene into a mixture of finely divided iron and chloride of sulphur. The iron acts as a catalytic. The liquid is kept cool, and as soon as the acetylene pa.s.ses through unabsorbed, its introduction is stopped and chlorine is pa.s.sed in. Acetylene and chlorine are then pa.s.sed in alternately until the liquid finally is saturated with acetylene. The tetrachlorethane, boiling at 147 C., is then distilled off, and the residual sulphur is reconverted to the chloride for use again in the process. A similar process in which the chlorine is used in excess is applicable also to the production of hexachlorethane.

Dependent upon price, again, are several uses for calcium carbide as a metallurgical or reducing reagent; but as those are uses for carbide only as distinguished from acetylene, they do not fall within the purview of the present book.

When discussing, in Chapter III., methods for disposing of the lime sludge coming from an acetylene generator, it was stated that on occasion a use could be found for this material. If the carbide has been entirely decomposed in an apparatus free from overheating, the waste lime is recovered as a solid ma.s.s or as a cream of lime practically pure white in colour. Sometimes, however, as explained in Chapter II., the lime sludge is of a bluish grey tint, even in cases where the carbide decomposed was of good quality and there was no overheating in the generator. Such discoloration is of little moment for most of the uses to which the sludge may be put. The residue withdrawn from a carbide-to-water generator is usually quite fluid; but when allowed to rest in a suitable pit or tank, it settles down to a semi-solid or pasty ma.s.s which contains on a rough average 47 per cent. of water and 53 per cent. of solid matter, the amount of lime present, calculated as calcium oxide, being about 40 per cent. Since 64 parts by weight of pure calcium carbide yield 74 parts of dry calcium hydroxide, it may be said that 1 part of ordinary commercial carbide should yield approximately 1.1 parts of dry residue, or 2.1 parts of a sludge containing 47 per cent. of moisture; and sludge of this character has been stated by Vogel to weigh about 22.5 cwt. per cubic yard.

Experience has shown that those pasty carbide residues can be employed very satisfactorily, and to the best advantage from the maker's point of view, by builders and decorators for the preparation of ordinary mortar or lime-wash. The mortar made from acetylene lime has been found equal in strength and other properties to mortar compounded from fresh slaked lime; while the distemper prepared by diluting the sludge has been used most successfully in all places where a lime-wash is required, _e.g._, on fruit-trees, on cattle-pens, farm-buildings, factories, and the "offices" of a residence. Many of the village installations abroad sell their sludge to builders for the above-mentioned purposes at such a price that their revenue accounts are materially benefited by the additional income. The sludge is also found serviceable for softening the feed-water of steam boilers by the common liming process; although it has been stated that the material contains certain impurities--notably "fatty matter"--which becomes hydrolysed by the steam, yielding fatty acids that act corrosively upon the boiler-plates. This a.s.sertion would appear to require substantiation, but a patent has been taken out for a process of drying the sludge at a temperature of 150 to 200 C. in order to remove the harmful matter by the action of the steam evolved. So purified, it is claimed, the lime becomes fit for treating any hard potable or boiler- feed water. It is very doubtful, however, whether the intrinsic value of acetylene lime is such in comparison with the price of fresh lime that, with whatever object in view, it would bear the cost of any method of artificial drying if obtained from the generators in a pasty state.

When, on the other hand, the residue is naturally dry, or nearly so, it is exactly equal to an equivalent quant.i.ty of quick or slaked lime as a dressing for soil. In this last connexion, however, it must be remembered that only certain soils are improved by an addition of lime in any shape, and therefore carbide residues must not be used blindly; but if a.n.a.lysis indicates that a particular plot of ground would derive benefit from an application of lime, acetylene lime is precisely as good as any other description. Naturally a residue containing unspent carbide, or contaminated with tarry matter, is essentially valueless (except as mentioned below); while it must not be forgotten that a solid residue if it is exposed to air, or a pasty residue if not kept under water, will lose many of its useful properties, because it will be partially converted into calcium carbonate or chalk.

Nevertheless, in some respects, the residue from a good acetylene generator is a more valuable material, agriculturally speaking, than pure lime. It contains a certain amount of sulphur, &c., and it therefore somewhat resembles the spent or gas lime of the coal-gas industry. This sulphur, together, no doubt, with the traces of acetylene clinging to it, renders the residue a valuable material for killing the worms and vermin which tend to infest heavily manured and under-cultivated soil. Acetylene lime has been found efficacious in exterminating the "finger-and-toe" of carrots, the "peach-curl" of peach-trees, and in preventing cabbages from being "clubbed." It may be applied to the ground alone, or after admixture with some soil or stable manure. The residue may also be employed, either alone or mixed with some agglomerate, in the construction of garden paths and the like.

If the residues are suitably diluted with water and boiled with (say) twice their original weight of flowers of sulphur, the product consists of a mixture of various compounds of calcium and sulphur, or calcium sulphides--which remain partly in solution and partly in the solid state.

This material, used either as a liquid spray or as a moist dressing, has been said to prove a useful garden insecticide and weed-killer.

There are also numerous applications of the acetylene light, each of much value, but involving no new principle which need be noticed. The light is so actinic, or rich in rays acting upon silver salts, that it is peculiarly useful to the photographer, either for portraiture or for his various positive printing operations. Acetylene is very convenient for optical lantern work on the small scale, or where the oxy-hydrogen or oxy-coal-gas light cannot be used. Its intensity and small size make its self-luminous flame preferable on optical grounds to the oil-lamp or the coal-gas mantle; but the illuminating surface is nevertheless too large to give the best results behind such condensers as have been carefully worked to suit a source of light scarcely exceeding the dimensions of a point. For lantern displays on very large screens, or for the projection of a powerful beam of light to great distances in one direction (as in night signalling, &c.), the acetylene blowpipe fed with pure oxygen, or with air containing more than its normal proportion of oxygen, which is discussed in Chapter IX., is specially valuable, more particularly if the ordinary cylinder of lime is replaced by one of magnesia, zirconia, or other highly refractory oxide.

CHAPTER XIII

PORTABLE ACETYLENE LAMPS AND PLANT

It will be apparent from what has been said in past chapters that the construction of a satisfactory generator for portable purposes must be a problem of considerable complexity. A fixed acetylene installation tends to work the more smoothly, and the gas evolved therefrom to burn the more pleasantly, the more technically perfect the various subsidiary items of the plant are; that is to say, the more thoroughly the acetylene is purified, dried, and delivered at a strictly constant pressure to the burners and stoves. Moreover, the efficient behaviour of the generator itself will depend more upon the mechanical excellence and solidity of its construction than (with one or two exceptions) upon the precise system to which it belongs. And, lastly, the installation will, broadly speaking, work the better, the larger the holder is in proportion to the demands ever made upon it; while that holder will perform the whole duty of a gasholder more effectually if it belongs to the rising variety than if it is a displacement holder. All these requirements of a good acetylene apparatus have to be sacrificed to a greater or less extent in portable generators; and since the sacrifice becomes more serious as the generator is made smaller and lighter in weight, it may be said in general terms that the smaller a portable (or, indeed, other) acetylene apparatus is, the less complete or permanent satisfaction will it give its user. Again, small portable apparatus are only needed to develop intensities of light insignificant in comparison with those which may easily be won from acetylene on a larger scale; they are therefore fitted with smaller burners, and those burners are not merely small in terms of consumption and illuminating power, but not infrequently are very badly constructed, and are relatively deficient in economy or duty. Thus any comparisons which may be made on lines similar to those adopted in Chapter I., or between unit weights, volumes, or monetary equivalents of calcium carbide, paraffin, candles, and colza oil, become utterly incorrect if the carbide is only decomposed in a small portable generator fitted with an inefficient jet; first, because the latent illuminating power of the acetylene evolved is largely wasted; secondly, because any gas produced over and above that capable of instant combustion must be blown off from a vent-pipe; and thirdly, because the carbide itself tends to be imperfectly decomposed, either through a defect in the construction of the lamp, or through the brief and interrupted requirements of the consumer.

In several important respects portable acetylene apparatus may be divided into two cla.s.ses from a practical point of view. There is the portable table or stand lamp intended for use in an occupied room, and there is the hand or supported lamp intended for the illumination of vehicles or open-air s.p.a.ces. Economy apart, no difficulty arises from imperfect combustion or escape of unburnt gas from an outdoor lamp, but in a room the presence of unburnt acetylene must always be offensive even if it is not dangerous; while the combustion products of the impurities--and in a portable generator acetylene cannot be chemically purified--are highly objectionable. It is simply a matter of good design to render any form of portable apparatus safe against explosion (employment of proper carbide being a.s.sumed), for one or more vent-pipes can always be inserted in the proper places; but from an indoor lamp those vent-pipes cannot be made to discharge into a place of safety, while, as stated before, a generator in which the vent-pipes come into action with any frequency is but an extravagant piece of apparatus for the decomposition of so costly a material as calcium carbide. Looked at from one aspect the holder of a fixed apparatus is merely an economical subst.i.tute for the wasteful vent- pipe, because it is a place in which acetylene can be held in reserve whenever the make exceeds the consumption in speed. It is perhaps possible to conceive of a large table acetylene lamp fitted with a water- sealed rising holder; but for vehicular purposes the displacement holder is practically the only one available, and in small apparatus it becomes too minute in size to be of much service as a store for the gas produced by after-generation. Other forms of holder have been suggested by inventors, such as a collapsible bag of india-rubber or the like; but rubber is too porous, weak, and perishable a material to be altogether suitable. If it is possible, by bringing carbide and water into mutual contact in predetermined quant.i.ties, to produce gas at a uniform rate, and at one which corresponds with the requirements of the burner, in a small apparatus--and experience has shown it to be possible within moderately satisfactory limits--it is manifest that the holder is only needed to take up the gas of after-generation; and in Chapters II. and III. it was pointed out that after-generation only occurs when water is brought into contact with an excess of carbide. If, then, the opposite system of construction is adopted, and carbide is fed into water mechanically, no after-generation can take place; and provided the make of gas can be controlled in a small carbide-feed generator as accurately as is possible in a small water-to-carbide generator, the carbide-feed principle will exhibit even greater advantages in portable apparatus than it does in plant of domestic size. Naturally almost every variety of carbide-feeding gear, especially when small, requires or prefers granulated (or granulated and "treated") carbide; and granulated carbide must inevitably be considerably more expensive per unit of light evolved than the large material, but probably in the application to which the average portable acetylene apparatus is likely to be put, strict economy is not of first consequence. In portable acetylene generators of the carbide-feed type, the supply is generally governed by the movements of a mushroom-headed or conical valve at the mouth of a conical carbide vessel; such movements occurring in sympathy with the alterations in level of the water in the decomposing chamber, which is essentially a small displacement holder also, or being produced by the contraction of a flexible chamber through which the gas pa.s.ses on its way to the burner.

So far as it is safe to speak definitely on a matter of this kind, the carbide-feed device appears to work satisfactorily in a stationary (_e.g._, table) lamp; but it is highly questionable whether it could be applied to a vehicular apparatus exposed to any sensible amount of vibration. The device is satisfactory on the table of an occupied room so far, be it understood, as any small portable generators can be: it has no holder, but since no after-generation occurs, no holder is needed; still the combustion products contaminate the room with all the sulphur and phosphorus of the crude acetylene.

For vehicular lamps, and probably for hand lanterns, the water-to-carbide system has practically no alternative (among actual generators), and safety and convenience have to be gained at the expense of the carbide.

In such apparatus the supply of water is usually controlled ultimately by pressure, though a hand-operated needle-valve is frequently put on the water tube. The water actually reaches the carbide either by dropping from a jet, by pa.s.sing along, upwards or downwards, a "wick" such as is used in oil-lamps, or by percolating through a ma.s.s of porous material like felt. The carbide is held in a chamber closed except at the gas exit to the burner and at the inlet from the water reservoir: so that if gas is produced more rapidly than the burner takes it, more water is prevented from entering, or the water already present is driven backwards out of the decomposing chamber into some adjoining receptacle. It is impossible to describe in detail all the lamps which have been constructed or proposed for vehicular use; and therefore the subject must be approached in general terms, discussing simply the principles involved in the design of a safe portable generator.

In all portable apparatus, and indeed in generators of larger dimensions, the decomposing chamber must be so constructed that it can never, even by wrong manipulation, be sealed hermetically against the atmosphere. If there is a c.o.c.k on the water inlet tube which is capable of being completely shut, there must be no c.o.c.k between the decomposing chamber and the burner. If there is a c.o.c.k between the carbide vessel and the burner, the water inlet tube must only be closed by the water, being water-sealed, in fact, so that if pressure rises among the carbide the surplus gas may blow the seal or bubble through the water in the reservoir. If the water-supply is mainly controlled by a needle-valve, it is useful to connect the burner with the carbide vessel through a short length of rubber tube; and if this plan is adopted, a c.o.c.k can, if desired, be put close to the burner. The rubber should not be allowed to form a bend hanging down, or water vapour, &c., may condense and extinguish the flame. In any case there should be a steady fall from the burner to the decomposing chamber, or to some separate catch-pit for the products of condensation. Much of the success attainable with small generators will depend on the water used. If it is contaminated with undissolved matter, the dirt will eventually block the fine orifices, especially the needle-valve, or will choke the pores of the wick or the felt pad. If the water contains an appreciable amount of "temporary hardness," and if it becomes heated much in the lamp, fur will be deposited sooner or later, and will obviously give trouble. Where the water reservoir is at the upper part of the lamp, and the liquid is exposed to the heat of the flame, fur will appear quickly if the water is hard. Considerable benefit would accrue to the user of a portable lamp by the employment of rain water filtered, if necessary, through fabric or paper. The danger of freezing in very severe weather may be prevented by the use of calcium chloride, or preferably, perhaps, methylated spirit in the water (_cf._ Chapter III., p. 92). The disfavour with which cycle and motor acetylene lamps are frequently regarded by nocturnal travellers, other than the users thereof, is due to thoughtless design in the optical part of such lamps, and is no argument against the employment of acetylene. By proper shading or deflection of the rays, the eyes of human beings and horses can be sufficiently protected from the glare, and the whole of the illumination concentrated more perfectly on the road surface and the lower part of approaching objects--a beam of light never reaching a height of 5 feet above the ground is all that is needed to satisfy all parties.

As the size of the generator rises, conditions naturally become more suited to the construction of a satisfactory apparatus; until generators intended to supply light to the whole of (say) a railway carriage, or the head and cab lamps of a locomotive, or for the outside and inside lighting of an omnibus are essentially generators of domestic dimensions somewhat altered in internal construction to withstand vibration and agitation. As a rule there is plenty of s.p.a.ce at the side of a locomotive to carry a generator fitted with a displacement holder of sufficient size, which is made tall rather than wide, to prevent the water moving about more than necessary. From the boiler, too, steam can be supplied to a coil to keep the liquid from freezing in severe weather. Such apparatus need not be described at length, for they can be, and are, made on lines resembling those of domestic generators, though more compactly, and having always a governor to give a constant pressure. For carriage lighting any ordinary type of generator, preferably, perhaps, fitted with a displacement holder, can be erected either in each corridor carriage, or in a brake van at the end of the train. Purifiers may be added, if desired, to save the burners from corrosion; but the consumption of unpurified gas will seldom be attended by hygienic disadvantages, because the burners will be contained in closed lamps, ventilating into the outside air. The generator, also, may conveniently be so constructed that it is fed with carbide from above the roof, and emptied of lime sludge from below the floor of the vehicle. It can hardly be said that the use of acetylene generated on board adds a sensible risk in case of collision. In the event of a subsequent fire, the gas in the generator would burn, but not explode; but in view of the greater illuminating power per unit volume of carbide than per equal volume of compressed oil- gas, a portable acetylene generator should be somewhat less objectionable than broken cylinders of oil-gas if a fire should follow a railway accident of the usual kind. More particularly by the use of "cartridges"

of carbide, a railway carriage generator can be constructed of sufficient capacity to afford light for a long journey, or even a double journey, so that attention would be only required (in the ordinary way) at one end of the line.

Pa.s.sing on from the generators used for the lighting of vehicles and for portable lamps for indoor lighting to the considerably larger portable generators now constructed for the supply of acetylene for welding purposes and for "flare" lamps, it will be evident that they may embody most or all of the points which are essential to the proper working of a fixed generator for the supply of a small establishment. The holder will generally be of the displacement type, but some of these larger portable generators are equipped with a rising holder. The generators are, naturally, automatic in action, but may be either of the water-to-carbide or carbide-to-water type--the latter being preferable in the larger sizes intended for use with the oxy-acetylene blow-pipe for welding, &c., for which use a relatively large though intermittent supply of acetylene is called for. The apparatus is either carried by means of handles or poles attached to it, or is mounted on a wheelbarrow or truck for convenience of transport to the place where it is to be used. The so called "flare"

lamps, which are high power burners mounted, with or without a reflector, above a portable generator, are extremely useful for lighting open s.p.a.ces where work has to be carried on temporarily after nightfall, and are rapidly displacing oil-flares of the Lucigen type for such purposes.

The use of "cartridges" of calcium carbide has already been briefly referred to in Chapters II. and III. These cartridges are usually either receptacles of thin sheet-metal, say tin plate, or packages of carbide wrapped up in grease proof paper or the like. If of metal, they may have a lid which is detached or perforated before they are put into the generator, or the generator (when automatic and of domestic size) may be so arranged that a cartridge is punctured in one or more places whenever more gas is required. If wrapped in paper, the cartridges may be dropped into water by an automatic generator at the proper times, the liquid then loosening the gum and so gaining access to the interior; or one spot may be covered by a drape of porous material (felt) only, through which the water penetrates slowly. The substance inside the cartridge may be ordinary, granulated, or "treated" carbide. Cartridges or "sticks" of carbide are also made without wrappings, either by moistening powdered carbide with oil and compressing the whole into moulds, or by compressing dry carbide dust and immersing the sticks in oil or molten grease. The former process is said to cause the carbide to take up too much oil, so that sticks made by the second method are reputed preferable. All these cartridges have the advantage over common carbide of being more permanent in damp air, of being symmetrical in shape, of decomposing at a known speed, and of liberating acetylene in known quant.i.ty; but evidently they are more expensive, owing to the cost of preparing them, &c. They may be made more cheaply from the dust produced in the braking of carbide, but in that case the yield of gas will be relatively low.

It is manifest that, where s.p.a.ce is to spare, purifiers containing the materials mentioned in Chapter V. can be added to any portable acetylene apparatus, provided also that the extra weight is not prohibitive. Cycle lamps and motor lamps must burn an unpurified gas unpurified from phosphorus and sulphur; but it is always good and advisable to filter the acetylene from dust by a plug of cotton wool or the like, in order to keep the burners as clear as may be. A burner with a screwed needle for cleaning is always advantageous. Formerly the burners used on portable acetylene lamps were usually of the single jet or rat-tail, or the union jet or fish tail type, and exhibited in an intensified form, on account of their small orifices, all the faults of these types of burners for the consumption of acetylene (see Chapter VIII.). Now, however, there are numerous special burners adapted for use in acetylene cycle and motor lamps, &c., and many of these are of the impinging jet type, and some have steat.i.te heads to prevent distortion by the heat. One such cycle- lamp burner, as sold in England by L. Wiener, of Fore Street, London, is shown in Fig. 21. A burner constructed like the "Kona" (Chapter VIII.) is made in small sizes (6, 8 and 10 litres per hour) for use in vehicular lamps, under the name of the "Konette," by Falk, Stadelmann and Co., Ltd., of London, who also make a number of other small impinging jet burners. A single jet injector burner on the "Phos" principle is made in small sizes by the Phos Co., of London, specially for use in lamps on vehicles.

[Ill.u.s.tration: FIG. 21.--CYCLE-LAMP BURNER NO. 96042A.]

Nevertheless, although satisfactory medium-sized vehicular lamps for the generation of acetylene have been constructed, the best way of using acetylene for all such employments as these is to carry it ready made in a state of compression. For railway purposes, where an oil-gas plant is in existence, and where it is merely desired to obtain a somewhat brighter light, the oil-gas may be enriched with 20 per cent. of acetylene, and the mixed gas pumped into the same cylinders to a pressure of 10 atmospheres, as mentioned in Chapter XI.; the only alteration necessary being the subst.i.tution of suitable small burners for the common oil-gas jets. As far as the plant is concerned, all that is required is a good acetylene generator, purifier, and holder from which the acetylene can be drawn or forced through a meter into a larger storage holder, the meter being connected by gearing with another meter on the pipe leading from the oil-gas holder to the common holder, so that the necessary proportions of the two gases shall be introduced into the common holder simultaneously. From this final holder the enriched gas will be pumped into the cylinders or into a storage cylinder, by means of a thoroughly cooled pump, so that the heat set free by the compression may be safely dissipated.

Whenever still better light is required in railway carriages, as also for the illumination of large, constantly used vehicles, such as omnibuses, the acetone process (_cf._ Chapter XI.) exhibits notable advantages.

The light so obtained is the light of neat acetylene, but the gas is acetylene having an upper limit of explosibility much lower than usual because of the vapour of acetone in it. In all other respects the presence of the acetone will be unnoticeable, for it is a fairly pure organic chemical body, which burns in the flame completely to carbon dioxide and water, exactly as acetylene itself does. If the acetylene is merely compressed into porous matter without acetone, the gas burnt is acetylene simply; but per unit of volume or weight the cylinders will not be capable of developing so much light.

In the United States, at least one railway system (The Great Northern) has a number of its pa.s.senger coaches lighted by means of plain acetylene carried in a state of compression in cylinders without porous matter. The gas is generated, filtered from dust, and stored in an ordinary rising holder at a factory alongside the line; being drawn from this holder through a drier to extract moisture, and through a safety device, by a pump which, in three stages, compresses the acetylene into large storage reservoirs. The safety device consists of a heavy steel cylinder filled with some porous substance which, like the similar material of the acetone cylinders, prevents any danger of the acetylene contained in the water-sealed holder being implicated in an explosion starting backwards from the compression, by extinguis.h.i.+ng any spark which might be produced there. The plant on the trains comprises a suitable number of cylinders, filled by contact with the large stores of gas to a pressure of 10 atmospheres, pipes of fusible metal communicating with the lamps, and ordinary half-foot acetylene burners. The cylinders are provided with fusible plugs, so that, in the event of a fire, they and the service- pipes would melt, allowing the gas to escape freely and burn in the air, instead of exploding or dissociating explosively within the cylinders should the latter be heated by any burning woodwork or the like. It is stated that this plan of using acetylene enables a quant.i.ty of gas to be carried under each coach which is sufficient for a run of from 53 to 70 hours' duration, or of over 3600 miles; that is to say, enables the train, in the conditions obtaining on the line in question, to make a complete "round trip" without exhaustion of its store of artificial light. The system has been in operation for some years, and appears to have been so carefully managed that no accident has arisen; but it is clear that elements of danger are present which are eliminated when the cylinders are loaded with porous matter and acetone. The use of a similar system of compressed acetylene train lighting in South America has been attended with a disastrous explosion, involving loss of life.

It may safely be said that the acetone system, or less conveniently perhaps the mere compression into porous matter, is the best to adopt for the table-lamp which is to be used in occupied rooms Small cylinders of such shapes as to form an elegant base for a table-lamp on more or less conventional lines would be easy to make. They would be perfectly safe to handle. If accidentally or wilfully upset, no harm would arise. By deliberate ill-treatment they might be burst, or the gas-pipe fractured below the reducing valve, so that gas would escape under pressure for a time; but short of this they would be as devoid of extra clangor in times of fire as the candle or the coal-gas burner. Moreover, they would only contaminate the air with carbon dioxide and water vapour, for the gas is purified before compression; and modern investigations have conclusively demonstrated that the ill effects produced in the air of an imperfectly ventilated room by the extravagant consumption of coal-gas depend on the acc.u.mulation of the combustion products of the sulphur in the gas rather than upon the carbon dioxide set free.

One particular application of the portable acetylene apparatus is of special interest. As calcium carbide evolves an inflammable gas when it merely comes into contact with water, it becomes possible to throw into the sea or river, by hand or by ejection from a mortar, a species of bomb or portable generator which is capable of emitting a powerful beam of light if only facilities are present for inflaming the acetylene generated; and it is quite easy so to arrange the interior of such apparatus that they can be kept ready for instant use for long periods of time without sensible deterioration, and that they can be recharged after employment. Three methods of firing the gas have been proposed. In one the shock or contact with the water brings a small electric battery into play which produces a spark between two terminals projecting across the burner orifice; in the second, a cap at the head of the generator contains a small quant.i.ty of metallic pota.s.sium, which decomposes water with such energy that the hydrogen liberated catches fire; and in the third a similar cap is filled with the necessary quant.i.ty of calcium phosphide, or the "carbophosphide of calcium" mentioned in Chapter XI., which yields a flame by the immediate ignition of the liquid phosphine produced on the attack of water. During the two or three seconds consumed in the production of the spark or pilot flame, the water is penetrating the main charge of calcium carbide in the interior of the apparatus, until the whole is ready to give a bright light for a time limited only by the capacity of the generator. It is obvious that such apparatus may be of much service at sea: they may be thrown overboard to illuminate separate lifebuoys in case of accident, or be attached to the lifebuoys they are required to illuminate, or be used as lifebuoys themselves if fitted with suitable chains or ropes; they may be shot ahead to illuminate a difficult channel, or to render an enemy visible in time of war. Several such apparatus have already been constructed and severely tested; they appear to give every satisfaction. They are, of course, so weighted that the burner floats vertically, while buoyancy is obtained partly by the gas evolved, and partly by a hollow portion of the structure containing air. Cartridges of carbide and caps yielding a self- inflammable gas can be carried on board s.h.i.+p, by means of which the torches or lifebuoys may be renewed after service in a few minutes' time.