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 classes 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 spaces. 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 assumed), 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 substitute 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 quantities, 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 passes 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 passing along, upwards or downwards, a "wick" such as is used in oil-lamps, or by percolating through a mass 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 cock on the water inlet tube which is capable of being completely shut, there must be no cock between the decomposing chamber and the burner. If there is a cock 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 cock 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 space 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.
Passing 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 spaces 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 quantity; 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 space 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 steatite 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 "Phôs" principle is made in small sizes by the Phôs Co., of London, specially for use in lamps on vehicles.
[Illustration: 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 substitution 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 passenger 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 extinguishing 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 quantity 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 accumulation 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 quantity of metallic potassium, which decomposes water with such energy that the hydrogen liberated catches fire; and in the third a similar cap is filled with the necessary quantity 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 ship, by means of which the torches or lifebuoys may be renewed after service in a few minutes' time.