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 butterflies 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 glass, 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 astonishing 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 glass, 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 glass.
Methods of spraying with carbide dust have been found useful in treating mildew in vines; while a process of burying small quantities 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-extinguishing appliances in which the pressure set up by the evolution of acetylene in a closed space 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 cock 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 flashing 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 glasses, or any desired optical arrangements round the flames; or, in other situations, periodically to open and close a cock 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 cock 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 passed, and the gas, standing at its limit of safety, immediately dissociates, yielding a quantitative 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 quantities 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-Stassfurt consists in passing 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 passes through unabsorbed, its introduction is stopped and chlorine is passed in. Acetylene and chlorine are then passed 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 mass 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 mass 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 assertion 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 quantity 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 analysis 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.