Chapter V: The Treatment of Acetylene After Generation

IMPURITIES IN CALCIUM CARBIDE.--The calcium carbide manufactured at the present time, even when of the best quality commercially obtainable, is by no means a chemically pure substance; it contains a large number of foreign bodies, some of which evolve gas on treatment with water. To a considerable extent this statement will probably always remain true in the future; for in order to make absolutely pure carbide it would be necessary for the manufacturer to obtain and employ perfectly pure lime, carbon, and electrodes in an electric furnace which did not suffer attack during the passage of a powerful current, or he would have to devise some process for simultaneously or subsequently removing from his carbide those impurities which were derived from his impure raw materials or from the walls of his furnace--and either of these processes would increase the cost of the finished article to a degree that could hardly be borne. Beside the impurities thus inevitably arising from the calcium carbide decomposed, however, other impurities may be added to acetylene by the action of a badly designed generator or one working on a wrong system of construction; and therefore it may be said at once that the crude gas coming from the generating plant is seldom fit for immediate consumption, while if it be required for the illumination of occupied rooms, it must invariably be submitted to a rigorous method of chemical purification.

IMPURITIES OF ACETYLENE.--Combining together what may be termed the carbide impurities and the generator impurities in crude acetylene, the foreign bodies are partly gaseous, partly liquid, and partly solid. They may render the gas dangerous from the point of view of possible explosions; they, or the products derived from them on combustion, may be harmful to health if inspired, injurious to the fittings and decorations of rooms, objectionable at the burner orifices by determining, or assisting in, the formation of solid growths which distort the flame and so reduce its illuminating power; they may give trouble in the pipes by condensing from the state of vapour in bends and dips, or by depositing, if they are already solid, in angles, &c., and so causing stoppages; or they may be merely harmful economically by acting as diluents to the acetylene and, by having little or no illuminating value of themselves, causing the gas to emit less light than it should per unit of volume consumed, more particularly, of course, when the acetylene is not burnt under the mantle. Also, not being acetylene, or isomeric therewith, they require, even if they are combustible, a different proportion of oxygen for their perfect combustion; and a good acetylene jet is only calculated to attract precisely that quantity of air to the flame which a gas having the constitution C_2H_2 demands. It will be apparent without argument that a proper system of purification is one that is competent to remove the carbide impurities from acetylene, so far as that removal is desirable or necessary; it should not be called upon to extract the generator impurities, because the proper way of dealing with them is, to the utmost possible extent, to prevent their formation. The sole exception to this rule is that of water-vapour, which invariably accompanies the best acetylene, and must be partially removed as soon as convenient. Vapour of water almost always accompanies acetylene from the generator, even when the apparatus does not belong to those systems of working where liquid water is in excess, this being due to the fact that in a generator where the carbide is in excess the temperature tends to rise until part of the water is vapourised and carried out of the decomposing chamber before it has an opportunity of reacting with the excess of carbide. The issuing gas is therefore more or less hot, and it usually comes from the generating chamber saturated with vapour, the quantity needed so to saturate it rising as the temperature of the gas increases. Practically speaking, there is little objection to the presence of water-vapour in acetylene beyond the fear of deposition of liquid in the pipes, which may accumulate till they are partially or completely choked, and may even freeze and burst them in very severe weather. Where the chemical purifiers, too, contain a solid material which accidentally or intentionally acts as a drier by removing moisture from the acetylene, it is a waste of such comparatively expensive material to allow gas to enter the purifier wetter than need be.

EXTRACTION OF MOISTURE.--In all large plants the extraction of the moisture may take place in two stages. Immediately after the generator, and before the washer if the generator requires such an apparatus to follow it, a condenser is placed. Here the gas is made to travel somewhat slowly through one or more pipes surrounded with cold air or water, or is made to travel through a space containing pipes in which cold water is circulating, the precise method of constructing the condenser being perfectly immaterial so long as the escaping gas has a temperature not appreciably exceeding that of the atmosphere. So cooled, however, the gas still contains much water-vapour, for it remains saturated therewith at the temperature to which it is reduced, and by the inevitable law of physics a further fall in temperature will be followed by a further deposition of liquid water from the acetylene. Manifestly, if the installation is so arranged that the gas can at no part of the service and on no occasion fall to a lower temperature than that at which it issues from the condenser, the removal of moisture as effected by such a condenser will be sufficient for all practical purposes; but at least in all large plants where a considerable length of main is exposed to the air, a more complete moisture extractor must be added to the plant, or water will be deposited in the pipes every cold night in the winter. It is, however, useless to put a chemical drier, or one more searching in its action than a water-cooled condenser, at so early a position in the acetylene plant, because the gas will be subsequently stored in a water- sealed holder, where it will most probably once again be saturated with moisture from the seal. When such generators are adopted as require to have a specific washer placed after them in order to remove the water- soluble impurities, _e.g._, those in which the gas does not actually bubble through a considerable quantity of liquid in the generating chamber itself, it is doubtful whether a separate condenser is altogether necessary, because, as the water in the washer can easily be kept at the atmospheric temperature (by means of water circulating in pipes or otherwise), the gas will be brought to the atmospheric temperature in the washer, and at that temperature it cannot carry with it more than a certain fixed proportion of moisture. The notion of partially drying a gas by causing it to pass through water may appear paradoxical, but a comprehension of physical laws will show that it is possible, and will prove efficient in practice, when due attention is given to the facts that the gas entering the washer is hot, and that it is subsequently to be stored over water in a holder.

GENERATOR IMPURITIES.--The generator impurities present in the crudest acetylene consist of oxygen and nitrogen, _i.e._, the main constituents of air, the various gaseous, liquid, and semi-solid bodies described in Chapter II., which are produced by the polymerising and decomposing action of heat upon the carbide, water, and acetylene in the apparatus, and, whenever the carbide is in excess in the generator, some lime in the form of a very fine dust. In all types of water-to-carbide plant, and in some automatic carbide-feed apparatus, the carbide chamber must be disconnected and opened each time a fresh charge has to be inserted; and since only about one-third of the space in the container can be filled with carbide, the remaining two-thirds are left full of air. It is easy to imagine that the carbide container of a small generator might be so large, or loaded with so small a quantity of carbide, or that the apparatus might in other respects be so badly designed, that the gas evolved might contain a sufficient proportion of air to render it liable to explode in presence of a naked light, or of a temperature superior to its inflaming-point. Were a cock, however, which should have been shut, to be carelessly left open, an escape of gas from, rather than an introduction of air into, the apparatus would follow, because the pressure in the generator is above that of the atmosphere. As is well known, roughly four-fifths by volume of the air consist of nitrogen, which is non-inflammable and accordingly devoid of danger- conferring properties; but in all flames the presence of nitrogen is harmful by absorbing much of the heat liberated, thus lowering the temperature of that flame, and reducing its illuminating power far more seriously. On the other hand, a certain quantity of air in acetylene helps to prevent burner troubles by acting as a mere diluent (albeit an inferior one to methane or marsh-gas), and therefore it has been proposed intentionally to add air to the gas before consumption, such a process being in regular use on the large scale in some places abroad. As Eitner has shown (Chapter VI.) that in a 3/4-inch pipe acetylene ceases to be explosive when mixed with less than 47.7 per cent. of air, an amount of, say, 40 per cent. or less may in theory be safely added to acetylene; but in practice the amount of air added, if any, would have to be much smaller, because the upper limit of explosibility of acetylene-air mixtures is not rigidly fixed, varying from about 50 per cent. of air when the mixture is in a small vessel, and fired electrically to about 25 per cent. of air in a large vessel approached with a flame. Moreover, safely to prepare such mixtures, after the proportion of air had been decided upon, would require the employment of some additional perfectly trustworthy automatic mechanism to the plant to draw into the apparatus a quantity of air strictly in accordance with the volume of acetylene made --a pair of meters geared together, one for the gas, the other for the air--and this would introduce extra complexity and extra expense. On the whole the idea cannot be recommended, and the action of the British Home Office in prohibiting the use of all such mixtures except those unavoidably produced in otherwise good generators, or in burners of the ordinary injector type, is perfectly justifiable. The derivation and effect of the other gaseous and liquid generator impurities in acetylene were described in Chapter II. Besides these, very hot gas has been found to contain notable amounts of hydrogen and carbon monoxide, both of which burn with non-luminous flames. The most plausible explanation of their origin has been given by Lewes, who suggests that they may be formed by the action of water-vapour upon very hot carbide or upon carbon separated therefrom as the result of previous dissociation among the gases present; the steam and the carbon reacting together at a temperature of 500° C. or thereabouts in a manner resembling that of the production of water-gas. The last generator impurity is lime dust, which is calcium oxide or hydroxide carried forward by the stream of gas in a state of extremely fine subdivision, and is liable to be produced whenever water acts rapidly upon an excess of calcium carbide. This lime occasionally appears in the alternative form of a froth in the pipes leading directly from the generating chamber; for some types of carbide-to-water apparatus, decomposing certain kinds of carbide, foam persistently when the liquid in them becomes saturated with lime, and this foam or froth is remarkably difficult to break up.

FILTERS.--It has just been stated that the purifying system added to an acetylene installation should not be called upon to remove these generator impurities; because their appearance in quantity indicates a faulty generator, which should be replaced by one of better action. On the contrary, with the exception of the gases which are permanent at atmospheric temperature--hydrogen, carbon monoxide, nitrogen, and oxygen-- and which, once produced, must remain in the acetylene (lowering its illuminating value, but giving no further trouble), extraction of these generator impurities is quite simple. The dust or froth of lime will be removed in the washer where the acetylene bubbles through water--the dust itself can be extracted by merely filtering the gas through cotton-wool, felt, or the like. The least volatile liquid impurities will be removed partly in the condenser, partly in the washer, and partly by the mechanical dry-scrubbing action of the solid purifying material in the chemical purifier. To some extent the more volatile liquid bodies will be removed similarly; but a complete extraction of them demands the employment of some special washing apparatus in which the crude acetylene is compelled to bubble (in finely divided streams) through a layer of some non-volatile oil, heavy mineral lubricating oil, &c.; for though soluble in such oil, the liquid impurities are not soluble in, nor do they mix with, water; and since they are held in the acetylene as vapours, a simple passage through water, or through water-cooled pipes, does not suffice for their recovery. It will be seen that a sufficient removal of these generator impurities need throw no appreciable extra labour upon the consumer of acetylene, for he can readily select a type of generator in which their production is reduced to a minimum; while a cotton-wool or coke filter for the gas, a water washer, which is always useful in the plant if only employed as a non-return valve between the generator and the holder, and the indispensable chemical purifiers, will take out of the acetylene all the remaining generator impurities which need, and can, be extracted.

CARBIDE IMPURITIES.--Neglecting very minute amounts of carbon monoxide and hydrogen (which may perhaps come from cavities in the calcium carbide itself), as being utterly insignificant from the practical point of view, the carbide impurities of the gas fall into four main categories: those containing phosphorus, those containing sulphur, those containing silicon, and those containing gaseous ammonia. The phosphorus in the gas comes from calcium phosphide in the calcium carbide, which is attacked by water, and yields phosphoretted hydrogen (or phosphine, as it will be termed hereafter). The calcium phosphide, in its turn, is produced in the electric furnace by the action of the coke upon the phosphorus in phosphatic lime--all commercially procurable lime and some varieties of coke (or charcoal) containing phosphates to a larger or smaller extent. The sulphur in the gas comes from aluminium sulphide in the carbide, which is produced in the electric furnace by the interaction of impurities containing aluminium and sulphur (clay-like bodies, &c.) present in the lime and coke; this aluminium sulphide is attacked by water and yields sulphuretted hydrogen. Even in the absence of aluminium compounds, sulphuretted hydrogen may be found in the gases of an acetylene generator; here it probably arises from calcium sulphide, for although the latter is not decomposed by water, it gradually changes in water into calcium sulphydrate, which appears to suffer decomposition. When it exists in the gas the silicon is derived from certain silicides in the carbide; but this impurity will be dealt with by itself in a later paragraph. The ammonia arises from the action of the water upon magnesium, aluminium, or possibly calcium nitride in the calcium carbide, which are bodies also produced in the electric furnace or as the carbide is cooling. In the gas itself the ammonia exists as such; the phosphorus exists mainly as phosphine, partly as certain organic compounds containing phosphorus, the exact chemical nature of which has not yet been fully ascertained; the sulphur exists partly as sulphuretted hydrogen and partly as organic compounds analogous, in all probability, to those of phosphorus, among which Caro has found oil of mustard, and certain bodies that he regards as mercaptans. [Footnote: It will be convenient to borrow the phrase used in the coal-gas industry, calling the compounds of phosphorus other than phosphine "phosphorus compounds," and the compounds of sulphur other than sulphuretted hydrogen "sulphur compounds." The "sulphur compounds" of coal-gas, however, consist mainly of carbon bisulphide, which is certainly not the chief "sulphur compound" in acetylene, even if present to any appreciable extent.] The precise way in which these organic bodies are formed from the phosphides and sulphides of calcium carbide is not thoroughly understood; but the system of generation employed, and the temperature obtaining in the apparatus, have much to do with their production; for the proportion of the total phosphorus and sulphur found in the crude gas which exists as "compounds" tends to be greater as the generating plant yields a higher temperature. It should be noted that ammonia and sulphuretted hydrogen have one property in common which sharply distinguishes them from the sulphur "compounds," and from all the phosphorus compounds, including phosphine. Ammonia and sulphuretted hydrogen are both very soluble in water, the latter more particularly in the lime-water of an active acetylene generator; while all the other bodies referred to are completely insoluble. It follows, therefore, that a proper washing of the crude gas in water should suffice to remove all the ammonia and sulphuretted hydrogen from the acetylene; and as a matter of fact those generators in which the gas is evolved in presence of a large excess of water, and in which it has to bubble through such water, yield an acetylene practically free from ammonia, and containing nearly all the sulphur which it does contain in the state of "compounds." It must also be remembered that chemical processes which are perfectly suited to the extraction of sulphuretted hydrogen and phosphine are not necessarily adapted for the removal of the other phosphorus and sulphur compounds.

WASHERS.--In designing a washer for the extraction of ammonia and sulphuretted hydrogen it is necessary to see that the gas is brought into most intimate contact with the liquid, while yet no more pressure than can possibly be avoided is lost. Subdivision of the gas stream may be effected by fitting the mouth of the inlet-pipe with a rose having a large number of very small holes some appreciable distance apart, or by bending the pipe to a horizontal position and drilling it on its upper surface with numbers of small holes. Another method is to force the gas to travel under a series of partitions extending just below the water- level, forming the lower edges of those partitions either perfectly horizontal or with small notches like the teeth of a saw. One volume of pure water only absorbs about three volumes of sulphuretted hydrogen at atmospheric temperatures, but takes up some 600 volumes of gaseous ammonia; and as ammonia always accompanies the sulphuretted hydrogen, the latter may be said to be absorbed in the washer by a solution of ammonia, a liquid in which sulphuretted hydrogen is much more soluble. Therefore, since water only dissolves about an equal volume of acetylene, the liquid in the washer will continue to extract ammonia and sulphuretted hydrogen long after it is saturated with the hydrocarbon. For this reason, _i.e._, to avoid waste of acetylene by dissolution in the clean water of the washer, the plan is sometimes adopted of introducing water to the generator through the washer, so that practically the carbide is always attacked by a liquid saturated with acetylene. Provided the liquid in the generator does not become seriously heated, there is no objection to this arrangement; but if the water is heated strongly in the generator it loses much or all of its solvent properties, and the impurities may be driven back again into the washer. Clearly if the waste lime of the generator occurs as a dry or damp powder, the plan mentioned is not to be recommended; but when the waste lime is a thin cream--water being in large excess--it may be adopted. If the generator produces lime dust among the gas, and if the acetylene enters the washer through minute holes, a mechanical filter to remove the dust must be inserted between the generator and the washer, or the orifices of the leading pipe will be choked. Whenever a water-cooled condenser is employed after the generator, in which the gas does not come in contact with the water, that liquid may always be used to charge the generator. For compactness and simplicity of parts the water of the holder seal is occasionally used as the washing liquid, but unless the liquid of the seal is constantly renewed it will thus become offensive, especially if the holder is under cover, and it will also act corrosively upon the metal of the tank and bell. The water-soluble impurities in acetylene will not be removed completely by merely standing over the holder seal for a short time, and it is not good practice to pass unnecessarily impure gas into a holder. [Footnote: This is not a contradiction of what has been said in Chapter III. about the relative position of holder and chemical purifiers, because reference is now being made to ammonia and sulphuretted hydrogen only.]

HARMFULNESS OF IMPURITIES.--The reasons why the carbide impurities must be removed from acetylene before it is burned have now to be explained. From the strictly chemical point of view there are three compounds of phosphorus, all termed phosphoretted hydrogen or phosphine: a gas, PH_3; a liquid, P_2H_4; and a solid, P_4H_2. The liquid is spontaneously inflammable in presence of air; that is to say, it catches fire of itself without the assistance of spark or flame immediately it comes in contact with atmospheric oxygen; being very volatile, it is easily carried as vapour by any permanent gas. The gaseous phosphine is not actually spontaneously inflammable at temperatures below 100° C.; but it oxidises so rapidly in air, even when somewhat diluted, that the temperature may quickly rise to the point of inflammation. In the earliest days of the acetylene industry, directly it was recognised that phosphine always accompanies crude acetylene from the generator, it was believed that unless the proportion were strictly limited by decomposing only a carbide practically free from phosphides, the crude acetylene might exhibit spontaneously inflammable properties. Lewes, indeed, has found that a sample of carbide containing 1 per cent of calcium phosphide gave (probably by local decomposition--the bulk of the phosphide suffering attack first) a spontaneously inflammable gas; but when examining specimens of commercial carbide the highest amount of phosphine he discovered in the acetylene was 2.3 per cent, and this gas was not capable of self-inflammation. According to Bullier, however, acetylene must contain 80 per cent of phosphine to render it spontaneously inflammable. Berdenich has reported a case of a parcel of carbide which yielded on the average 5.1 cubic foot of acetylene per lb., producing gas which contained only 0.398 gramme of phosphorus in the form of phosphine per cubic metre (or 0.028 per cent. of phosphine) and was spontaneously inflammable. But on examination the carbide in question was found to be very irregular in composition, and some lumps produced acetylene containing a very high proportion of phosphorus and silicon compounds. No doubt the spontaneous inflammability was due to the exceptional richness of these lumps in phosphorus. As manufactured at the present day, calcium carbide ordinarily never contains an amount of phosphide sufficient to render the gas dangerous on the score of spontaneous inflammability; but should inferior material ever be put on the markets, this danger might have to be guarded against by submitting the gas evolved from it to chemical analysis. Another risk has been suggested as attending the use of acetylene contaminated with phosphine (and to a minor degree with sulphuretted hydrogen), viz., that being highly toxic, as they undoubtedly are, the gas containing them might be extremely dangerous to breathe if it escaped from the service, or from a portable lamp, unconsumed. Anticipating what will be said in a later paragraph, the worst kind of calcium carbide now manufactured will not yield a gas containing more than 0.1 per cent. by volume of sulphuretted hydrogen and 0.05 per cent. of phosphine. According to Haldane, air containing 0.07 per cent. of sulphuretted hydrogen produces fatal results on man if it is breathed for some hours, while an amount of 0.2 per cent. is fatal in 1- 1/2 minutes. Similar figures for phosphine cannot be given, because poisoning therewith is very rare or quite unknown: the cases of "phossy- jaw" in match factories being caused either by actual contact with yellow phosphorus or by inhalation of its vapour in the elemental state. However, assuming phosphine to be twice as toxic as sulphuretted hydrogen, its effect in crude acetylene of the above-mentioned composition will be equal to that of the sulphuretted hydrogen, so that in the present connexion the gas may be said to be equally toxic with a sample of air containing 0.2 per cent. of sulphuretted hydrogen, which kills in less than two minutes. But this refers only to crude acetylene undiluted with air; and being a hydrocarbon--being in fact neither oxygen nor common air--acetylene is irrespirable of itself though largely devoid of specific toxic action. Numerous investigations have been made of the amount of acetylene (apart from its impurities) which can be breathed in safety; but although these point to a probable recovery after a fairly long-continued respiration of an atmosphere charged with 30 per cent. of acetylene, the figure is not trustworthy, because toxicological experiments upon animals seldom agree with similar tests upon man. If crude acetylene were diluted with a sufficient proportion of air to remove its suffocating qualities, the percentage of specifically toxic ingredients would be reduced to a point where their action might be neglected; and short of such dilution the acetylene itself would in all probability determine pathological effects long before its impurities could set up symptoms of sulphur and phosphorus poisoning.

Ammonia is objectionable in acetylene because it corrodes brass fittings and pipes, and because it is partially converted (to what extent is uncertain) into nitrous and nitric acids as it passes through the flame. Sulphur is objectionable in acetylene because it is converted into sulphurous and sulphuric anhydrides, or their respective acids, as it passes through the flame. Phosphorus is objectionable because in similar circumstances it produces phosphoric anhydride and phosphoric acid. Each of these acids is harmful in an occupied room because they injure the decorations, helping to rot book-bindings, [Footnote: It is only fair to state that the destruction of leather bindings is commonly due to traces of sulphuric acid remaining in the leather from the production employed in preparing it, and is but seldom caused directly by the products of combustion coming from gas or oil.] tarnishing "gold-leaf" ornaments, and spoiling the colours of dyed fabrics. Each is harmful to the human system, sulphuric and phosphoric anhydrides (SO_3, and P_4O_10) acting as specific irritants to the lungs of persons predisposed to affections of the bronchial organs. Phosphorus, however, has a further harmful action: sulphuric anhydride is an invisible gas, but phosphoric anhydride is a solid body, and is produced as an extremely fine, light, white voluminous dust which causes a haze, more or less opaque, in the apartment. [Footnote: Lewes suggests that ammonia in the gas burnt may assist in the production of this haze, owing to the formation of solid ammonium salts in the state of line dust.] Immediately it comes in contact with atmospheric moisture phosphoric anhydride is converted into phosphoric acid, but this also occurs at first as a solid substance. The solidity and visibility of the phosphoric anhydride and acid are beneficial in preventing highly impure acetylene being unwittingly burnt in a room; but, on the other hand, being merely solids in suspension in the air, the combustion products of phosphorus are not so easily carried away from the room by the means provided for ventilation as are the products of the combustion of sulphur. Phosphoric anhydride is also partly deposited in the solid state at the burner orifices, perhaps actually corroding the steatite jets, and always assisting in the deposition of carbon from any polymerised hydrocarbons in the acetylene; thus helping the carbon to block up or distort those orifices. Whenever the acetylene is to be burnt on the incandescent system under a mantle of the Welsbach or other type, phosphorus, and possibly sulphur, become additionally objectionable, and rigorous extraction is necessary. As is well known, the mantle is composed of the oxides of certain "rare earths" which owe their practical value to the fact that they are non-volatile at the temperature of the gas-flame. When a gas containing phosphorus is burnt beneath such a mantle, the phosphoric anhydride attacks those oxides, partially converting them into the respective phosphates, and these bodies are less refractory. A mantle exposed to the combustion products of crude acetylene soon becomes brittle and begins to fall to pieces, occasionally showing a yellowish colour when cold. The actual advantage of burning acetylene on the incandescent system is not yet thoroughly established-- in this country at all events; but it is clear that the process will not exhibit any economy (rather the reverse) unless the plant is provided with most capable chemical purifiers. Phosphorus, sulphur, and ammonia are not objectionable in crude acetylene because they confer upon the gas a nauseous odour. From a well-constructed installation no acetylene escapes unconsumed: the gas remains wholly within the pipes until it is burnt, and whatever odour it may have fails to reach the human nostrils. A house properly piped for acetylene will be no more conspicuous by its odour than a house properly piped for coal-gas. On the contrary, the fact that the carbide impurities of acetylene, which, in the absolutely pure state, is a gas of somewhat faint, hardly disagreeable, odour, do confer upon that gas a persistent and unpleasant smell, is distinctly advantageous; for, owing to that odour, a leak in the pipes, an unclosed tap, or a fault in the generating plant is instantly brought to the consumer's attention. A gas wholly devoid of odour would be extremely dangerous in a house, and would have to be scented, as is done in the case of non-carburetted water-gas when it is required for domestic purposes.

AMOUNTS OF IMPURITIES AND SCOPE OF PURIFICATION.--Partly for the reason which has just been given, and partly on the ground of expense, a complete removal of the impurities from crude acetylene is not desirable. All that need be done is to extract sufficient to deprive the gas of its injurious effects upon lungs, decorations, and burners. As it stands, however, such a statement is not sufficiently precise to be useful either to consumers of acetylene or to manufacturers of plant, and some more or less arbitrary standard must be set up in order to define the composition of "commercially pure" acetylene, as well as to gauge the efficiency of any process of purification. In all probability such limit may be reasonably taken at 0.1 milligramme of either sulphur or phosphorus (calculated as elementary bodies) per 1 litre of acetylene, _i.e._, 0.0-1.1 grain per cubic foot; a quantity which happens to correspond almost exactly with a percentage by weight of 0.01. Owing to the atomic weights of these substances, and the very small quantities being considered, the same limit hardly differs from that of 0.01 per cent. by weight of sulphuretted hydrogen or of phosphine--it being always recollected that the sulphur and phosphorus do not necessarily exist in the gas as simple hydrides. Keppeler, however, has suggested the higher figure of 0.15 milligramme of either sulphur or phosphorus per litre of acetylene (=0.066 grain per cubic foot) for the maximum amount of these impurities permissible in purified acetylene. He adopts this standard on the basis of the results of observations of the amounts of sulphur and phosphorus present in the gas issuing from a purifier charged with heratol at the moment when the last layer of the heratol is beginning to change colour. No limit has been given for the removal of the ammonia, partly because that impurity can more easily, and without concomitant disadvantage, be extracted entirely; and partly because it is usually removed in the washer and not in the true chemical purifier.

According to Lewes, the maximum amount of ammonia found in the acetylene coming from a dripping generator is 0.95 gramme per litre, while in carbide-to-water gas it is 0.16 gramme: 417 and 70.2 grains per cubic foot respectively. Rossel and Landriset have found 4 milligrammes (1.756 grains [Footnote: Milligrammes per litre; grains per cubic foot. It is convenient to remember that since 1 cubic foot of water weighs 62.321 x 16 - 997.14 avoirdupois ounces, grammes per litre are approximately equal to oz. per cubic foot; and grammes per cubic metre to oz. per 1000 cubic feet.]) to be the maximum in water-to-carbide gas, and none to occur in carbide-to-water acetylene. Rossel and Landriset return the minimum proportion of sulphur, calculated as H_2S, found in the gaseous state in acetylene when the carbide has not been completely flooded with water at 1.18 milligrammes per litre, or 0.52 grain per cubic foot; and the corresponding maxima at 1.9 milligrammes, or 0.84 grain. In carbide-to- water gas, the similar maxima are 0.23 milligramme or 0.1 grain. As already stated, the highest proportion of phosphine yet found in acetylene is 2.3 per cent. (Lewes), which is equal to 32.2 milligrammes of PH_3 per litre or 14.13 grains per cubic foot (Polis); but this sample dated from 1897. Eitner and Keppeler record the minimum proportion of phosphorus, calculated as PH_3, found in crude acetylene, as 0.45 milligramme per litre, and the maximum as 0.89 milligramme per litre; in English terms these figures are 0.2 and 0.4 grain per cubic foot. On an average, however, British and Continental carbide of the present day may be said to give a gas containing 0.61 milligramme of phosphorus calculated as PH_3 per litre and 0.75 milligramme of sulphur calculated as H_2S. In other units these figures are equal to 0.27 grain of PH_3 and 0.33 grain of H_2S per 1 cubic foot, or to 0.041 per cent. by volume of PH_3 and 0.052 per cent. of H_2S. Yields of phosphorus and sulphur much higher than these will be found in the journals and books, but such analytical data were usually obtained in the years 1896-99, before the manufacture of calcium carbide had reached its present degree of systematic control. A commercial specimen of carbide was seen by one of the authors as late as 1900 which gave an acetylene containing 1.12 milligramme of elementary sulphur per litre, i.e., 0.096 per cent, by volume, or 0.102 per cent, by volume of H_2S; but the phosphorus showed the low figure of 0.36 milligramme per litre (0.031 per cent, of P or 0.034 per cent, of PH_3 by volume).

The British Acetylene Association's regulations relating to carbide of calcium (_vide_ Chap. XIV.) contain a clause to the effect that "carbide which, when properly decomposed, yields acetylene containing from all phosphorus compounds therein more than 0.05 per cent, by volume of phosphoretted hydrogen, may be refused by the buyer." This limit is equivalent to 0.74 milligramme of phosphorus calculated as PH_3 per litre. A latitude of 0.01 per cent, is, however, allowed for the analysis, so that the ultimate limit on which carbide could be rejected is: 0.06 volume per cent. of PH_3, or 0.89 milligramme of phosphorus per litre.

The existence in appreciable quantity of combined silicon as a normal impurity in acetylene seems still open to doubt. Calcium carbide frequently contains notable quantities of iron and other silicides; but although these bodies are decomposed by acids, yielding hydrogen silicide, or siliciuretted hydrogen, they are not attacked by plain water. Nevertheless Wolff and Gerard have found hydrogen silicide in crude acetylene, and Lewes looks upon it as a common impurity in small amounts. When it occurs, it is probably derived, as Vigouroux has suggested, from "alloys" of silicon with calcium, magnesium, and aluminium in the carbide. The metallic constituents of these substances would naturally be attacked by water, evolving hydrogen; and the hydrogen, in its nascent state, would probably unite with the liberated silicon to form hydrogen silicide. Many authorities, including Keppeler, have virtually denied that silicon compounds exist in crude acetylene, while the proportion 0.01 per cent. has been given by other writers as the maximum. Caro, however, has stated that the crude gas almost invariably contains silicon, sometimes in very small quantities, but often up to the limit of 0.8 per cent.; the failure of previous investigators to discover it being due to faulty analytical methods. Caro has seen one specimen of (bad) carbide which gave a spontaneously inflammable gas although it contained only traces of phosphine; its inflammability being caused by 2.1 per cent. of hydrogen silicide. Practically speaking, all the foregoing remarks made about phosphine apply equally to hydrogen silicide: it burns to solid silicon oxide (silica) at the burners, is insoluble in water, and is spontaneously inflammable when alone or only slightly diluted, but never occurs in good carbide in sufficient proportion to render the acetylene itself inflammable. According to Caro the silicon may be present both as hydrogen silicide and as silicon "compounds." A high temperature in the generator will favour the production of the latter; an apparatus in which the gas is washed well in lime-water will remove the bulk of the former. Fraenkel has found that magnesium silicide is not decomposed by water or an alkaline solution, but that dilute hydrochloric acid acts upon it and spontaneously inflammable hydrogen silicide results. If it may be assumed that the other silicides in commercial calcium carbide also behave in this manner it is plain that hydrogen silicide cannot occur in crude acetylene unless the gas is supposed to be hurried out of the generator before the alkaline water therein has had time to decompose any traces of the hydrogen silicide which is produced in the favouring conditions of high temperature sometimes prevailing. Mauricheau-Beaupré has failed to find silica in the products of combustion of acetylene from carbide of varying degrees of purity. He found, however, that a mixture of strong nitric and hydrochloric acids (_aqua regia_), if contaminated with traces of phosphoric acid, dissolved silica from the glass of laboratory vessels. Consequently, since phosphoric acid results from the phosphine in crude acetylene when the gas is passed through aqua regia, silica may be found on subsequently evaporating the latter. But this, silica, he found, was derived from the glass and not through the oxidation of silicon compounds in the acetylene. It is possible that some of the earlier observers of the occurrence of silicon compounds in crude acetylene may have been misled by the solution of silica from the glass vessels used in their investigations. The improbability of recognisable quantities of silicon compounds occurring in acetylene in any ordinary conditions of generation is demonstrated by a recent study by Fraenkel of the composition of the deposit produced on reflectors exposed to the products of combustion of a sample of acetylene which afforded a haze when burnt. The deposit contained 51.07 per cent. of phosphoric acid, but no silica. The gas itself contained from 0.0672 to 0.0837 per cent. by volume of phosphine.

PURIFYING MATERIALS.--When acetylene first began to be used as a domestic illuminant, most generator builders denied that there was any need for the removal of these carbide impurities from the gas, some going so far as to assert that their apparatus yielded so much purer an acetylene than other plant, where purification might be desirable, that an addition of a special purifier was wholly unnecessary. Later on the more responsible members of the trade took another view, but they attacked the problem of purification in a perfectly empirical way, either employing some purely mechanical scrubber filled with some moist or dry porous medium, or perhaps with coke or the like wetted with dilute acid, or they simply borrowed the processes adopted in the purification of coal-gas. At first sight it might appear that the more simple methods of treating coal-gas should be suitable for acetylene; since the former contains two of the impurities--sulphuretted hydrogen and ammonia--characteristic of crude acetylene. After removing the ammonia by washing with water, therefore, it was proposed to extract the sulphur by passing the acetylene through that variety of ferric hydroxide (hydrated oxide of iron) which is so serviceable in the case of coal-gas. The idea, however, was quite unsound: first, because it altogether ignores the phosphorus, which is the most objectionable impurity in acetylene, but is not present in coal- gas; secondly, because ferric hydroxide is used on gasworks to extract in a marketable form the sulphur which occurs as sulphuretted hydrogen, and true sulphuretted hydrogen need not exist in well-generated and well- washed acetylene to any appreciable extent; thirdly, because ferric hydroxide is not employed by gasmakers to remove sulphur compounds (this is done with lime), being quite incapable of extracting them, or the analogous sulphur compounds of crude acetylene.

About the same time three other processes based on somewhat better chemical knowledge were put forward. Pictet proposed leading the gas through a strong solution of calcium chloride and then through strong sulphuric acid, both maintained at a temperature of -20° to -40° C., finally washing the gas in a solution of some lead salt. Proof that such treatment would remove phosphorus to a sufficient degree is not altogether satisfactory; but apart from this the necessity of maintaining such low temperatures, far below that of the coldest winter's night, renders the idea wholly inadmissible for all domestic installations. Willgerodt suggested removing sulphuretted hydrogen by means of potassium hydroxide (caustic potash), then absorbing the phosphine in bromine water. For many reasons this process is only practicable in the laboratory. Bergé and Reychler proposed extracting both sulphuretted hydrogen and phosphine in an acid solution of mercuric chloride (corrosive sublimate). The poisonousness of this latter salt, apart from all other objections, rules such a method out.

BLEACHING POWDER.--The next idea, first patented by Smith of Aberdeen, but fully elaborated by Lunge and Cedercreutz, was to employ bleaching- powder [Footnote: Bleaching-powder is very usually called chloride of lime; but owing to the confusion which is constantly arising in the minds of persons imperfectly acquainted with chemistry between chloride of lime and chloride of calcium--two perfectly distinct bodies--the less ambiguous expression "bleaching-powder" will be adopted here.] either in the solid state or as a liquid extract. The essential constituent of bleaching-powder from the present aspect is calcium hypochlorite, which readily oxidises sulphuretted hydrogen, and more particularly phosphine, converting them into sulphuric and phosphoric acids, while the acetylene is practically unattacked. In simple purifying action the material proved satisfactory; but since high-grade commercial bleaching-powder contains some free chlorine, or some is set free from it in the purifier under the influence of the passing gas, the issuing acetylene was found to contain chlorine, free or combined; and this, burning eventually to hydrochloric acid, is hardly less harmful than the original sulphur compounds. Moreover, a mixture of acetylene, chlorine, and air is liable to catch fire of itself when exposed to bright sunlight; and therefore the use of a bleaching-powder purifier, or rather the recharging thereof, was not unattended by danger in the early days. To overcome these defects, the very natural process was adopted of diluting the bleaching-powder, such diluent also serving to increase the porosity of the material. A very unsuitable substance, however, was selected for the purpose, viz., sawdust, which is hygroscopic organic, and combustible. Owing to the exothermic chemical action between the impurities of the acetylene and the bleaching-powder, the purifying mass became heated; and thus not only were the phenomena found in a bad generator repeated in the purifying vessel, but in presence of air and light (as in emptying the purifier), the reaction proceeded so rapidly that the heat caused inflammation of the sawdust and the gas, at least on one occasion an actual fire taking place which created much alarm and did some little damage. For a time, naturally, bleaching-powder was regarded as too dangerous a material to be used for the purification of crude acetylene; but it was soon discovered that danger could be avoided by employing the substance in a proper way.

HERATOL, FRANKOLINE, ACAGINE AND PURATYLENE.--Setting aside as unworthy of attention certain compositions offered as acetylene purifying materials whose constitution has not been divulged or whose action has not been certified by respectable authority, there are now three principal chemical reagents in regular use. Those are chromic acid, cuprous chloride (sub- or proto-chloride of copper), and bleaching- powder. Chromic acid is employed in the form of a solution acidified with acetic or hydrochloric acid, which, in order to obtain the advantages (_see_ below) attendant upon the use of a solid purifying material, is absorbed in that highly porous and inert description of silica known as infusorial earth or "kieselguhr." This substance was first recommended by Ullmann, and is termed commercially "heratol" As sold it contains somewhere about 136 grammes of chromic acid per kilo. Cuprous chloride is used as a solution in strong hydrochloric acid mixed with ferric chloride, and similarly absorbed in kieselguhr. From the name of its proposer, this composition is called "frankoline." It will be shown in