It cannot altogether be granted that the value of a process for diluting acetylene with carbon dioxide has been established, except in so far as the mere presence of the diluent may somewhat diminish the tendency of the acetylene to polymerise as it passes through a hot burner (cf. Chapter VIII.). Certainly as a fuel-gas the mixture would be less efficient, and the extra amount of carbon dioxide produced by each flame is not wholly to be ignored. Moreover, since properly generated and purified acetylene can be consumed in proper burners without trouble, all reason for introducing carbon dioxide has disappeared.
MIXTURES OF ACETYLENE AND AIR.--A further proposal for diluting acetylene was the addition to it of air. Apart from questions of explosibility, this method has the advantage over that of adding carbon dioxide that the air, though not inflammable, is, in virtue of its contained oxygen, a supporter of combustion, and is required in a flame; whereas carbon dioxide is not only not a supporter of combustion, but is actually a product thereof, and correspondingly more objectionable. According to some experiments carried out by Dufour, neat acetylene burnt under certain conditions evolved between 1.0 and 1.8 candle-power per litre- hour; a mixture of 1 volume of acetylene with 1 volume of air evolved 1.4 candle-power; a mixture of 1 volume of acetylene with 1.2 volumes of air, 2.25 candle-power; and a mixture of 1 volume of acetylene with 1.3 volumes of air, 2.70 candle-power per litre-hour of acetylene in the several mixtures. Averaging the figures, and calculating into terms of acetylene (only) burnt, Dufour found neat acetylene to develop 1.29 candle-power per litre-hour, and acetylene diluted with air to develop 1.51 candle-power. When, however, allowance is made for the cost and trouble of preparing such mixtures the advantage of the process disappears; and moreover it is accompanied by too grave risks, unless conducted on a largo scale and under most highly skilled supervision, to be fit for general employment.
Fouché, however, has since found the duty, per cubic foot of neat acetylene consumed in a twin injector burner at the most advantageous rate of 3.2 inches, to be as follows for mixtures with air in the proportions stated:
Percentage of air 0 17 27 33.5
Candles per cubic feet 38.4 36.0 32.8 26.0
At lower pressures, the duty of the acetylene when diluted appears to be relatively somewhat higher. Figures which have been published in regard to a mixture of 30 volumes of air and 70 volumes of acetylene obtained by a particular system of producing such a mixture, known as the "Molet- Boistelle," indicate that the admixture of air causes a slight increase in the illuminating duty obtained from the acetylene in burners of various sizes. The type of burner and the pressure employed in these experiments were not, however, stated. This system has been used at certain stations on the "Midi" railway in France. Nevertheless even where the admixture of air to acetylene is legally permissible, the risk of obtaining a really dangerous product and the nebulous character of the advantages attainable should preclude its adoption.
In Great Britain the manufacture, importation, storage, and use of acetylene mixed with air or oxygen, in all proportions and at all pressures, with or without the presence of other substances, is prohibited by an Order in Council dated July 1900; to which prohibition the mixture of acetylene and air that takes place in a burner or contrivance in which the mixture is intended to be burnt, and the admixture of air with acetylene that may unavoidably occur in the first use or recharging of an apparatus (usually a water-to-carbide generator), properly designed and constructed with a view to the production of pure acetylene, are the solitary exceptions.
MIXED CARBIDES.--In fact the only processes for diluting acetylene which possess real utility are that of adding vaporised petroleum spirit or benzene to the gas, as was described in Chapter X. under the name of carburetted acetylene, and one other possible method of obtaining a diluted acetylene directly from the gas-generator, to which a few words will now be devoted. [Footnote: Mixtures of acetylene with relatively large proportions of other illuminating gases, such as are referred to on subsequent pages, are also, from one aspect, forms of diluted acetylene.] Calcium carbide is only one particular specimen of a large number of similar metallic compounds, which can be prepared in the electric furnace, or otherwise. Some of those carbides yield acetylene when treated with water, some are not attacked, some give liquid products, and some yield methane, or mixtures of methane and hydrogen. Among the latter is manganese carbide. If, then, a mixture of manganese carbide and calcium carbide is put into an ordinary acetylene generator, the gas evolved will be a mixture of acetylene with methane and hydrogen in proportions depending upon the composition of the carbide mixture. It is clear that a suitable mixture of the carbides might be made by preparing them separately and bulking the whole in the desired proportions; while since manganese carbide can be won in the electric furnace, it might be feasible to charge into such a furnace a mixture of lime, coke, and manganese oxide calculated to yield a simple mixture of the carbides or a kind of double carbide. Following the lines which have been adopted in writing the present book, it is not proposed to discuss the possibility of making mixed carbides; but it may be said in brief that Brame and Lewes have carried out several experiments in this direction, using charges of lime and coke containing (a) up to 20 per cent. of manganese oxide, and (b) more than 60 per cent. of manganese oxide. In neither case did they succeed in obtaining a material which gave a mixture of acetylene and methane when treated with water; in case (a) they found the gas to be practically pure acetylene, so that the carbide must have been calcium carbide only; in case (b) the gas was mainly methane and hydrogen, so that the carbide must have been essentially that of manganese alone. Mixed charges containing between 20 and 60 per cent. of manganese oxide remain to be studied; but whether they would give mixed carbides or no, it would be perfectly simple to mix ready-made carbides of calcium and manganese together, if any demand for a diluted acetylene should arise on a sufficiently large scale. It is, however, somewhat difficult to appreciate the benefits to be obtained from forms of diluted acetylene other than those to which reference is made later in this chapter.
There is, nevertheless, one modification of calcium carbide which, in a small but important sphere, finds a useful rôle. It has been pointed out that a carbide containing much calcium phosphide is usually objectionable, because the gas evolved from it requires extra purification, and because there is the (somewhat unlikely) possibility that the acetylene obtained from such material before purification may be spontaneously inflammable. If, now, to the usual furnace charge of lime and coke a sufficient quantity of calcium phosphate is purposely added, it is possible to win a mixture of calcium phosphide and carbide, or, as Bradley, Read, and Jacobs call it, a "carbophosphide of calcium," having the formula Ca_5C_6P_2, which yields a spontaneously inflammable mixture of acetylene, gaseous phosphine, and liquid phosphine when treated with water, and which, therefore, automatically gives a flame when brought into contact with the liquid. The value of this material will be described in Chapter XIII.
GAS-ENRICHING.--Other methods of diluting acetylene consist in adding a comparatively small proportion of it to some other gas, and may be considered rather as processes for enriching that other gas with acetylene. Provided the second gas is well chosen, such mixtures exhibit properties which render them peculiarly valuable for special purposes. They have, usually, a far lower upper limit of explosibility than that of neat acetylene, and they admit of safe compression to an extent greatly exceeding that of acetylene itself, while they do not lose illuminating power on compression. The second characteristic is most important, and depends on the phenomena of "partial pressure," which have been referred to in Chapter VI. When a single gas is stored at atmospheric pressure, it is insensibly withstanding on all sides and in all directions a pressure of roughly 15 lb. per square inch, which is the weight of the atmosphere at sea-level; and when a mixture of two gases, X and Y, in equal volumes is similarly stored it, regarded as an entity, is also supporting a pressure of 15 lb. per square inch. But in every 1 volume of that mixture there is only half a volume of X and Y each; and, ignoring the presence of its partner, each half-volume is evenly distributed throughout a space of 1 volume. But since the volume of a gas stands in inverse ratio to the pressure under which it is stored, the half-volume of X in the 1 volume of X + Y apparently stands at a pressure of half an atmosphere, for it has expanded till it fills, from a chemical and physical aspect, the space of 1 volume: suitable tests proving that it exhibits the properties which a gas stored at a pressure of half an atmosphere should do. Therefore, in the mixture under consideration, X and Y are both said to be at a "partial pressure" of half an atmosphere, which is manifestly 7.5 lb. per square inch. Clearly, when a gas is an entity (either an element or one single chemical compound) partial and total pressure are identical. Now, it has been shown that acetylene ceases to be a safe gas to handle when it is stored at a pressure of 2 atmospheres; but the limit of safety really occurs when the gas is stored at a partial pressure of 2 atmospheres. Neat acetylene, accordingly, cannot be compressed above the mark 30 lb. shown on a pressure gauge; but diluted acetylene (if the diluent is suitable) may be compressed in safety till the partial pressure of the acetylene itself reaches 2 atmospheres. For instance, a mixture of equal volumes of X and Y (X being acetylene) contains X at a partial pressure of half the total pressure, and may therefore be compressed to (2 / 1/2 =) 4 atmospheres before X reaches the partial pressure of 2 atmospheres; and therewith the mixture is brought just to the limit of safety, any effect of Y one way or the other being neglected. Similarly, a mixture of 1 volume of acetylene with 4 volumes of Y may be safely compressed to a pressure of (2 / 1/5 =) 10 atmospheres, or, broadly, a mixture in which the percentage of acetylene is x may be safely compressed to a pressure not exceeding (2 / x/100) atmospheres. This fact permits acetylene after proper dilution to be compressed in the same fashion as is allowable in the case of the dissolved and absorbed gas described above.
If the latent illuminating power of acetylene is not to be wasted, the diluent must not be selected without thought. Acetylene burns with a very hot flame, the luminosity of which is seriously decreased if the temperature is lowered. As mentioned in Chapter VIII., this may be done by allowing too much air to enter the flame; but it may also be effected to a certain extent by mixing with the acetylene before combustion some combustible gas or vapour which burns at a lower temperature than acetylene itself. Manifestly, therefore, the ideal diluent for acetylene is a substance which possesses as high a flame temperature as acetylene and a certain degree of intrinsic illuminating power, while the lower the flame temperature of the diluent and the less its intrinsic illuminating power, the less efficiently will the acetylene act as an enriching material. According to Love, Hempel, Wedding, and others, if acetylene is mixed with coal-gas in amounts up to 8 per cent. or thereabouts, the illuminating power of the mixture increases about 1 candle for every 1 per cent. of acetylene present: a fact which is usually expressed by saying that with coal-gas the enrichment value of acetylene is 1 candle per 1 per cent. Above 8 per cent., the enrichment value of acetylene rises, Love having found an increase in illuminating power, for each 1 per cent. of acetylene in the mixture, of 1.42 candles with 11.28 per cent. of acetylene; and of 1.54 candles with 17.62 per cent. of acetylene. Theoretically, if the illuminating power of acetylene is taken at 240 candles, its enrichment value should be (240 / 100 =) 2.4 candles per 1 per cent.; and since, in the case of coal-gas, its actual enrichment value falls seriously below this figure, it is clear that coal-gas is not an economical diluent for it. Moreover, coal-gas can be enriched by other methods much more cheaply than with acetylene. Simple ("blue") water-gas, according to Love, requires more than 10 per cent. of acetylene to be added to it before a luminous flame is produced; while a mixture of 20.3 per cent. of acetylene and 79.7 per cent. of water-gas had an illuminating power of 15.47 candles. Every addition to the proportion of acetylene when it amounted to 20 per cent. and upwards of the mixture had a very appreciable effect on the illuminating power of the latter. Thus with 27.84 per cent. of acetylene, the illuminating power of the mixture was 40.87 candles; with 38.00 per cent. of acetylene it was 73.96 candles. Acetylene would not be an economical agent to employ in order to render water-gas an illuminating gas of about the quality of coal-gas, but the economy of enrichment of water-gas by acetylene increases rapidly with the degree of enrichment demanded of it. Carburetted water-gas which, after compression under 16 atmospheres pressure, had an illuminating power of about 17.5 candles, was enriched by additions of acetylene. 4.5 per cent. of acetylene in the mixture gave an illuminating power of 22.69 candles; 8.4 per cent., 29.54 candles; 11.21 per cent., 35.05 candles; 15.06 per cent., 42.19 candles; and 21.44 per cent., 52.61 candles. It is therefore evident that the effect of additions of acetylene on the illuminating power of carburetted water-gas is of the same order as its effect on coal-gas. The enrichment value of the acetylene increases with its proportion in the mixture; but only when the proportion becomes quite considerable, and, therefore, the gas of high illuminating power, does enrichment by acetylene become economical. Methane (marsh-gas), owing to its comparatively high flame temperature, and to the fact that it has an intrinsic, if small, illuminating power, is a better diluent of acetylene than carbon monoxide or hydrogen, in that it preserves to a greater extent the illuminative value of the acetylene.