Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/57

 ACETYLENE 37 It lias also been observed by Cailletet and later by Villard gas, acetylene forms an explosive mixture. Clowes has that when allowed to stand in the presence of water at a shown that it has a wider range of explosive low temperature a solid hydrate is formed. Acetylene is proportions when mixed with air than any of BxP'osive readily decomposed by heat, polymerizing under the other combustible gases, the limiting per- m,Xlures‘ ^a^tart ^n^uence f°nri an enormous number of centages being as follows :— Ing. point in ?rganic compounds; indeed the gas, which can Acetylene 3 to 82 synthesis, itself be directly prepared from its constituents, Hydrogen 5 to 72 carbon and hydrogen, under the influence of the Carbon monoxide. . . . . 13 to 75 Ethylene 4 to 22 electric arc, can be made the starting-point for the construcMethane 5 to 13 tion of an enormous number of different organic compounds of a complex character. In contact with nascent hydrogen The methods which can be and have been employed it builds up ethylene; ethylene acted upon by sulphuric from time to time for the formation of acetylene ■acid yields sulphovinic acid; this can again be decom- in small quantities are exceedingly numerous. Methods of posed in the presence of water to yield alcohol, and it Before the commercial production of calcium t[on“C‘ has also been proposed to manufacture sugar from this carbide made it one of the most easily obtainremarkable body. Picric acid can also be obtained from it able gases, the processes which were most largely adopted by first treating acetylene with sulphuric acid, converting for its preparation in laboratories were :—first, the decomthe product into phenol by solution in potash, and then position of ethylene bromide by dropping it slowly into a treating the phenol with fuming nitric acid. boiling solution of alcoholic potash, and purifying the Acetylene is one of those bodies the formation of which evolved gas from the volatile bromethylene by washing is attended with the disappearance of heat, and it through a second flask containing a boiling solution of Detonait is for this reason termed an “ endothermic ” alcoholic potash, or by passing it over moderately heated tion. compound, in contra-distinction to those bodies soda lime; and, second, the more ordinarily adopted process •which evolve heat in their formation, and which are called of passing the products of incomplete combustion from a “exothermic.” Such endothermic bodies are nearly always Bunsen burner, the flame of which had struck back, found to show considerable violence in their decomposition through an ammoniacal solution of cuprous chloride, as the heat of formation stored up within them is then when the red acetylide of copper was produced. This on liberated as sensible heat, and it is undoubtedly this being washed and decomposed with hydrochloric acid property of acetylene gas which leads to its easy detonation yielded a stream of acetylene gas. This second method of by either heat or a shock from an explosion of fulminating production has the great drawback that, unless proper mercury when in contact with it under pressure. The precautions are taken to purify the gas obtained from the observation that acetylene can be resolved into its con- copper acetylide, it is always contaminated with certain stituents by detonation is due to Berthelot, who started chlorine derivatives of acetylene. Edmund Davy first made an explosive wave in it by firing a charge of OT gram of acetylene in 1836 from a compound produced during the mercury fulminate. It has since been shown, however, manufacture of potassium from potassium tartrate and that unless the gas is at a pressure of more than two charcoal, which under certain conditions yielded a black atmospheres this wrave soon dies out, and the decomposi- compound decomposed by water with considerable violence tion is only propagated a few inches from the detonator. and the evolution of acetylene. This compound was Heated in contact with air to a temperature of 480° C. afterwards fully investigated by Berzelius, who showed it acetylene ignites and burns with a flame, the Ignition appearance of which varies with the way in to be carbide of potassium. He also made the corresponding sodium compound and showed that it evolved the point. which it is brought in contact with the air. same gas, whilst in 1862 Wohler first made carbide of With the gas in excess a heavy lurid flame emitting dense calcium, and found that water decomposed it into lime volumes of smoke results, whilst if it be driven out in a and acetylene. It was not, however, until 1892 that the sufficiently thin sheet, it burns with a flame of intense almost simultaneous discovery was made by Willson in brilliancy and almost perfect whiteness, by the light of America and Moissan in France that if lime and carbon which colours can be judged as well as they can by daylight. be fused together at the temperature of the electric Having its ignition point below that of ordinary gas, furnace, the lime is reduced to calcium, which unites with it can be ignited by any red-hot carbonaceous matter, the excess of carbon present to form calcium carbide. such as the brightly glowing end of a cigar. For its The cheap production of this material and the easy complete combustion a volume of acetylene Combus- needs approximately twelve volumes of air, liberation by its aid of acetylene at once gave the gas a position of commercial importance. In the manufacture tion. forming as products of combustion carbon of calcium carbide in the electric furnace, lime dioxide and water vapour. When, however, the air is and hard metallurgical coke of the highest Manufacpresent in much smaller ratio the combustion is incom- possible degree of purity are employed. A ture°f plete, and carbon, carbon monoxide, carbon dioxide, good working mixture of these materials may carbide’. hydrogen, and water vapour are produced. This is well be taken as being 100 parts by weight of lime shown by taking a cylinder one-half full of acetylene and with 68 parts by weight of carbonaceous material. About one-half of air; on applying a light to the mixture a 1'8 Ih of this is used up for each pound of carbide lurid flame runs down the cylinder and a cloud of soot is produced. The two principal processes utilized in making thrown up, the cylinder also being thickly coated with it, calcium carbide by electrical power are the ingot process and often containing a ball of carbon. If now, after a and the tapping process. In the former, the coke and few moments’ interval to allow some air to diffuse into lime are finely ground and carefully mixed in the right the cylinder, a taper again be applied, an explosion takes proportions to suit the chemical actions involved. The place, due to a mixture of carbon monoxide and air. It arc is struck in a crucible into which the powdered is probable that when a flame is smoking badly, distinct mixture is allowed to flow, partially filling it. An ingot traces of carbon monoxide are being produced, but when gradually builds up from the bottom of the crucible, the an acetylene flame burns properly the products are as carbon electrode being raised from time to time autoharmless as those of coal gas, and, light for light, less in matically or by hand to suit the diminution of resistance amount. Mixed with air, like every other combustible due to the shortening of the arc by the rising ingot. The