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Book, page 311 / 445 a cylinder of known capacity containing (1) gaseous acetylene compressed to 10 atmospheres, or (2) containing the solution at the same pressure, if an explosion were to occur, in case (1) the whole contents would participate in the decomposition, whereas in case (2), as mentioned already, only the small quantity of gaseous acetylene above the solution would be dissociated. It is manifest that of the three varieties of compressed acetylene now under consideration, the solution in acetone is the only one fit for general employment; but it exhibits the grave defects (_a_) that the pressure under which it is prepared must be so small that the pressure in the cylinders can never approach 20 atmospheres in the hottest weather or in the hottest situation to which they may be exposed, (_b_) that the gas does not escape smoothly enough to be convenient from large vessels unless those vessels are agitated, and (_c_) that the cylinders must always be used in a certain position with the valve at the top, lest part of the liquid should run out into the pipes. For these reasons the simple solution of acetylene in acetone has not become of industrial importance; but the processes of absorbing either the gas, or better still its solution in acetone, in porous matter have already achieved considerable success. Both methods have proved perfectly safe and trustworthy; but the combination of the acetone process with the porous matter makes the cylinders smaller per unit volume of acetylene they contain. Several varieties of solid matter appear to work satisfactorily, the only essential feature in their composition being that they shall possess a proper amount of porosity and be perfectly free from action upon the acetylene or the acetone (if present). Lime does attack acetone in time, and therefore it is not a suitable ingredient of the solid substance whenever acetylene is to be compressed in conjunction with the solvent; so that at present either a light brick earth which has a specific gravity of 0.5 is employed, or a mixture of charcoal with certain inorganic salts which has a density of 0.3, and can be introduced through a small aperture into the cylinder in a semi-fluid condition. Both materials possess a porosity of 80 per cent., that is to say, when a cylinder is apparently filled quite full, only 20 per cent, of the space is really occupied by the solid body, the remaining 80 per cent, being available for holding the liquid or the compressed gas. If all comparisons as to degree of explosibility and effects of explosion are omitted, an analogy may be drawn between liquefied acetylene or its compressed solution in acetone and nitroglycerin, while the gas or
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