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Book, page 61 / 445 as a means for judging whether overheating has occurred in a generator.] and the odour of tar was as noticeable as that of crude acetylene. There is another effect of heat upon acetylene, more calculated to be dangerous than any of those just mentioned, which must not be lost sight of. Being an endothermic substance, acetylene is prone to decompose into its elements-- (7) C_2H_2 -> C_2 + H_2 whenever it has the opportunity; and the opportunity arrives if the temperature of the gas risen to 780 deg. C., or if the pressure under which the gas is stored exceeds two atmospheres absolute (roughly 30 lb. per square inch). It decomposes, be it carefully understood, in the complete absence of air, directly the smallest spark of red-hot material or of electricity, or directly a gentle shock, such as that of a fall or blow on the vessel holding it, is applied to any volume of acetylene existing at a temperature exceeding 780 deg. or at a gross pressure of 30 lb. per square inch; and however large that volume may be, unless it is contained in tubes of very small diameter, as will appear hereafter, the decomposition or dissociation into its elements will extend throughout the whole of the gas. Equation (7) states that 2 volumes of acetylene yield 2 volumes of hydrogen and a quantity of carbon which would measure 2 volumes were it obtained in the state of gas, but which, being a solid, occupies a space that may be neglected. Apparently, therefore, the dissociation of acetylene involves no alteration in volume, and should not exhibit explosive effects. This is erroneous, because 2 volumes of acetylene only yield exactly 2 volumes of hydrogen when both gases are measured at the same temperature, and all gases increase in volume as their temperature rises. As acetylene is endothermic and evolves much heat on decomposition, and as that heat must primarily be communicated to the hydrogen, it follows that the latter must be much hotter than the original acetylene; the hydrogen accordingly strives to fill a much larger space than that occupied by the undecomposed gas, and if that gas is contained in a closed vessel, considerable internal pressure will be set up, which may or may not cause the vessel to burst. What has been said in the preceding paragraph about the temperature at which acetylene decomposes is only true when the gas is free from any notable quantity of air. In presence of air, acetylene inflames at a much
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