Coal; its history and uses . ooks and corrode themetal fittings of rooms. The illuminating power of coal-gas entirely dependsupon the hydrocarbons other than marsh-gas which it con-tains; these are classed together as olefines in the foregoinganalyses. One of the most important of these hydrocar-bons is known as ethylene or ethene (C, H4); it burns witha highly bituminous flame, the light-giving power of whichis due to the ease with which a portion of its carbon isseparated in the free state. Other members of the sameseries of hydrocarbons, notably C3 Hg and G^ H,, are pro-bably also present,
Coal; its history and uses . ooks and corrode themetal fittings of rooms. The illuminating power of coal-gas entirely dependsupon the hydrocarbons other than marsh-gas which it con-tains; these are classed together as olefines in the foregoinganalyses. One of the most important of these hydrocar-bons is known as ethylene or ethene (C, H4); it burns witha highly bituminous flame, the light-giving power of whichis due to the ease with which a portion of its carbon isseparated in the free state. Other members of the sameseries of hydrocarbons, notably C3 Hg and G^ H,, are pro-bably also present, together with acetelene or ethine C^H^,and minute quantities ofhemene Cg Hg, all of which con-tribute to the illuminating value of the gas. The various inflammable constituents of coal-gas burn 208 COAL. at very different rates. All combustible mixtures of gasesmust possess a certain temperature depending on thenature of tbe gases, before combination takes place, andthe combination tbus set up requires a certain definite in-. FiG. 47.—Eefrigerators or Condensers. From Eoscoe andSohorlemmers Chemistry. terval of time to accomplish, also varying with the natureof the gases. A comparatively low temperature will ini-tiate the union of a mixture of oxygen and hydrogen of themaximum explosive power, that is, of a mixture which isentirely converted to water on burning, and the flame, thevisible sign of combination, travels through this mixture CHAP. VI. THE CHEMISTEY OF COAl. 209 at the rate of about thirty-four metres or thirty-sevenyards per second. A mixture of carbon monoxide andoxygen requires a still higher temperature to set upchemical combination under ordinary conditions, and the
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