The Romance of Modern Chemistry

MORE ABOUT FUEL wood, but the character of the products differs in some important particulars. The gas which is given off on heating coal is of much more use for illuminating pur- poses, and is, in fact, after purification, nothing else than the common coal gas used throughout our towns. This consists largely of hydrogen and marsh gas, together with some carbon monoxide and small quantities of heavy hydrocarbons which are responsible for the illuminating power. In the crude gas which comes from the retorts there are several undesirable constituents which must be removed before the gas can be supplied to the public. In addition to carbon, hydrogen, and oxygen, coal contains small quantities of the elements nitrogen and sulphur, and these appear to some extent in the coal gas in the form of ammonia—a compound of nitrogen and hydrogen—and sulphuretted hydrogen—a com- pound of sulphur and hydrogen. The ammonia collects mostly in the watery liquid, which accordingly becomes alkaline, in contrast with the acid watery liquid obtained in the destructive distillation of wood. The last traces of ammonia are removed from coal gas by “ scrubbers ”■— towers packed with coke or brushwood over which a constant stream of water is trickling. The current of gas goes in the opposite direction, and as ammonia is very soluble in water, it is all removed before the gas issues at the top. The sulphuretted hydrogen resulting from the above process, if it were allowed to remain in the coal gas, would on burning produce sulphur dioxide, and this, in anything more than a small quantity, would be a very objectionable addition to the atmosphere. The gas is accordingly passed through a series of purifiers, 145 k