The Romance of Modern Chemistry

VALUABLE SUBSTANCES amount of sulphur in coal is very small, only 1 to 2 per cent., equivalent to an average of about 35 lbs. per ton. Only about one-third of this amount reaches the gas purifiers as sulphuretted hydrogen, and yet so large is the quantity of coal which is treated in the gasworks of Great Britain that in the aggregate the recovered sulphur amounts to thousands of tons per annum. In some gasworks this recovered sulphur is used in the pre- paration of sulphuric acid, which in its turn is employed in fixing ammonia and forming ammonium sulphate. Besides the sulphuretted hydrogen, there is another im- purity in crude coal gas which has to be removed, and which at the same time is made to contribute to the cost of production. This is the poisonous compound of hydrogen, carbon, and nitrogen known as hydrocyanic acid. By suit- able chemical methods it is extracted from the crude gas and converted into potassium ferrocyanide, a substance which is perhaps better known as yellow prussiate of potash. From this product it is easy to prepare either Prussian blue, for the manufacture of printing-ink, or potassium cyanide. This latter compound is extensively employed in gold extraction and in electro-plating. Thus it is that, the objectionable impurity present in the crude coal gas to the extent of less than 1 part in 1000 is converted into useful products. Another valuable by-product of gas manufacture is sulphate of ammonia, obtained from the ammoniacal liquor. The amount of nitrogen in coal is 1 to 2 per cent., but only a part of this is obtained in the form of ammonia. Roughly speaking, we may say that for every ton of coal put into the gas retorts 25 to 30 lbs. of ammonium sulphate are recovered from the liquor. In Great Britain the annual output of ammonia from 281