The Romance of Modern Chemistry

MORE ABOUT FUEL value of gathering up the fragments; even the very impurities in the coal gas are made to contribute to the cost of its production. This remark covers also the ammonia which is found in crude coal gas. As has been stated already, the destructive distillation of coal converts some of the nitrogen which it contains into ammonia, and this has turned out to be a very valuable by-product of coal- gas manufacture. From the watery liquid, in which it mostly collects, the ammonia is driven out by a current of steam ; it is then passed into sulphuric acid, forming sulphate of ammonia, and the crystals of this substance are fished out from time to time. On the average, a ton of coal yields 20 lbs. of ammonium sulphate; the latter substance fetches a good price as a manure__ about .£10 per ton—and it makes, therefore, a very substantial contribution to the expense of producing the coal gas. Other by-products obtained in the manufacture of coal gas are tar and coke. From coal tar so many interesting and useful substances are prepared that a special chapter must be devoted to their consideration, where we shall see that even from this uninviting and unpromising material many beautiful products may be extracted. Coke is the residue in the retorts after all gas, tar, and ammonia have been driven off. The mineral matter or ash in the original coal is not volatile, so that it remains in the coke, which contains about ninety per cent, of carbon and small quantities of hydrogen, oxygen, nitrogen, and sulphur. Gas coke is used as a fuel, although the reader will understand that since the volatile combustible gases have been removed, it is rather