The Romance of Modern Chemistry

MORE ABOUT FUEL containing slaked lime and iron oxide. The reader is already familiar with the first of these as being an alkaline substance, in virtue of which it readily absorbs anything of an acid nature which passes through the purifiers. Now both carbon dioxide, a little of which is sure to be present in the gas, and sulphuretted hydrogen are substances of an acid nature, and one would therefore expect them to be fixed by the lime. Sulphuretted hydrogen, however, is not absorbed by lime when it is mixed with carbon dioxide, so in order to insure the complete removal of the former the coal gas must also be passed over iron oxide. This substance, generally in the form of Irish bog ore, is at first very active in holding back the sulphuretted hydrogen, but as it absorbs more and more it gets exhausted, being gradually converted into sulphide of iron. A course of fresh air, however, is found to have a beneficial effect on its activity, hence the exhausted or “spent’1 oxide of iron is taken out of the purifiers and spread on the ground for a time. During this “ rest cure ” the sulphide of iron enters into a chemical reaction with the oxygen of the air, with the result that the element sulphur is liberated and iron oxide is re-generated. The material is then again capable of actively absorbing sulphuretted hydrogen, and is therefore returned to the purifiers until exhausted a second time. This process of “ revivifying11 the iron oxide may be repeated a good many times until the material has picked up about half its own weight of sulphur. It will then have lost its effectiveness as a purifier of coal gas, and is accordingly sold to the sulphuric acid manufacturer. As the proportion of sulphur in the original coal is not more than one or two per cent., this is a very instructive instance of the 146