The Romance of Modern Chemistry

ACIDS AND ALKALIS composition chalk is as good as marble; it is only the poor brother in the family. By dropping a little acid on marble, limestone, or chalk, we can satisfy ourselves that they give an effer- vescence of carbon dioxide. As a matter of fact, carbonate of lime is just a neutral salt formed by the union of the alkaline lime and the acidic carbon dioxide. This salt, however, differs from other common salts, because when it is heated it gives off its acidic component, the carbon dioxide, while the lime remains behind. This chemical change is carried out on the large scale when limestone is strongly heated in lime-kilns ; the process is termed “lime-burning.” The reader must not suppose that lime burns in the sense that a piece of coal does ; the term refers only to the strong heating to which the limestone is subjected. The product of lime- burning is called “quick” lime, but for a great many purposes, such as the preparation of building mortar, this must be converted into “ slaked ” lime by the action of water. The slaking of lime is a beautiful example of the changes brought about by a simple chemical action; for if a little water is sprinkled on one of the hard lumps of quicklime obtained from a lime-kiln, some remarkable effects are observed. For a minute or two nothing is apparent, but presently steam rises from the lime, and if the observer touches the mass with his hand he will realise that much heat is being generated. The chemical forces at work are such that the hard lump of lime splits up and crumbles down to a soft powder, which is absolutely dry in spite of the added water. The secret of this striking phenomenon is that a new chemical compound has been formed; the water 92