The Romance of Modern Chemistry

NATURAL WATERS dissolving from these the carbonate of lime which they contain, as well as more soluble substances. When this water comes to the surface again in a well or spring it is found to be hard. But the simple process of boiling may render it comparatively soft, since in this operation the carbon dioxide with which the water is charged is boiled out, and the carbonate of lime, being no longer soluble, is deposited. That the “ furring ” in the aforementioned kitchen kettle is caused by a carbonate is shown by the effervescence which occurs on the addition of a little hydrochloric acid—“spirits of salt,” as it is commonly called. Apart from actual boiling, mere exposure of such a hard water to the air will deprive it of its carbon dioxide by evaporation, and in so far as the carbon dioxide is removed, in so far is a deposit of carbonate of lime pro- duced. This is the way in which these curious excrescences known as stalactites and stalagmites are formed. Water which has percolated through some depth of soil and rock, and become hard in the process, may arrive at the roof of some underground cavern. The drops which there form are subject to evaporation, and part of the carbon dioxide with which they are charged is removed. This leads to the deposition of chalk, and a tiny contribu- tion is made to the growth of the stalactite. A further quantity of carbon dioxide evaporates as the drops fall on the floor of the cavern, a further deposit of chalk is formed, and a tiny contribution is made to the growth of the stalagmite column. Another very interesting natural phenomenon closely related to the formation of stalactites and stalagmites is the action of what are known as “petrifying springs.11 If a wicker basket, for example, is exposed to the action 100