Page:Popular Science Monthly Volume 8.djvu/184

172 rocks have burst up through the chalk which forms a long succession of fine cliffs on the Antrim coast, this chalk has been so altered in texture as almost to resemble marble, all trace of its original nature being obliterated. Knowing, as we do, how much more extensive and potent must have been the agencies which were at work in metamorphosing the Palæozoic rocks, we have no difficulty in accounting for the fact that vast beds of our Carboniferous Limestone now show little or no trace of the organic texture which we believe them to have originally possessed. That you may better understand the nature of this metamorphosis, I shall now show you some of the chemical properties of carbonate of lime, which is the material of all calcareous rocks alike, whether showing the perfect crystalline form of calc-spar, the close minutely-crystalline arrangement of marble, the sub-crystalline texture of limestone, the "roe-stone" aggregation of oolite, or the fine powdery condition of chalk.

If we treat a piece of any one of these substances with dilute nitric or muriatic acid, an effervescence is immediately produced by the liberation of carbonic acid, while the lime is dissolved; and this gives a ready way of distinguishing a calcareous from any other rock. In "burning" limestone, on the other hand, the union of the carbonic acid and the lime is dissolved by heat; the carbonic acid is driven off, and the lime remains behind in the condition of "quicklime." This is very greedy (so to speak) of carbonic acid, and is always trying to get it back again. We can dissolve a small quantity of quicklime in water; and if we leave this with a large surface exposed to the air, it gradually recombines with the carbonic acid which it draws from the air; and, as the carbonate is nearly insoluble in water, it falls as a fine white powder, making the water turbid. We may do the same in a moment, by blowing through a pipe into a glass of lime-water, our breath containing a considerable quantity of carbonic acid; and we may then clear the liquid again, by a drop or two of nitric or muriatic acid. But, though insoluble in pure water, carbonate of lime is slightly soluble in water which is already charged with carbonic acid; and, as all rain-water brings down carbonic acid from the air, it is capable of taking up carbonate of lime from the soils and rocks through which it filters; and it thus happens that all springs and rivers, that rise in localities in which there is any kind of calcareous rock, become more or less charged with carbonate of lime kept in solution by an excess of carbonic acid. This is what gives the peculiar character lo water which is known as "hardness;" and a water hard enough to curdle soap may be converted into a very "soft" water (as the late Prof Clark, of Aberdeen, showed) by the simple addition of lime-water, which, by combining with the excess of carbonic acid, causes the precipitation of all the lime in solution in the form of insoluble carbonate, which gradually settles to the bottom, leaving the water clear. It is this solvent power of water charged with carbonic acid,