172 THE POPULAR SCIENCE MONTHLY, 



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 text- 

 ure as almost to resemble marble, all trace of its original nature be- 

 ing obliterated. Knowing, as we do, how much more extensive and 

 potent must have been the agencies which were at work in metamor- 

 phosing the Palaeozoic rocks, we have no difficulty in accounting for 

 the fact that vast beds of our Carboniferous Limestone now show lit- 

 tle 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 prop- 

 erties 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-crystal- 

 line 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 lib- 

 eration 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 

 iji 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 mu- 

 riatic 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 ca- 

 pable 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 thei-e 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 convert*! 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. 



