ao The Great World's Farm 



though it may be only a very minute quantity, of every- 

 thing through, or over, which it passes? Its dissolving 

 powers are greatly increased, too, by the addition of car- 

 bon dioxide — the gas we are now speaking of — which is 

 being constantly produced both in earth and air, by the 

 decay of vegetable matter in the one, and by the lungs of 

 animals, fires, and furnaces in the other. 



The rain as it descends from the clouds washes down 

 with it some of this gas, and if it comes in contact with 

 such a rock as limestone, soon makes an impression upon 

 it. Chalk, limestone, and marble are all composed of 

 carbonate of lime, softer or harder, the lime being already 

 united with a certain quantity of carbon dioxide. But in 

 this condition it dissolves so very slowly as to be called 

 insoluble in pure water. When it comes in contact with 

 the gas, however, whether in air or water, it takes up 

 double the quantity it had before, and is converted into a 

 double, or bicarbonate, which is easily dissolved and 

 washed away. 



Even rocks which consist only in part of carbonate of 

 lime are open to the attacks of carbon dioxide. For 

 instance, there are the sandstones. The grains are hard 

 enough, being composed of silica, and if they are cemented 

 together with silica, too, the stone is one of the most 

 durable that can be found, neither water nor gases, 

 together or separately, being able to make much impres- 

 sion upon it. But if the grains are cemented together by 

 iron oxide, or by carbonate of lime, it is quite another 

 matter. Oxygen or carbon dioxide may get to work on 

 the cement, and as that is removed the grains fall apart 

 and become sand. 



But nature's laborers proceed upon the principle that 



