Pioneer Labourers 23 



whether in air or water, it takes up double the quantity 

 it ■ had before, and is converted into a double, or 

 bi-carbonate, which is easily dissolved and washed 

 away. Marble slabs which are exposed to the weather 

 in such a climate as that of Edinburgh are com- 

 pletely destroyed in this way in less than a hundred 

 years. 



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 

 impression 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. 



Then, again, there are the granites, composed of three 

 minerals, quartz, mica, and felspar ; the first is silica, 

 like flint, and equally hard, and if the rock consist 

 mainly of quartz, as some granites do, it is of course 

 extremely durable, for, besides resisting the action of 

 the gases, it also admits very little water. If, on the 

 other hand, the felspar predominates, the rock is softer, 

 and water finds an easier entrance ; but, besides this, 

 the felspar — which is clay without the water, and in a 

 crystallized form — contains either potash or soda, and 

 often lime as well, and any one of these is easily 

 attacked and made soluble by union with carbon 

 dioxide. Once converted into carbonates, or bicar- 

 bonates, as the case may be, these are easily dissolved 



