1 6 The Great World's Farm 



of the two gases chlorine and fluorine. All these are 

 contained in the rocks; but as long as it is stored in the 

 rocks it might as well be locked up so far as most of them 

 are concerned, for they cannot get at it or make use of 

 it. The stone must be converted into soil before they 

 can turn it to account; and how is this to be accom- 

 plished.? 



If a man had to make his soil from the rocks before he 

 could grow his crops, he would have to begin with crow- 

 bars and pickaxes, if he did not first resort to blasting 

 with gunpowder or dynamite, and even then his progress 

 would be slow and laborious. 



Nature usually works in a much more quiet and unob- 

 trusive fashion, but there are times when she, too, has 

 recourse to blasting as a preliminary measure. She 

 mines the rocks and shatters them by means of the earth- 

 quake, compared with which the power even of dynamite 

 is insignificant; but it is the noiseless and often invisible 

 workers who accomplish most, for they are at work, some 

 or other of them, incessantly during every moment of 

 every hour, day and night, summer and winter, through- 

 out the whole year. 



Usually the first of the silent laborers to begin work 

 upon the rocks are also the invisible ones — the gases of 

 air and water, which wear away the very hardest rocks by 

 degrees. 



The two gases which do the chief part of the chemical 

 work are oxygen and carbon dioxide, formerly called car- 

 bonic acid. Rocks containing much iron are especially 

 open to the attacks of the one, and those containing lime, 

 potash, soda, to the attacks of the other. We are all 

 familiar with the fact that iron and steel become covered 



