16 AGRICULTURE 



is slaked the pail is no longer able to contain 

 it. Sometimes a farmer has an illustration 

 thrust upon his notice when he purchases 

 lime for agricultural purposes. The railway 

 waggon is filled at the lime-kiln, and is prob- 

 ably dispatched unprotected by a water- 

 proof covering. During the journey, occupy- 

 ing a day or more, heavy rain may fall, and the 

 lime will be more or less slaked, so that 

 when the waggon arrives at its destination it 

 may be found that a considerable amount of 

 lime has been spilt on the journey through 

 swelling up and falling over the side of the 

 waggon. Many other substances besides lime 

 are capable of combining with water in a 

 similar manner, and some of them are present 

 in rocks. One such substance is calcium 

 sulphate, which like lime is capable of chemi- 

 cally combining with water, and in the process 

 is converted into the material called gypsum, 

 undergoing in the change considerable in- 

 crease in volume. Rocks also in many cases 

 contain what are called anhydrous silicates, 

 namely, substances holding silica in com- 

 bination with some other material, but 

 possessing no water. These silicates can, 

 like burned lime, absorb water and increase 

 in volume; and it is evident that if a rock 

 contains any such substance it will be rapidly 



