THE FORMATION OF SOIL 15 



knows that when freshly burned lime is brought 

 into contact with water it absorbs a cer- 

 tain amount, and in the process swells up, and 

 finally crumbles down into a dusty, powdery 

 mass. The water, as such, has disappeared, 

 that is to say, we might add a gallon of water 

 to a heap of burned lime, and although the 

 water was manifestly absorbed by the lime 

 we should be left with lime as dry as the 

 substance with which we started. A small 

 proportion of the water may have escaped 

 into the air in the form of vapour, but most 

 has been taken up by the lime to form a new 

 chemical compound. We start with what 

 is known chemically as oxide of lime, or 

 anhydrous calcium oxide, a substance con- 

 taining only calcium and oxygen, and we end 

 with a substance which the chemist calls 

 hydrated lime, which is still calcium oxide 

 but now united chemically with a certain 

 amount of water. During this change two 

 things are to be noted, the one, that the 

 lime has undergone considerable increase in 

 volume, and the other, that it has crumbled 

 down in the process. That the slaking of 

 lime is accompanied by increase in volume is 

 evident from various familiar examples. For 

 instance, if we fill a pail with lime shells and 

 pour water upon it, we find that when the lime 



