48 AGRICULTURE 



materials constitutes the bulk of the mass. 

 In some soils the main ingredient is sand or 

 quartz, in others it is clay, in some it is 

 carbonate of lime, while in soils of the fourth 

 class the predominating material is humus. 



Soils whose main constituent is sand or 

 quartz grains has definite and easily recog- 

 nizable properties. In the first place it 

 undergoes little if any further change under 

 the processes of weather, for the reason that the 

 siliceous material which forms the bulk of its 

 mass is not decomposable by the action of 

 natural agents. It is said to be light, in 

 the sense that the labour of working it is 

 easy. It is generally dry, because its mechan- 

 ical condition permits of the easy drainage 

 of water. It is warm, partly because it has 

 a low specific heat, and its temperature 

 therefore rises quickly under the influence 

 of sun or warm air, and partly because it 

 holds little water, and therefore this substance 

 of high specific heat is not present to keep 

 it cool. Not only does water, in conse- 

 quence of its high specific heat, prevent the 

 temperature of soil from rising, but it also 

 tends to make soil cool, owing to the fact 

 that where one has water one has always 

 more or less evaporation, and the conversion 

 of water from the liquid into the gaseous 



