THE SOIL 48 



powdered material, with its enclosed air spaces, compared with 

 that of an equal volume of water. 



Specific heat, eqvxil weights^ is the ratio of the amount of 

 heat necessary to raise the temperature of a certain quantity 

 of the substance through, say, 10°, compared with that required 

 to raise an equal weight of water through the same range of 

 temperature. (See also chap, iv.) 



Specific heat, equal volumes, is easily understood — the relative 

 amounts of heat required to raise equal volumes of the material 

 and of water through a given range of temperature. 



Conductivity for heat is the quantity of heat which passes 

 through a cube of the substance when its opposite faces are 

 kept at different but constant temperatures, compared with 

 that which passes through a similar cube of another substance 

 under exactly similar conditions. In the table the numbers re- 

 ferred to silica =100 are given. The meaning of the figures 

 in the last column is suflSciently explained in its heading. 



It is to be noted that some of the above values will vary 

 with difference in degree of fineness of the material and other 

 circumstances. 



Sand is seen to have the greatest conductivity for heat and 

 the highest specific gravity, but the lowest specific heat and 

 lowest water-retaining power of all soil constituents. As a 

 plant food sand is practically valueless, except for the small 

 amounts of potassium, iron, and calcium sometimes present in 

 fragments of minerals occurring mixed with the true sand. Its 

 physical properties have great and often valuable effects upon 

 the character of a soil, particularly with regard to friability 

 and its relations towards water and heat. 



Clay, if pure, is free from plant food, but is usually well 

 supplied with potash because of the felspar present. Common 

 clay, however, often contains quartz and calcium carbonate 

 (as in marls) in addition to felspar, the true clay, or kaolin, 

 Alj03.2Si03.2H,0, acting as a kind of cement to the grains of 

 other minerals. 



It is thought that even in the purest clay there is a small 



