78 SOILS AND MANUEES 



the earth, i.e., when A D is parallel to B C and makes a 

 right angle with the direction of the rays. In the figure 

 this position of maximum is an angle of 30, because that 

 is the angle at which B C is drawn. 



Influence of Colour. Dark soils absorb more heat from 

 the sun's rays than those of lighter colour. Experiments 

 have shown that considerable difference of temperature 

 may be produced by artificially darkening or lightening the 

 surface of the soil. The effects are only noticeable in 

 bright sunshine, and are probably much greater in the 

 tropics than in more temperate climates. The darker 

 shades of colour are due mainly to the organic matter. 

 Some of the prairie soils in North America which contain 

 a large proportion of this constituent are quite black, and 

 it is stated that, on this account, these soils are more suit- 

 able for the production of maize, and crops which require a 

 large amount of heat, than lighter coloured soils situated 

 much further south. Under natural conditions the thermal 

 effects of colour are probably not of great practical impor- 

 tance in this country. 



Capacity for Heat. The capacity for heat ,or specific 

 heat of a body means the amount of heat jt v can hold or 

 store up in a definite quantity of the substance. It is 

 measured by the extent to which the temperature of the 

 body is raised by a given amount of heat. For example, 

 a given amount of heat will raise the temperature of a 

 pound of quartz five times as much as that of a pound of 

 water; or it will raise the temperature of five times as 

 much quartz to an equal extent. The capacity for heat 

 of quartz is therefore one -fifth of that of water. The 

 capacity for heat of water is greater than that of any other 

 substance and is taken as unity. 



The mineral constituents of the soil all exhibit much 

 the same capacity for heat about 0'2 or 0'25 but that of 

 organic matter is greater, viz., about 0*5 (water = 1). 



