v.] COLOUR AND TEMPERATURE 127 



when the air is laden with water vapour, because it 

 retains the radiations emitted by the surfaces heated by 

 the sun. Per contra, the temperature of the ground 

 falls more rapidly at night when the sky is clear and 

 the air dry, for then there is no blanket of cloud or 

 water vapour to arrest or reflect the radiations from the 

 surface. 



The power of soils to absorb the sun's rays depends 

 very much upon colour : with black soils the absorption 

 is almost complete ; it is greater for red than for yellow 

 soils, least of all for those which look distinctly white or 

 light coloured. It has already been shown that the 

 colour of soils depends mainly upon humus and 

 hydrated ferric oxide, the latter accounts for all the 

 red, yellow, and brown shades, the former for the black 

 coloration of the soil Deep-seated clays are often 

 blue or green, due to various ferrous silicates or to 

 finely divided iron pyrites, which afterwards oxidise 

 to brown ferric oxide. The more finely grained a 

 soil is the more surface it possesses, and the greater 

 amount of colouring matter that is required to pro- 

 duce a given colour; a coarse sand is often quite 

 black though it contains but a small percentage of 

 humus. 



Though the colour of a soil affects the rate at which 

 it absorbs heat, it does not follow that the dark soils 

 will lose with a corresponding rapidity when radiation 

 is taking place at night; the emissive power of the 

 substance for rays of low refrangibility, such as are 

 emitted at ordinary temperatures, is not affected by 

 colour. Hence, the extra heat gained by a dark soil 

 is retained and not lost by a corresponding increase in 

 its radiating power. 



The curves in the accompanying diagram (Fig. 10), 

 show the temperatures of the soil at a depth of 6 



