THE PROPERTIES OF SOIL 41 



the yield may be deficient. It is, therefore, 

 desirable to take steps to raise the specific 

 heat of such a substance as sand, and this 

 can be done by adding to the stock of humus 

 that it contains ; for not only does humus in 

 itself keep sand cool, but, being a powerful 

 absorbent of water, it enables the soil to 

 hold more water, and nothing depresses the 

 temperature so much as this substance. 



The temperature of a soil depends not only 

 on its specific heat, but also upon its power 

 to absorb or reflect heat, and this property 

 is influenced to a considerable extent by its 

 colour. Other things being equal, a white or 

 light-coloured soil is a cool soil, because the 

 sun's rays are not so readily absorbed by a 

 white surface as by a dark. If, for instance, 

 two vessels are filled with soil, the surface of 

 which, in the one case, is whitened by a 

 dusting of lime, while, in the other, it is 

 blackened with a sprinkling of soot, it will 

 be found, after exposure to sunlight, that 

 the temperature of the soil with the dark 

 surface is higher than the temperature of 

 the same soil which has been sprinkled 

 with lime. This property of soil and other 

 bodies, whereby their temperature is in- 

 fluenced by their colour, is illustrated in 

 practical life by the fact that human beings 



