30 THE CHEMISTRY OF THE FARM 



of the sun's rays is the prime cause of the immense 

 difference between an Equatorial climate and that of 

 Northern Europe. The effect is observed to some 

 extent in our own fields, and explains why certain 

 slopes or aspects are favourable to fertility. It is on 

 a slope facing the south that the soil will reach its 

 highest temperature during sunshine. 



The mean temperature of both soil and subsoil is 

 nearly the same as the mean temperature of the air 

 at the surface. Every circumstance affecting the tem- 

 perature of the air (as warm ocean currents) affects the 

 mean temperature of the soil. When, however, a soil 

 is freely exposed to the sky, the temperature at the 

 surface reaches a far higher maximum, and falls to a' 

 lower minimum than is reached by the air above it. 

 Schiibler determined for two years the temperature of 

 freely-exposed soil in his garden at TUbingen, at 3^2 

 inch below the surface, shortly after noon, on every 

 day when the weather w^as perfectly fine. The mean 

 of these determinations was above 120° Fahr. for 

 every month from April to September inclusive, and 

 in July reached 146° ; this latter temperature was 65" 

 above that of the air taken at the same time. 



A dark-coloured soil becomes hotter in the sun's rays 

 than a light-coloured one, a larger proportion of the 

 sun's energy being converted into heat ; the extreme 

 difference observed in the case of natural soils in 

 European climates is about 8°. No difference will be 

 observed on cloudy days. At night all soils will cool 

 to the same point. 



The quantity of heat required to produce the same 

 rise in temperature {specific heat) is very different for 

 the different constituents of soil, as will be seen from 

 the following table : — 



