v.1 COLOUR AND TEMPERATURE 143 



with the yellow colour of the young corn during a spell 

 of cold and drying east wind. 



Radiation, 



The main source of the soil warmth consists in the 



heat received from the sun by radiation ; this, according 



to Langley, amounts to about 1,000,000 calories per 



hour per square metre of surface from a vertical sun in 



a clear sky. Supposing this energy were wholly 



absorbed by a layer of dry soil 10 cm. thick, its 



temperature would rise by as much as 90 F. in an hour. 



Of course in nature many other factors are at work to 



reduce this temperature ; the sun is rarely vertical, 



the soil material does not completely absorb but reflects 



some of the sun's rays unchanged ; at the same time it 



is always radiating in its turn rays of lower pitch than 



the majority of those received. The latter rays are 



easily caught by many substances, glass and water 



vapour in particular, which are transparent to the rays 



of higher refrangibility proceeding from the sun. A 



greenhouse, for example, is practically a radiant heat 



trap ; the temperature inside runs up because the sun's 



rays of light and heat can penetrate the glass, whereas 



the obscure heat rays radiated back again from the 



warmed-up surfaces inside the house are not able to 



pass through the glass again. Just in the same way the 



temperature rises and the sun's heat becomes oppressive 



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 



