INFLUENCE OF COLOUR 163 



of heat by soil (Forsck. der Agrikulturphysik, xvii. 397), found 

 that while different soil constituents radiated heat with 

 different facility, these differences were quite independent of 

 their colour. 



The fact that the rays of heat emitted by the warm earth 

 are different in character from the rays emitted by the sun is 

 in several ways very important. The solar rays are but little 

 absorbed by the atmosphere ; the radiation from the earth is 

 completely absorbed by moist air. The heat received from 

 the sun is thus retained around the earth when the atmo- 

 sphere is moist, and night frosts only occur with a clear sky 

 and dry air. The vicinity of water is a great advantage where 

 early crops are to be grown, or where there is a danger of frost 

 in spring or early summer ; a small island or peninsula is thus 

 especially well suited for early market gardening. Land irri- 

 gated in the spring is also largely free from the effects of frost. 



The same facts serve to explain the high temperatures 

 easily obtained in glass-houses exposed to sunshine. The 

 sun's rays pass freely through the glass and are absorbed by 

 the wood, brick, and soil within ; these radiate back heat 

 rays of a different kind, which are absorbed by the moist 

 atmosphere in the house, and also by the glass roof. The 

 energy furnished by the sun is thus caught in a trap, it enters 

 the greenhouse easily, and leaves it with difficulty ; the tem- 

 perature within the house thus quickly rises. Frankland 

 (Pro. Eoy. Soc., xxii. 3 19) placed a thermometer in a box lined 

 with black cloth, with a sheet of plate glass as the lid. 

 Exposed to bright sunshine at Davos on December 22, the 

 thermometer rose to 221 F., or 9 above the ordinary tem- 

 perature of boiling water. Herschel, at the Cape of Good 

 Hope, cooked a beef-steak, and boiled eggs hard, by simple 



M 2 



