162 RELATIONS OF SOIL TO HEAT 



between the temperature of a black and white sand. Working 

 with natural soils, in a European climate, Schiibler, Oemler, 

 and Wollny always obtained the highest temperatures with 

 the darkest soils, humus always heading the list. The 

 extreme differences observed among natural soils did not 

 however exceed 6-8. No difference was observed on cloudy 

 days. During the night the dark and light soils cooled to 

 the same point. 



The facts just mentioned are easily explained. The white 

 or light coloured soils reflect a portion of the radiation from 

 the sun, while the black soils absorb the whole of it, the 

 total energy of the sun's rays appearing in this case as heat. 



It is sometimes erroneously supposed that the darkest soil 

 must cool most, and that the gain of temperature due to a 

 dark colour is followed at night by a loss of temperature due 

 to the same cause. This is not necessarily the case. The 

 absorption in sunlight is influenced by colour, the cooling 

 by radiation at night is not influenced by colour. The 

 absorption in day by black and white surfaces is different, 

 because these surfaces behave differently to rays of high 

 refrangibility ; at night both behave alike, because both are 

 then emitting similar rays of very low refrangibility. 

 Melloni found that when lamp-black and white-lead were 

 both exposed to the rays from a lamp flame their relative 

 absorption of heat was 100 : 53 ; but when the same surfaces 

 were exposed to the radiation from hot water, their capacity 

 for absorbing heat was identical. The emission of heat by 

 lamp-black and white-lead at the temperature of boiling 

 water would necessarily be as equal as their rate of absorption. 

 The extra heat obtained from the sun by a dark soil is thus 

 a substantial gain. Ahr, in his investigation on the radiation 



