3 4 SOILS. 



tain extent discussed. While it is true that, broadly speaking, 

 dark-colored soils absorb more of the sun's heat than light- 

 colored ones, other things being equal, it must still be under- 

 stood, that the nature of the color-giving substance exerts a 

 very material influence upon the amount of heat absorbed. 

 Thus charcoal is among all known substances the one absorb- 

 ing and radiating the sun's heat rays most powerfully, and all 

 kinds alike; so much so, that its absorbent power is taken as 

 100. But other substances which to the eye appear equally 

 black, have by no means the same absorbing power. The heat 

 absorption by black humus is high, though not quite equal to 

 that of charcoal ; and many gray soils, though appearing to the 

 eye of rather light tint, really absorb more heat than others 

 which, to our perception, have the darker tint, but are colored 

 by other substances. Gardeners and especially vine growers in 

 the colder portions of Europe often take advantage of the 

 powerful absorbing power of carbon by spreading charcoal 

 or black slate powder over the surface of the soil where early 

 maturity is specially desired; and slate powder is similarly 

 used by the peasants at Chamouni to hasten the melting of 

 the snow. 



Heat of High and Low Intensity. It must also be kept in 

 view that the surfaces, and especially the colors that favor ab- 

 sorption of the intense rays of the sun, may comport them- 

 selves quite differently toward heat rays of low intensity, such 

 as those thrown back from the soil at night when it cools. 

 \Yere this otherwise, a soil that absorbs much heat in the 

 daytime would lose it with corresponding rapidity at night. 

 But this is true only of charcoal ; in the case of most other 

 substances, there is a material difference in favor of the re- 

 tention of the heat, of low intensity, by slower radiation into 

 a " heat-trapping " atmosphere. 



Reflection vs. Dispersion of Heat. Theoretically, a smooth 

 surface reflects more heat than a rough one, and warms much 

 more slowly by absorption ; as is strikingly shown by the use 

 of polished metal screens placed on walls to prevent their being 

 overheated by a flue near by. In the case of soils, also, the 

 condition of the surface affects materially the absorption of 

 heat, but not in accordance with the above rule so far as the 



