164 RELATIONS OF SOIL TO HEAT 



exposure to the sun in a box covered with a pane of window 

 glass, and placed in another box so covered. 



The influence of the colour of the soil on its temperature 

 has been recognized by practical men. In some of the Rhine 

 vineyards it is usual to scatter fragments of black basalt on 

 the surface of the ground, with the object of gaining more 

 heat to ripen the grapes. In parts of Spain, on the other 

 hand, having a much hotter climate, it is only on white soils 

 that certain grapes can be successfully cultivated. 



Colour ceases to have a considerable influence on the tem- 

 perature of the soil when soils are very wet, the extra heat 

 received by dark soils being consumed in the evaporation of 

 water. The power of humus to warm a soil is thus seriously 

 diminished, humus always favouring the retention of water at 

 the surface. 



Specific Heat of Soils. Different substances require different 

 quantities of heat to bring them to the same temperature ; or, 

 in other words, different substances receiving the same quan- 

 tity of heat will rise to different temperatures. Thus if a 

 pound of iron and a pound of tin are both placed in boiling 

 water till they have gained that temperature, and are then 

 each of them placed in a similar bulk of cold water, it will 

 be found that the cold water is raised to a higher temperature 

 by the iron than by the tin. By proceeding in this way the 

 relative amounts of heat contained by substances at the same 

 temperature may be measured. As water is of all ordinary 

 substances the one requiring most heat to raise a given weight 

 to a given temperature it is taken as the standard. The 

 numbers representing the specific heat of other substances 

 express the fraction of a unit of heat which is required to 

 raise them to the same temperature that would be reached if 



