220 PHYSICAL PROPERTIES OF SOIL. 



power. Hence sands retain heat longest, three times 

 longer than geine, and half as long again as clay. 

 Hence, the dryness and heat of sandy plains. Sand, 

 clay, and peat, are to each other as 1, 2, and 3 in 

 their power of retaining heat. But while the capac- 

 ity of soil to retain heat, depends on the absolute 

 weight, the power to be warmed, another very im- 

 portant physical character depends on four principal 

 circumstances : first, the colour ; second, the damp- 

 ness ; third, the materials ; fourth, the angle at 

 which the sun's rays fall. First, colour, the blacker 

 the colour, the easier warmed. White sand and 

 gray, differ almost 50 per cent., in the degree of 

 heat acquired in a given time. As peat and the va- 

 rieties of geine, are almost all of a black, or dark 

 brown colour, it is seen how easily they may become 

 warm soils, when dry ; for secondly, dampness modi- 

 fies the influence of colour, so that a dry light colour- 

 ed soil will become hotter, sooner than a dark wet 

 one. As long as evaporation goes on, a difference 

 of 10 or 12 degrees will be found between a dry 

 and a wet soil of the same colour. Thirdly, the dif- 

 ferent materials of which soils are composed, exert 

 but very little influence on their power of being heat- 

 ed by the sun's rays. Indeed, if sand, peat, clay, 

 garden mould, all equally dry, are sprinkled with 



