PHYSICAL PROPKRTIES OF SOIL. 245 



color, so that a dry, light-colored 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 be- 

 tween a dry and a wet soil of the same color. Thirdly, the 

 different materials of which soils are composed, exert but 

 very little influence on their power of being heated by the 

 sun's rays. Indeed, if sand, peat, clay, garden mould, all 

 equally dry, are sprinkled with chalk, making their surfaces 

 all of a color, and then exposed to the sun's rays, the differ- 

 ences of their temperature will be found inconsiderable. 

 Color and dryness, then, exert a most powerful influence on 

 the capacity of soil to be warmed. 



Fourthly, the last circumstance to be noticed, is the differ- 

 ent angle at which the sun's rays fall. The more perpen- 

 dicular, the greater the heat. The effect is less in propor- 

 tion, as these rays by falling more slanting, spread their 

 light out over a greater surface. But this point, which seems 

 external to soil, need not be enlarged on. Now, the great 

 fact to be observed, in this relation, of soil to heat is, that 

 geine exerts the most marked influence. If soil heats quickly, 

 it is because it has a large proportion of geine. Does it cool 

 quickly 1 It is the geine which gives up heat quickly, refer- 

 ring here to the soil in a dry state, the modification pro- 

 duced by dampness having been already considered. 



294. The relation of soil to moisture and gas, is not less 

 important than that of heat. All soil, except pure siliceous 

 sands, absorb moisture, but in different degrees. Geine 

 possesses the greatest power of absorption, and no variety 

 of geine equals in its absorptive power, that from animal 

 manure. The others rank in the following order, garden 

 mould, clay, loam, sandy clay, arable soil. They all saturate 

 themselves with moisture by a few days' exposure. It is a 

 very interesting question, Does soil give up this absorbed 



