PHYSICAL PROPERTIES OF SOIL. 221 



chalk, making their surfaces all of a colour, and then 

 exposed to the sun's rays, the differences of their 

 temperature, will be found inconsiderable. Colour 

 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 

 different angle at which the sun's rays fall. The 

 more perpendicular, the greater the heat. The ef- 

 fect is less in proportion, 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 propor- 

 tion of geine. Does it cool quickly ? it is the geine 

 which gives up heat quickly, referring here to the 

 soil in a dry state, the modification produced 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, 

 19* 



