PHYSICAL PROPERTIES OF SOILS. 15 



heated, is one of the best constituted soils. But " stiff 

 clays, which take up the greatest quantity of water, when 

 it is poured upon them in a fluid form, are not the soils 

 which absorb most moisture from the atmosphere 

 in dry weather; they cake and present only a small sur- 

 face to the air, and the vegetation on them is generally 

 burnt up almost as readily as on sands. The soils that 

 are most efficient in supplying the plant with water by 

 atmospheric absorption, are those in which there is a 

 due mixture of sand, finely divided clay, and carbonate of 

 lime, with some animal or vegetable matter, and which 

 are so loose and light as to be freely permeable to the 

 atmosphere. With respect to this quality, carbonate of 

 lime and vegetable matter are of great use in soils; they 

 give absorbent power to the soil without likewise giving 

 it tenacity; sand, which also destroys tenacity, on the 

 contrary, gives it little absorbent power." In accordance, 

 then, with these observations, Ave find that the materials 

 which are most influential in soils may be arranged in 

 the following order, when their relations to moisture 

 are considered: — organic matter, marls, clays, loams, and 

 sands. 



The Temperature of a soil depends very much upon 

 its humidity. Dry land absorbs heat more quickly and 

 loses it more slowly than that which is wet, and thus the 

 summer temperature of our undrained districts will be 

 lower than if they had been drained. 



The temperature of drained land is in summer occa- 

 sionally three degrees Fah. above that of undrained land. 

 The greatest difference between the temperature of the soil 

 and the air occurs in spring, the soil acquiring the proper 

 temperature for the coming vegetation rather slowly, in 

 consequence of the evaporation required in order to dry 

 it sufficiently. In the autumn, it seems to have acquired 

 a stock of heat which is sufficient for some time without 



