218 CONSERVATIVE ORGANS. [LECT. V. 



1000 parts of water : 2000 parts of pure water 

 hold one of magnesia in solution. 



AIR is, also, a necessary component of soils. 

 Atmospheric air is absolutely necessary, as we 

 know, for carrying on the process of germina- 

 tion ; the more pulverulent, therefore, the soil 

 is, the more air it is capable of containing, and 

 consequently is the better adapted for support- 

 ing vegetation. But a soil which is too sandy, 

 the water not being retained, although it appears 

 to be loose, yet does not contain so much air en- 

 veloped in it as is required ; for the small par- 

 ticles of which it is composed apply more closely to 

 each other, and lie in a smaller compass than the 

 aggregated masses of a better soil, which touch 

 at a few points only, and, therefore, leave more 

 and larger interstices between them. When the 

 soil is too retentive, the water which remains on 

 its surface evaporates in summer, and deposit- 

 ing the clayey particles which it held suspended, 

 a kind of paste is left, which hardening, by 

 being baked, as it were, in the heat of the sun, 

 no air can penetrate to the parts beneath it ; nor 

 can that which has been already used in the ve- 

 getative process, and which is unfit to carry it fur- 

 ther on, escape : and we know that as atmosphe- 

 rical air is vitiated by the roots of growing plants, 

 and during the germination of seeds, a constant 

 renewal of it is requisite for supporting the vigour 



