C i6i 3 



merits of which were united in the substance by che- 

 mical combination. , 



When pipe-clay dried at the temperature of the 

 atmosphere is brought in contact with water, the fluid 

 is rapidly absorbed ; this is owing to cohesive attrac- 

 tion. Soils in general, vegetable, and animal sub- 

 stances, that have been dried at a heat below that of 

 boiling water, increase in weight by exposure to air, 

 owing to their absorbing water existing in the state of 

 vapour in the air, in consequence of cohesive attrac- 

 tion. 



The water chemically combined amongst the ele- 

 ments of soils, unless in the case of the decomposition 

 of animal or vegetable substances, cannot be absorbed, 

 by the roots of plants ; but that adhering to the parts 

 of the soil is in constant use in vegetation. Indeed 

 there are few mixtures of the earths found in soils, 

 that contain any chemically combined water ; water 

 is expelled from the earths by most substances that 

 combine with them. Thus, if a combination of lime 

 and water be exposed to carbonic acid, the carbonic 

 acid takes the place of water ; and compounds of alu- 

 mina and silica, or other compounds of the earths, do 

 not chemically unite with water : and soils, as it has 

 been stated, are formed either by earthy carbonates, 

 or compounds of the pure earths and metallic oxides. 



When saline substances exist in soils, they may 

 be united to water both chemically and mechanically j 

 but they are always in too small a quantity to influence 

 materially the relations of the soil to water, 



