MECHANICAL EFFECTS OF BURNING. 



259 



I shall state my views of the effects of burning upon clays 

 in a separate section. 



4. Mechanical and chemical effects of burning upon a clay. 

 How it afterwards acts when applied to the soil. 



In considering the effects of burning upon clay, it is of im- 

 portance to bear in mind that the action of the heat must 

 neither be too great nor too prolonged, otherwise the clay will 

 be overburned, and rendered comparatively useless for agri- 

 cultural purposes. This fact is of especial importance in refer- 

 ence to the chemical changes which the clay undergoes, and 

 the chemical action it afterwards exercises in the soil. 



1. The mechanical effects of such burning are simple and 

 easy to be understood. The clay is hardened to a certain 

 degree, without being made compact or melted. It has become 

 porous, and, though it falls under the action of the weather, 

 it does not again become adhesive, or tenacious and plastic. It 

 thus tends to open and render lighter stiff clay soils to which 

 it may be applied. It is chiefly to this mechanical action that 

 the greater friability of heavy land, after being dressed with 

 burned clay, is to be ascribed, and the smaller amount of labour 

 which is afterwards required to work it. 



The beneficial action of burned clay has been ascribed alto- 

 gether to its porosity. This enables the burned lumps, it is 

 said, to absorb the ammonia of the atmosphere, and thus to 

 bring within the reach of plants from a natural source what 

 they might otherwise be unable to obtain in sufficient abundance. 

 It is certain that all porous bodies do absorb gases and vapours 

 in comparatively large proportion, and, by condensing all the 

 constituents of the atmosphere in their pores, the pieces of 

 burned clay may really be useful to the growth of plants. It is 

 a fancy, however, to suppose that the clay exercises this absorp- 

 tive power, especially upon the ammonia, which exists, it is true, 

 but in an almost infinitesimal quantity in the air. It may take 

 in traces of ammonia along with the oxygen and nitrogen of the 

 atmosphere, but these alone cannot justly be regarded as likely 

 to influence in a sensible degree the fertility of the soil to which 



