260 CHEMICAL EFFECTS OF BURNING. 



it is applied. Besides, as soon and as often as the pores are filled 

 with water, they will cease to absorb gaseous matter, and thus 

 their useful function ought then to cease. 



It may be, however, that the porous quality of the clay may 

 also facilitate evaporation from the surface of the soil, and may 

 thus aid in keeping it dry. It will dry sooner in the air during 

 the day than the tenacious unburned soil ; and again absorbing 

 moisture from the latter, it may both hasten the drying of the 

 whole, and prevent its baking so hard together as it usually 

 does. 



These mechanical influences of burned clay are by no means 

 to be undervalued, but its fertilising effects are in a very much 

 greater degree to be ascribed to the chemical changes which the 

 constituents of the clay undergo during the process of burning. 



2. These chemical changes are of such a kind as to render the 

 constituents of the clay more soluble, that is, soluble to a 

 greater extent than before the burning, both in water and in 

 acids. 



In experiments made upon this subject, the details of which 

 I need not insert here, I have found that, after burning, pure 

 water will often dissolve twice as much from the clay as it would 

 before burning, and with much more ease. Acids, also, such as 

 a mixture of nitric and muriatic acids, will leave from 5 to 8 

 per cent less of insoluble matter after they have been burned 

 than when they are digested in the same acids for the same time 

 in their natural state. This greater solubility must render them 

 much more capable of yielding to the roots of plants the several 

 mineral substances contained in them, and which plants can only 

 derive from the soil. 



That this is the true general explanation of the benefits of 

 burning, is proved, I think, by the concurrent testimony 



a Of experience that, by over-burning, clays lose their fer- 

 tilising virtues ; and, 



b Of chemical analysis that, by such over-burning, new 

 changes among the constituents of the clay take place, by which 

 they are again rendered less soluble than when moderately 

 burned. 



It is quite clear, that if such be the true nature of the useful 



