



ARE ALL CLAYS EQUALLY EFFICACIOUS ? 257 



This clay possessed the quality so often seen in the Oxford 

 clay, of being like bird-lime in wet weather, and in dry sum- 

 mers like a stone, requiring a pick-axe to break it. 



But there are three things, in regard to burned clay, which 

 are far from being cleared up. 



1. Are all clays equally efficacious, when burned in a similar 

 way? 



2. If not equally efficacious, in what respect do the good 

 differ from the bad clays, and by what qualities or characters are 

 they to be distinguished? 



3. How do they act in improving the soil or the 

 crops ? 



I shall briefly consider each of these questions. 



1. Are all clays equally efficacious when burned in a similar 

 way. I believe the correct answer to this question is, that they 

 are not. It is true that similar clays, in the same neighbour- 

 hood, often produce very different effects, according as they are 

 well or ill burned; but experience appears to indicate that, 

 with the most skilful treatment, there are many clays which 

 cannot be profitably burned by the practical man. 



The practice of paring and burning the surface, or of burn- 

 ing, in various ways, the scourings of ditches and refuse vege- 

 table matter of different kinds, is not to be confounded with 

 that of true clay-burning. Both practices, however, have these 

 things in common that the heaps or kilns must contain, or 

 must be supplied with, a sufficient quantity of vegetable matter 

 to enable them to burn thoroughly that the burning should 

 be slowly conducted, and with little access of air, a method 

 which is well described by the epithet of s^e-burning and 

 that the heat should not be permitted to become so great as to 

 produce what is called over-burning. But in regard to true 

 stiff clays 



2. In what respects , physical or chemical, do the good differ 

 from the bad those which are improved by burning from those 

 which are not. These points have not as yet been sufficiently 

 investigated or described. In general, I believe that such as 

 burn most profitably are very impervious to water and air, are 

 exceedingly tenacious, and harden very much under the influ- 



