t6o A TREATISE ON THE CONNECTION OF 



SOILS, 

 ARGILLACEOUS OR CLAYEY. 



There is no clayey soil that is pure and free from 

 ^and ; and there are but few clays that are free from a 

 mixture of calcareous matter, magnesia, vegetable and 

 anirrial matters, mineral oil, and other mineral or metal- 

 lic substances : some clay? are of a much more unctuous, 

 and, as it were, greasy nature, than others. They do not 

 differ more in this respect, than they do in the appear- 

 ance they assume when submitted to a moderate degree 

 of heat. Those clays which are the most undluous and 

 greasy to the touch, are by calcination changed to a black 

 .colour. This must be owing either to their containing 

 animal or vegetable matter, although previous to calci- 

 nation it escapes observation ; or the inflammable matter 

 in the clay may exist in the state of a colourless mineral 

 oil, adhering obstinately to the clay, and not capable of 

 being separated from, it by water, with which oil can 

 Oiold no union ; yet capable of being changed into a black 

 carbonaceous matter by the aclion of fire. A due mix- 

 ture of clay serves the important purposes of retaining 



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