Argillaceous Genut. 73 



clays are more or lefs eafily diffufible in wa- 

 ter, which they rnbibe with more or lefs avi- 

 dity, according to their previous compadlnefs, 

 and if they contain much air they froth with 

 water, burft and tali to pieces : when they 

 have imb.bed it rhey are more or leis dutil 

 and vicid, according as the fand they are mixed 

 with is more or lefs fine. They contract ia 

 drying, and crack when heated, they con- 

 trad: ftill m"> re, and harden to fuch a degree 

 as to give fire with fteel. They do not effer- 

 vefce with ack!s, un lefs they contain ma^nefia 

 or calcareous Earth. The finer clays have 

 a fmooth, and in lome meafure, a foapy feel. 



Thefe are the properties of pure argilla- 

 ceous Earths ; yet moft clays contain but from 

 35 to 48 per cent, of this Earth, the remain- 

 der confifts of fine filiceous Earth, and a little 

 water. On the fubtility and purity of the fili- 

 ceous part, the finenefs of clay principally 

 depend. 



Many clays contain a little volatile alkali, 

 and fome the vitriolic acid ; probably this lat- 

 ter is united to the argill in the ftate of em- 

 bryon allum, that is, in fuch proportion as 

 not to render it foluble in water. Moft of 

 them contain alfo a finall proportion of iron. 



Fufible 



