72 Different Characters of Clay. [Book VI* 



plants die that grew there ; for the clay being previ- 

 oufly mixed with the water, and being then compreffed 

 by the feet of the cattle, the ground is rendered too. 

 tough for the vegetables to Ipring up through it s 

 efpecialiy when the cky thus trodden together is af- 

 terwards dried by the heat of the fyn. Clays differ, 

 much in confiftency. Some melt in the mouth, 

 others are gritty, and get between the teeth. They 

 are white, blue, grey, red, yellow, or black. Some 

 are much lefs yifcid than others. Some refift a very, 

 violent heat without undergoing any change; but 

 moft of them, in very intenfe heat, melt into a vitri- 

 fied mafs. Some of them effervefce with acids, others 

 not. All thefe varieties have but one fpecies of earth 

 for their bafis, and are nothing more than mixtures of 

 pure clay with heterogeneous fubftances. Thus we 

 often find, flrata, which contain a large mixture of 

 fandy and gritty particles. All clays which are fandy, 

 are fo from fan4, or calcareous earth: when mix,ed 

 with the latter, they efFervefce. The variety pf theis- 

 colour depends on an admixture of iron; though 

 fometimes of inflammable matter. The clay becoming 

 white in the fire, is the fureft fign of its purity. We 

 can actually extract iron from moft pf thefe clays^ 

 efpecialiy thofe which burn to a red colour, which 

 colour is always affumed by the calces or ruft of iron. 

 When they effervefce with acids, they clafs with marles. 

 Clay, united with vitriolic acid, forms that common 

 and well known fubftance called AL UM. 



The Sruirrrioia of the Greeks, and the alumen of 

 the Romans, was a native fubftance, and differed much 

 from the fait of which I am now treating. The 

 varieties mentioned by Diofcorides refer to ftalactites, 

 which contained very little if any alum, and that 

 completely enveloped by a vitreous matter. The 



defcriptions 



