184 Chemistry. [Lecture 30. 



in the mouth, others are gritty, and get between 

 the teeth ; they are white, yellow, blue, grey, 

 or black. Some are less viscid than others. 

 Some resist a very violent heat, without under- 

 going any change; but most of them, in very 

 intense heat, melt into a vitrified mass. Some 

 of them effervesce with acids, others not. All 

 these varieties, as I intimated before, have but 

 one species of earth for their basis, and are no- 

 thing more than a mixture of pure alumina witli 

 heterogeneous substances. 



The variety of colour usually depends on an 

 admixture of iron. We can actually extract 

 iron from most of these clays, especially those 

 that burn to a red colour, which colour is 

 always assumed by the calces or rust of iron ; 

 when they effervesce, which is from a mixture 

 of lime, they class with marls. 



As alumina is obtained from alum, so, on the 

 other hand, by mixing clay, while it is heated 

 in a crucible almost to redness, with strong sul- 

 phuric acid, gradually affused, a very perfect 

 combination may be effected, and plenty of 

 alum procured by lixiviation with an addition of 

 potass. 



The quantities of alum employed in the arts 

 are not, however, prepared in this way. Alum 

 is sometimes met with in small quantities in 

 mineral springs, but is mostly prepared from 

 certain fossils, or ores of alum, as they are called. 

 These ores are generally found in strata, which 



