78 THE SOIL. 



less like slate, and, by their crumbling and decomposition, 

 seem to have given rise to clay or argillaceous earth. 



Clay is silicate of alumina; a chemical compound of 

 silicic acid, alumina, and water. Clay usually contains 

 also silicates of potash, of soda, and of lime. It forms a 

 compact, fatty earth, which is soft to the touch, adheres 

 somewhat closely to the tongue, and exhales a peculiar 

 odor, which is perceived when it or clay-slate is breathed 

 upon. 



Pure clay is white ; but clay, as ordinarily found, is 

 colored blue, brown or red, by oxides of iron. It absorbs 

 a great deal of water, and parts with it very reluctantly ; 

 and it has a strong attraction for ammonia and for the 

 very richest portions of manure. 



When completely wet, it becomes a thick paste, almost 

 impenetrable to water and to air, which it prevents from 

 percolating or penetrating farther into the earth. Under 

 the effect of drought, it cracks and becomes excessively 

 hard. From the action of frost, on the contrary, it swells 

 and crumbles into powder, from the water s expanding, 

 as it freezes, and thus breaking up whatever contains it. 

 Hence the usual humidity of clayey lands, the difficulty 

 of ploughing them in a very wet or a very dry season, 

 and the beneficial effects of freezing. 



274. There are many kinds of clay, and most of them 

 are of great value in the plastic arts. All the varieties 

 of porcelain, pottery, stone ware, earthen ware, tiles and 

 bricks, arc made wholly or chiefly of clay. The celebrated 

 kaolin, or pure white porcelain clay of China, is mouldered 

 felspar ; and the petuntze of the Chinese potter is another 

 kind of felspar containing potash. Clay is also the mate 

 rial commonly used by the statuary, in which to shape 

 the first draught or model of his figures, and often by the 



