^ DRAWING SLATE OR BLACK CHALK. 181 



tongue. Feels more or less greasy. Soft, some- 

 times approaching to friable. Not particularly 

 heavy, passing into light. Sp. Gr. of earthy 

 Potter's Clay = 1-800 ... 2-000, KIRWAN ; 

 = 2-085, KARSTEN; Slate-clay = 2-600 ... 

 2-680, KIRWAN ; = 2-636, KARSTEN. 



Common Clay has been divided into Loam, Potter's Clay, 

 Variegated Clay, and Slate-Clay ; and Potter's clay again 

 into earthy and slaty ; which division rests either upon par- 

 ticular, though accidental properties, or upon the employ- 

 ment made of the varieties. 



Clay is a mixture of decomposed minerals, and hence it 

 is little constant in its composition. Several varieties 

 soften in water, and allow themselves to be kneaded and 

 formed into moulds, a property to which they owe their 

 well known employment. Some are easily fusible, others 

 refractory ; some acquire particular tints of colour, others 

 lose theirs, and become white when exposed to a strong 

 heat ; upon all of which properties their applicability 

 depends. They occur in beds near the surface of the 

 earth, or covered by the soil in the formations of brown and 

 black coal. In the latter they very often contain remains 

 of vegetables, and are called slate-clay, which is intimately 

 related to bituminous shale and alum-earth. 



The appropriate varieties of clay are of various import- 

 ant applications in pottery, in manufacturing stone-ware 

 and porcelain, in constructing furnaces for metallurgic ope- 

 rations, &c. 



DRAWING SLATE OR BLACK CHALK. 



Drawing-Slate or Black Chalk. JAM. Syst. Vol. II. 

 p. 273. Man. p. 443. Black Chalk. PHILL. p. 55. 



Massive. Composition impalpable. Principal frac- 



