DißinBive Charakters of Earihs. 197 Of Umber. A brown or blackifh fubftance, very light, which moderately ftains the fingers, and ftrongly adheres to the tongue; has longbeen taken for a fort of clay. but Mr. Hupfch, in the Memoirs of Berlin for 1777 ( and > long before that, Dr. Hill, P. 63) has Ihewn it to confift chiefly ot particles of decayed wood, mixed with bitumen. This name hath alfo been given to an ochre of the fame colour, which becomes red when flightly heated; but in a ftronger heat again becomes brown; and in a ftill ftronger melts mto black elafs. It is therefore a compound of clay and fron, but in which the iron is the prevalent in- gredient. It does not effervefce with acids before torrefaäion; but after that Operation the martial part is eafily afted on. PlFFERENCES OF THE FOREGOING EARTHS. As the diftinftive charafters of earths arefound varioufly combined in the earths above deferibed, their diferimination will be much affifted by foew- ing the properties that diftinguiflt each lrom all the others. Native Argill is difliuguilhed, From Porcelain eartb, often by its colour, or ef- fervefeing with acids, and its cryllalline appcarance when viewed through a lens, O 3 its