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2°4 Argillaceous Genus. Luftre o, except in the rifts, and there 2. Tranfparency, o. Fradture, esrthy, yet unequal, difcovering ihick curved laminse irregularly compadted. Fragments, obtufe-angled. Hardnefs from 4 to 5. Sp. gr. inconfiderable. It ftains flightly, adheres fomewhat to the fongue, and feels fomewhat greafy. Befides argill, it contains fome raagnefxa, and falcarcous earth. Mifcellaneous Obfervations. Coloured clays of the common or coarfer kinds, which are by French writers called glaife, were found by Mr. Morveau to contain fome contami- nation of the vitriolic acid; but in white clays he could difcover none. 1 Encyclop. Chym. 122. This acid is probably united to the ferruginous particles that are found in thofe clays, and to which they owe their colour and fufibility. The yelLw, however, fometimes derive their colour from Regulus of antimony, bifmuth, lead, filver, or zinc. Rinman, Hift. Ferri § 197. Blue clays and marls fometimes whiten when heated, and confequently owe their colour to vola- tile matter, coal, or bitumen ; but fome redden, and contain pyrites. This trial fliould be made be- fore they are ufed as a manure ; as the firft are in- nocent, the laft hurtful. Neither Beame nor Woulfe could find any vitriolic acid in fome blue clays, moft probably in thofe of the firft kind, and pof- fibly not even in the fecond, as the barytic teft,
