COLORIFIC earths, in mineralogy, a class or tribe of earths, in the arrangement of Kirwan, described by him as strongly staining the fingers. Of these he enume rates four families, viz. red, yellow, black, and green ; the red is the reddle, of dark cochineal red colour, or intermediate be. tween brick and blood red,having neither lustre nor transparency ; fracture, earthy, sometimes conchoidal ; fragments, 1 ; hard ness, 4 ; sp. gr. inconsiderable ; adhering pretty strongly to the tongue ; feeling rough ; assuming a polish from the nail ; strongly stainingthe fingers ; falling imme diately into powder in water, and not be comingductile ; not effervescing, nor easi ly dissolving in acids. When heated to redness, crackling and growing black ; at 159°the specimen melted into a dark gree nish yellow frothy enamel. It differs from red ochres only by containing more argil. The red colour proceeds from oxygena tion, and the ab.,ence of acid. The more air of water is expelled by heat, the brown er it grows. The yellow is of an ochre yellow colour ; as to lustre, externally it often has some gloss, but internally none ; it is not transparent ; fracture earthy, often inclining to the conchoidal ; no specific gravity ; fragments, inconsi derable ; adheres strongly to the tongue ; feels smooth, or somewhat greasy ; takes a high polish from the nail ; strongly stains the fingers ; in water it immediately falls to pieces with some hissing; and after wards to powder, without diffusing itself through it ; does not effervesce with acids, nor is easily soluble in them ; heated to redness it crackles, hardens, and acquires a red colour, and gives a reddish streak. At Mr. Kirwan melted a specimen into a liver-brown porous porcelain mass. This yellow earth differs from ochres on ly in containing a greater proportion of argil ; the yellow colour proceeds from the cal' of iron, highly oxygenated, and probably containing both water and acid.
Those earths which contain a large pro portion of iron have rather an orange co lour. According to the analysis of M. Sage of Paris, who has the merit of pre serving to his countrymen the immense gains acquired by the Dutch from con verting this yelloNN earth into what is there called" English red," it contains 50 per cent. argil, 40 oxide of iron, 10 of water, acidulated by sulphuric acid. The 3d fami ly, or black ; black chalk is of a greyish black colour; fracture imperfectly curved slaty : fragments partly flat, partly long splintery ; adheres slightly to the tongue, feels smooth, assumes a polish from a knife ;, gives a black streak, and marks black: in water does not readily moulder, but if taken out cracks in a short time ; does not effervesce with acids, nor easily dissolve in them ; heated .to redness, it crackles and becomes reddish grey, and contains somewhat vitriolic. The 4th fami ly, green earth, is of a greyish green co lou• ; found generally in lumps in the ca vities of other stones, or externally invest ing them : fracture, earthy, sometimes uneven,sometimesvergingtothe conchoi dal ; sp. gr. 2.637, sometimes feels smooth, does not assume a polish from the knife, nor adhere to the tongue, nor stain the fingers, nor mark while dry, and when wet but lightly in water, it often crumbles after standing about LW an hour ; does not effervesce with acids, nor is easily soluble in them ; heated to redness, it crackles and becomes of a dark reddish cream colour; at 147°, a specimen was melted into a black compact glass, re sembling that of basalt ; which strews it to consist of silex, argil, iron, not much oxygenated, and oxyde of nickel, from which the green colour is derived, be water.