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produced this effect in a greater or less degree ; nitre, alum,tartar, very weakly ; sea salt more strongly ; fixed alcalinesalts still more so; sal ammoniac the most strongly of all.Metallic solutions, made in acids, and diluted so as not tocorrode the paper, acted in the same manner.

7. Besides the simple blacks, there are a multitudeof compound ones, inclining more or less to other colours.Thus the painters have blue-blacks, brown-blacks, Scc.which may be made by mixing pigments of the respectivecolours with simple black ones, in greater or less quantity,according to the shade required. The dyers also havedifferent blacks, and often darken other colours by slightlypassing them through the black dying liquor; but the termbrown-black is in this business unknown, brown and blackbeing here looked upon as opposite to one another. Ineffect, the colour called brown-black is no other than thatwhich ill dyed black clothes change to in wearing: nowonder then that it is excluded from the catalogue of thedyers colours.

8. The true or simple blacks, mixed with white, formdifferent shades of grey, lighter or darker according as thewhite or black ingredient prevails in the mixt. Theblack pigments, spread thin upon a white ground, have alike effect.

9. Hence the painter, with one true black pigment, canproduce on white paper, or on other white bodies, all theshades of grey and black, from the slightest discolorationof the paper, up to a full black : and the dyer producesthe lame effect on white wool, silk, or cloth, by continu-ing the subjects for a shorter or longer time in theblack bath, or making the bath itself weaker or stronger.

10. Hence also the dilution of black pigments withwhite, or the spreading of them thin upon a white ground,affords a ready method of judging of the quality or species

of