The painter naturally classes pigments according to their colour, or more exactly according to what are called the constants of colour - namely, hue, brightness and purity. He also takes into account transparency and opacity, although these terms are conventional and comparative only, since no pigment is perfectly transparent, none perfectly opaque. Another basis of classification, and a very important one, that of stability, will be considered in the next chapter; at present we are concerned with none of these methods of grouping pigments, but with others founded either upon their origin, or their physical characters, or their chemical composition.

Pigments are often classified into two groups - the mineral, and the organic. It is necessary to divide these groups further, in some such way as this:

Such a classification brings into prominence one marked distinction between the two groups, for, in accordance with one's expectation, the mineral pigments are, as a rule, characterized by a much higher degree of permanence than those of organic origin. [The chemist looks upon all compounds containing carbon, save the carbonates, as organic; but at the same time the distinction between organic and inorganic, or organic and mineral, is nothing more than a convenient convention.]

Other bases of classification are afforded by physical or mechanical characters. Pigments may be fixed or volatile, soluble or insoluble, crystalline or amorphous, substantive or adjective. There are difficulties in carrying out these schemes of classification, and it will be found that distinctions of physical character are utilized to the best purpose when connected with such a chemical classification as is offered below.

The simplest chemical classification is this:

Elements ; as graphite, lamp-black, gold. Compounds ; as aureolin, viridian, vermilion. Mixtures ; as yellow ochre, brown pink, rose madder.

After considerable expansion and some rearrangement, the differences just indicated afford a reasonable basis for a chemical classification which will prove of real service in judging of the degrees of stability, and of the possible interaction of pigments.* The mixed and

* In a pamphlet by M. Paul de Lapparent there is an ingeniously constructed diagram giving, at one view, a list of pigments which, in the judgment of the author, react upon one another. If we omit from the list red lead, we shall be able to cancel nearly one-third of the black marks assigned to mixtures of two pigments; but we shall still include two conspicuous offenders, namely, pale cadmium and Prussian blue. Pale cadmium is stated to be incompatible with Indian red, Venetian red, burnt sienna, the native earths, ivory black, Prussian blue, and cobalt violet; while Prussian blue is marked as affecting, or as affected by, Venetian red, burnt sienna, deep cadmium, orange cadmium, pale cadmium, aurora yellow, even indefinite character of many pigments, and the chemical solitariness of others, preclude the formation of groups having equal rank and precise group-characters: thus the proposed chemical classification, though convenient, has no pretension to completeness. I suggest the following nine groups:

Group I. - Elements

Group II. - Oxides

Group III. - Sulphides

the native earths, ivory black, white lead, zinc white, and viridian. On the other hand, M. de Lapparent allows that vine black is an entirely innocuous and permanent pigment. My experience is not in general agreement with the judgments of M. de Lapparent as embodied in the diagram under consideration. For instance, I do not find that aureolin reacts with white lead or with ultramarine, nor can I admit that Prussian blue deserves the bad character assigned to it. Possibly the purity of the particular pigments with which our author dealt was not assured in all cases. Might there not have been free sulphur in his pale cadmium and free acid in his Prussian blue?

Group IV. - Hydrates

Group V. - Carbonates

Group VI. - Silicates

Group VII. - Chromates

Group VIII. - Various Inorganic Salts

Group IX. - Organic Compounds

Indian yellow. Yellow lake. Gamboge. Pure orange. Rose madder. Madder carmine. Rubens madder. Madder red. Purple madder.

Brown madder. Scarlet alizarin. Crimson alizarin. Carmine. Crimson lake. Scarlet lake. Purple lake. Sap green. Verdigris.

Emerald green.

Indigo.

Prussian blue.

Antwerp blue.

Bitumen.

Bistre.

Sepia.

Vandyke brown B.