COLORING MATTERS. This name. ought to include every substance, organic or in organic, which is the cause of color in another, but in practice it is restricted to the coloring matters of vegetables and animals, to the dyes that are used for coloring fabrics. The reason of this restriction probably is that these coloring matters are distinctly different from the tissues or fluids which contain them, whereas the color of a mineral is not in general due to a separable body, but is peculiar to the mineral itself. It is impossible, for instance, to take away the blue color of a copper compound, or the green or yellow of one of chromium. In the mineral world the analogy to the coloring of plants is found rather in rocks; for example, in a sandstone colored with oxide of iron, where the coloring matter may be removed without the rock mass being destroyed.
The organic coloring matters derived from vegetables are both important from their uses in the arts and interesting from their character and decompositions. They may be divided into two classes, those which exist ready formed in the plant, and those which are obtained by the spontaneous or artificial decomposition of some principle in the plant. Of those belong ing to the first class, chlorophyll, the green coloring matter of leaves, and the colors in flowers, are the most obvious. Those however, which are used in the arts are not at first sight apparent, being contained in the seed, bark, stem or roots, from which they can be extracted by water, alcohol, ether, dilute etc. The second class includes bodies which re sult by oxidation or other chemical change from some usually colorless matters, to which the name color-producers, has been given; and, indeed, the question has been raised whether all coloring matters, even in plants, have not been produced from prior chromogens. Although coloring matters have been subjected to numberless investigation by chemists, very little is known about their real constitution. Coloring matters are generally odorless, with a rough taste, soluble, some in water, others in alcohol. Indigo and alizarine can be sublimed but most are decomposed by a comparatively slight elevation of temperature. They all con sist of carbon, hydrogen and oxygen, to which in some cases nitrogen is added. Some exhibit a weak acid tendency, combining with the oxides of lead, copper and especially tin, iron and aluminum, and forming insoluble colored compounds called °lakes.' Some have the power of attaching themselves permanently to different vegetable and animal fibres, as silk, wool, cotton and linen; others are unable to give a color which will not redissolve in water ; in such cases the fibre is mordanted, that is, treated with one of the metallic oxides just mentioned, and then, when immersed in the color, the insoluble lake is precipitated in the fibre. Many coloring matters are liable to change by exposure to daylight; they are also affected by a number of chemical reagents — certain blues, for example, are turned green by alkalis, and red by acids; they are destroyed by nitric acid, bleached by chlorine, decolorized, but not always permanently destroyed, by sul phurous acid, sulphuretted hydrogen, etc.
Of the vegetable coloring matters the yel lows are the most abundant, and different shades are obtained from different plants : fustic, turmeric, quercitron, Persian berries, morindin, saffron, arnotto, purree, chrysophanic acid and others; of the blues, indigo and litmus are the most familiar; and of the reds and purples, madder, logwood, Brazil-wood, safflower and a few more. Most of these colors require com
plicated operations to separate them in the pure state.
The only green coloring matter known, of no importance as a dye, but indispensable to the life of the plant, is chlorophyll. It was for tnerly supposed to be a simple substance. It was obtained from an alcoholic extract of leaves by adding lime, then decomposing the lime-chlorophyll compound with an acid, and agitating with ether, from which the chlor ophyll was got by evaporation. But by another process it was found that it could be separated into two bodies, one yellow, the other blue; and by the application of the spectroscope, Stokes showed that chlorophyll contains four coloring matters, two yellow and two green, differing in optical properties. By further study in the same direction Mr. Sorby thinks he has proved that besides the greens there are four or five distinct yellow coloring matters, to which he has given special names. It is quite obvious, if this be so, that our knowledge of the nature of chlorophyll is just beginning, for each col oring matter will become an object of chemical and physiological investigation, and not till then will it be possible to say how chlorophyll is produced and acts in a plant. Experiments have been recently made to elucidate precisely the fading of chlorophyll when exposed to light, a change which is accompanied by altered spectrum bands, but in their present state they are too inconclusive to be detailed here.
The chief animal coloring matters are those of the blood, the bile, the urine, the retina, of the muscle and of the skin. The blood's color ing matter is the well-known hemoglobin, while the pigments of the bile, bilimbim and itg oxida tion product, bilivcrdin, are derived from the blood pigments. Biliprasin and fuscin, bilicy anin, bilipurpurin and bilixanthin, are other bile pigments. They may be regarded as various products of oxidation and reduction of the ini tial bile pigments. Urochrome is the general name applied to the urinary pigments. Its exact composition is by no means definitely under stood. In the retina there are a number of pig ments or chromophenes. They are mostly lipochromes, or fatty pigments, and have been named rhodophane, chlorophane and xantho phane, these being red, green and yellow re spectively. There is a further black pigment in the eye, fuscin, allied to the melanins of the skin and hair. The muscle pigments are identi cal with the blood pigments. The skin pigments belong to a group lcnown as the melanins. It is undecided whether these are iron pigtnents or not.
The artificial coloring matters may be di vided into two classes, those which exist ready formed in nature, as many of the common red and brown earths, or which are formed by the mechanical mixture of such naturally. existing colors, and those which do not exist in nature, but are produced by chemical operations. The latter are of mineral or of organic origin, ex amples of the first class being afforded by Scheele's, Guignet's and other greens, artificial ultramanne, smalt and many others., and of the latter by Prussian blue and especially by the aniline colors. The artificial colors are more particularly described in the articles COAL-TAR