_Artificial Indigo._—The exact composition of indigo was first determined some sixty years ago, and from that time on some of the greatest chemists of the world have been attempting to prepare it, artificially, from some comparatively inexpensive source, obtained from coal tar or elsewhere. As early as 1875 the problem was solved, at least from a scientific standpoint, but the process proved too expensive for commercial purposes. During the last five years, however, at least two of the great German firms have discovered methods for making, in any desired quantities and at very reasonable expense, absolutely pure indigo from some of the important coal-tar derivatives. And since that time the cultivation of the indigo plant has proved so unprofitable that it has been almost entirely abandoned, and the land formerly used for this crop is being turned over to other and, at present, more useful purposes. This synthesis—i.e., chemical formation—of indigo from coal-tar products has been justly regarded as one of the great triumphs of modern science. Right here let me impress upon my readers this fact: the real dyestuff, indigo, is absolutely the same material, whether it comes mixed with a great mass of impurities, as in the woad; or whether it contains from 5 to 25 per cent. of foreign matter of little or no value, as in the Bengal or natural indigo; or whether we get it from Metz or the Badische Company, chemically pure, either in the dry state or, thinned with water, in the form of a 20 per cent. paste. It is positively the same dye; and being absolutely without contamination of any kind, the artificial or synthetic dyestuff presents advantages in the matter of purity of shade, ease and surety of manipulation, and permanence of the color produced, which could never be obtained before its introduction. =Application of Indigo.=—The principles of indigo dyeing are the same now as with the Egyptians, the only difference being in the means used to bring about the chemical changes involved. Indigo itself is a blue solid, insoluble in water, acids, and alkalies, and practically unaffected by sunlight. If, however, the element hydrogen be added to it, or, as the chemist would say, if it is “reduced” by the action of any one of numerous deoxidizing or reducing agents, the indigo blue is changed to a new substance, indigo white, which is almost colorless, and which dissolves, in the presence of alkalies, to a bright yellow liquid. If cotton, wool, paper, wood, or indeed almost any solid materials (noticeably the fingers and nails, as some of my readers may find out), are immersed in the solution, they will absorb some of this indigo white, and then, on exposure to the air, the white indigo will rapidly take up oxygen, and become converted into the insoluble blue coloring matter.