Its uses are manifold. It serves for water-pipes, for vessels fit for the reception of alkaline or acid liquids which would corrode metal or wood, for surgical implements, for boxes, baskets, combs, and a variety of other articles. The wonder of the age, submarine telegraphy, could hardly have been realised without it, as it is only by being cased in so isolating a substance, and one so impermeable by water, that the metallic wire is able to transmit the galvanic stream through the depths of ocean from land to land.
Of all the dyeing substances which the tropical zone produces in such endless variety, none is more important in a commercial point of view than indigo. Various species of plants producing this beautiful cerulean colour are found growing spontaneously in the warmer countries of both hemispheres, but the _Indigofera tinctoria_ is most generally cultivated. The knotty shrubby plant rises about two feet from the ground; the leaves are winged like those of the acacia, smooth and soft to the touch, furrowed above, and of a darker colour on the upper than the under side. The small reddish flowers which grow in ears from the axillæ of the leaves have no smell, and are succeeded by long crooked brown pods, which contain small yellow seeds. The plant requires a smooth rich soil, well tilled, and neither too dry nor too moist. A child of the sun, it cannot be advantageously cultivated anywhere except within the tropics, a higher mean temperature than 60° being absolutely necessary for its vegetation. The seed is sown in furrows a foot apart from each other, and two or three inches in depth. Sufficient moisture causes it to shoot above the surface in three or four days, and it is usually fit for gathering at the end of two months. When it begins to flower it is cut with a sickle a few inches above the roots, and furnishes, after six or eight weeks, a second crop. The cultivation of indigo would thus seem to be extremely profitable, but the sun, which so rapidly improves and invigorates the plant, calls forth at the same time a multitude of insects and caterpillars, that prey upon the valuable leaves, and frequently disappoint the planter’s expectations.
[Illustration: CUTTING THE INDIGO PLANT.]