Perhaps we have not, in the opinion of some, answered the want expressed in the first paragraph. No pigment with the requisites of durability and cheapness will resist the attacks of strong acids on iron. The first we have mentioned will--all such as may float in our air from factories or chemical works. Chemically it is converted by nitric acid and chlorine into an insoluble substance--plumbic acid or the cyanide of lead. An experience of more than three years, with almost unlimited means at our command for experiment, demonstrates to us that we have indicated the means of filling the other requisites asked for. It may be that something new will be discovered, but we doubt it. Let any one tread the road we have trod, investigate and experiment where and as much as we have, and, if that place is, where we have not, and their experience will be the same as ours. * * * * * THE BANANAS AND PLANTAINS OF THE TROPICS. [For the Scientific American.] Poets have celebrated the banana plant for its beauty, its luxuriance, the majesty of its leaves, and the delicacy of its fruit; but never have they sufficiently praised the utility of this tropical product. Those who have never lived in southern countries are unable to fully appreciate its value. Some look even with indifference upon the gigantic clusters of this fruit, as they are unloaded from the steamers and sailing vessels; and yet they deserve special attention and admiration, for they are to the inhabitants of the torrid zone, what bread and potatoes are to those of the north temperate zone. The banana tree is one of the most striking illustrations of tropical fertility and exuberance. A plant, which in a northern climate, would require many years to gain strength and size, is there the production of ten or twelve months. The native of the South plants a few grains, taken from an old tree, in a moist and sandy soil, along some river or lake; they develop with the greatest rapidity, and at the end of ten months the first crop may be gathered, though the cluster and bananas are yet small; but the following year one cluster alone will weigh some sixty or more pounds. Even in the South they are always cut down when green, as they lose much of their flavor when left to ripen or soften on the tree. The trunk of the tree, if it may be so called, and which grows to a hight of some fifteen feet, is formed only by the fleshy part of the large leaves, some of which attain a length of eighteen feet, and are two and a half feet in width. While from an upper sprout you perceive the large yellow flowers, or already formed fruits, you see underneath a cluster, which is bending the tree by its weight.