The value of indigo as a dyeing material is due to the great stability of the blue color, and the derivatives from blue, which it gives to fabrics, especially of wool and cotton. It is not sufficient that a dyed fabric should preserve its color when submitted to violent tests, as when acted upon by vegetable or mineral acids or alkaline or soapy baths: the only stable dyes are those which resist air and light, the two destructive agents of vegetable colors. Indigo, from the remarkable manner in which its color becomes fixed upon a fabric, to be hereafter explained, possesses properties of resistance and stability in a higher degree than any blue dye. And when we consider that this blue has not only its own hue, but is the best foundation for blacks, greens, purples, and even browns, the importance of these properties cannot be over-estimated. Says M. de Kæppelin, a chemist and manufacturer of Mulhouse, in one of a series of articles furnished to the _Annales du gênie Civil_, 1864: “So high are the properties of resistance and stability which indigo possesses, that it is perhaps to be regretted for the art of the dyer and manufacturer of printed calicoes, that the use of indigo becomes more and more rare, and that the recent discoveries which modern science has placed at the service of industry are daily eliminating it from our factories. I have observed that whenever we have to dye stuffs of a high price, it is indigo which always serves as a base for the foundation of all the blue colors, or of those which are derived from blue. It is the same for the fabrication of printed tissues, which serve for the poorer classes, whose colors should have great stability without much increase of cost. But of late years, especially, we find a tendency to employ colors of little stability, and to prefer them, even in the class of fabrics first referred to, to those which are more fast, on account of their vivacity and freshness of tone. It is this tendency, which the consumer partakes of even while complaining of it, that the textile manufacturers ought to seek to combat. How often have I heard the greatest manufacturers of Alsace deplore the obligation which they felt that they were under of printing their tissues by means of colors so fugacious and so little resistant as those composed from aniline. We must hope, then, in the interest of that industry, that while adopting the marvellous discoveries which science is every day making, there shall be made a less general application of them, and that we shall return to the fabrication of the styles which necessitate the more constant employment of coloring materials,—less brilliant, it is true, but more adherent to the tissues, and less alterable by air and light. It seems to me, also, that taste would lose nothing; and that printed stuffs, colored in a manner less brilliant, but more harmonious, would be perhaps more appreciated, especially by those who use them.”