The old European woad vat process is that used in all our establishments. Mr. Henderson of the Washington Mills, whose experience as a practical dyer of wool is exceptionally large, informs us that he has found no work so instructive upon this process as Napier’s “Chemistry of Dyeing” (published by Henry Carey Baird, of Philadelphia, 1869). Napier’s description of the process is extracted from Dumas’s “Lectures on Dyeing.” The appreciation expressed by so competent a judge induces us to reprint Dumas’s description in an appendix to this article. That we may give at least a general view of the whole subject, we will proceed to consider indigo in some relations not yet adverted to. In Part I. of our notes we have treated only of the application of this substance in dyeing by means of reduction through the indigo vat. Indigo may be applied by means of reduction in the printing of fabrics, as well as in dyeing them. A true scientific arrangement would compel us next in order to consider this other application of indigo by means of reduction. But the more natural and practical order is to pursue the subject of dyeing, and to consider next the applications of the derivatives from indigo in dyeing proper. SULPHURIC DERIVATIVES,—SAXON BLUE, &C. The powerful action of sulphuric acid upon indigo, and the bright and lively blue color thereby produced, had been observed by chemists long ago; but no person appears to have applied this color upon cloth, until it was done about the year 1740, by Counsellor Barth, at Grossenhein, in Saxony. The vividness of the dye, and the facility with which it was applied, brought it into great vogue under the name of Saxon blue, from its origin. Its popularity in former times is evinced by the words of the old song, “The Blue Bells of Scotland:”— “In what clothes, in what clothes is your Highland laddie clad? His bonnet’s of the Saxon blue, his waistcoat of the plaid.”[6] ------------------------------------------------------------- [6] First sung by Mrs. Jordan, about the year 1799. ------------------------------------------------------------- The Saxon blue consists simply of a solution of indigo, the Guatemala blue indigo being preferred, in sulphuric acid suitably diluted with water. The result of this reaction is not a single chemical substance, but two acids giving different tints, one called _sulpho-purpuric_ acid or _phenicine_, and the other _sulpho-indigotic_ acid; the first giving to wool a reddish-violet color, and the other a pure blue. A third compound has been indicated by Berzelius, the nature of which has not been determined. Whether one or the other of the two named acids, or the two combined, shall be produced by the reaction between the sulphuric acid and the indigo, depends upon the duration of the contact, the temperature of the mixture, and the nature and proportion of the acid used. Persoz gives the following general receipt:—