William Inglis Clark in 1879 submitted to the Edinburgh University a thesis entitled "An Attempt to Place the Manufacture of Ink on a Scientific Basis," and which very justly received the commendation of the University authorities. His researches and rational deductions are of the greatest possible value judged from a scientific standpoint. The introduction of blue-black ink is a phase of the development towards modern methods which he discusses at much length. The object of adding a dye in moderation, he asserts, is to give temporary color to the ink and where indigo-paste is used, it has been assumed that it kept the iron gallo-tannate in solution, whereas any virtue of this kind which indigo-paste possesses is more likely due to the sulphuric acid which it contains than to the indigo itself. The essential part of the paste required is the sulpho-indigodate of sodium, now commonly called indigo-carmine. He further remarks that the stability of an ink precipitate depends upon the amount of iron which it contains and which on no account should be less than eight per cent; he adds rightly, if gallic acid be preferably used in substitution for tannin, "no precipitate is obtained under precisely similar conditions." This point followed up explains in a measure why a gall infusion prepared with hot water is not suitable for a blue-black, while a cold water infusion is. In the latter case a comparatively small percentage of tannin is extracted from the galls, while much is extracted with hot water and the consequence is, on adding the indigo blue the color is not brought out as it should be. Substantially the same thing occurs with ink made with the respective acids, although the blue color remains for a time unimpaired in the tannin ink, apparently due to the fact that ferrous-tannate reduces indigo blue to indigo white, a change which the low reducing power of ferrous- gallate does little to effect. The vegetable matter in common inks facilitates the destruction, or rather alteration and precipitation of the indigo, for the dye appears in the iron precipitate and may be extracted from it with boiling water. Dr. Clark's investigations seek to demonstrate the superiority of tannin and gallic acid over infusions of the natural galls, and he undertakes to determine the correct ratio of tannin and sulphate of iron to be used as ink. His experiments in this line show that: 1. The amount of precipitate increases as the proportion of iron to tannin is increased. 2. The composition of the precipitate is so valuable as to preclude the possibility of its being a definite body. Increase of iron in the solution has not at first any effect on the composition of the precipitate, but afterwards iron is found in it in greater but not proportional amount. 3. At one point the proportions of iron in the precipitate and in solution are the same, and this is at between 6 and 10 parts of iron to 100 parts of tannin.