“With respect to the chemical constitution of ink, I may remark that, although as usually prepared it is a combination of the metallic salt or oxide with all the four vegetable principles mentioned above, yet I am induced to believe that the last three of them, so far from being essential, are the principal {309} cause of the difficulty that we meet with in the formation of a perfect and durable ink.
“I endeavored to prove this point by a series of experiments, of which the following is a brief extract.
“Having prepared a cold infusion of galls, I allowed a portion of it to remain exposed to the atmosphere, in a shallow capsule, Until it was covered with a thick stratum of mould, the mould was removed by filtration, and the proper proportion of sulphate of iron being added to the clear fluid, a compound was formed of a deep black color, which showed no further tendency to mould, and which remained for a long time without experiencing any further alteration. Another portion of the same infusion of galls had solution of isinglass added to it, until it no longer produced a precipitate; by employing the sulphate of iron, a black compound was produced, which, although paler than that formed from the entire fluid, appeared to be a perfect and durable ink.
“Lastly, a portion of the infusion of galls, was kept for some time at the boiling temperature, by which means a part of its contents became insoluble; this was removed by filtration, when, by addition of sulphate of iron, a very perfect and durable ink was produced.
“In the above three processes, I conceive that a considerable part of the mucilage, of the tan, and the extract, were respectively removed from the infusion, whilst the greatest part of the gallic acid would be left in solution.
“The three causes of deterioration in ink, the moulding, the precipitation of black matter, and loss of color, as they are distinct operations, so we may presume that they depend on the operation of different proximate principles.
“It is probable that the moulding more particularly depends ©n the mucilage, and the precipitation on the extract, from the property, which extractive matter possesses of forming insoluble compounds with metallic oxides.
“As to the operation of the tan, from its affinity for metallic salt we may conjecture that, in the first instance, it forms a {310} triple compound with the gallic acid and the iron, and that in consequence of the decomposition of the tan, this compound is afterwards destroyed. Owing to the difficulty, if not impossibility, of entirely depriving the infusion of galls of any one of its ingredients without in some degree affecting the others, I was not able to obtain any results which can be regarded as decisive; but the general result of my experiments favors the above opinion, and leads me to conclude that, in proportion as ink consists merely of the gallate of iron it is less liable to decomposition or to experience any kind of change. The experiments to which I have alluded above, consisted in forming a standard solution by macerating the powder of galls in five times its weight of water, and comparing this with other infusions, which had either been suffered to mould, from which the tan had been extracted by jelly, or which had been kept for some time at the boiling temperature, and by adding to each of these respectively both the recent solution of the sulphate of iron, and a solution which had been exposed for some time to the atmosphere.
“The nature of the black compound produced was examined, by putting portions of it into cylindrical jars and observing the changes which they experienced with respect either to the formation of mould, the deposition of their contents, or any change of color. The fluids were also compared by dropping portions of them upon white tissue paper, in which way both their color and their consistence might be minutely ascertained. A third method was to add together the respective infusions, and the solutions of the sulphate of iron, in a very diluted state, by which I was enabled to form a more correct comparison of the quantity and of the shade of the coloring matter, and of the degree of its solubility.
“The practical conclusions which I think myself warranted in drawing from these experiments are as follows:—In order to procure an ink which may be little disposed either to mould or to deposit its contents, and which at the same time may possess a deep black color not liable to fade, the galls should be {311} macerated for some hours in hot water, and the fluid filtered; it should then be exposed for about fourteen days to a warm atmosphere, when any mould which may have been produced must be removed. A solution of sulphate of iron is to be employed which has been exposed for some time to the atmosphere, and which consequently contains a certain quantity of the red oxide diffused through it. I should recommend the infusion of galls to be made of considerably greater strength than is generally directed, and I believe that an ink formed in this manner will not necessarily require the addition of any mucilaginous substance to render it of a proper consistence.