into an insoluble salt, of an intense black color. Dissolved in tannic acid to the condition of a soluble salt, in proportions which vary according to the degree of hardness to be given to the wood, it is rapidly transformed under the influence of air, and is deposited in the cells of the wood in a solid state, which it petrifies, so to speak, thus increasing the preservative effects of the tannic acid.
Thus we find resolved, in a manner simple and practical, the question of the introduction of an insoluble salt into wood. We may operate by injecting successively tannic acid, and then a soluble salt of iron; or, by means of a single operation, inject, sheltered from the air, the tannate of protoxide of iron, prepared in advance.
Results of experiments demonstrate the efficacy of this system. In fact, we frequently find in ferruginous soils very old oaks of a black color, and in a state of perfect preservation. I will cite an example that is quite remarkable.
In 1830 there were found at Rouen pieces of oak-wood, from some piles of a bridge built in the year 1150. This wood resembles ebony, of which it has acquired the hardness and the color. Chemical analysis has demonstrated that this modification was owing to the presence of tannate of peroxide of iron (Berthier). Argument and experience, therefore, agree in deciding in favor of the process which I propose.
The bark of most trees, the young branches and leaves, especially of the oaks, birches, elms, sumacs, chestnuts, and walnuts, the roots of the tormentil, and snake-weed, the green shells of horse-chestnuts, and the extracts of exotic woods, contain a large proportion of tannin, to which they owe their astringent properties.
The First Cost of the Materials used for Injection.—In the present state of industry, we can procure tannin at one franc, at most, per kilogramme, in the form of extracts of wood that are employed in dyeing; but we may remark that these products, employed now only for dyeing and tanning, utilize only a very small quantity of the resources of the vegetable kingdom. There is no doubt that a considerable consumption of this product would lead to the establishment of numerous factories, principally in the poor regions, where chestnut and other suitable kinds of trees occur. The industrial wealth of the country would thus be augmented, and, at the same time, the price of this new product would be considerably reduced. Even at the present price, six hundred grammes being sufficient for the injection of one sleeper, the cost of the tannic acid would not exceed 60⁄100 of a franc per sleeper. As to the salts of iron, they are so low in price that it is hardly worth while to estimate their cost. The salts of the protoxide of iron, soluble in tannic acid, carbonate, sulphate, protochloride, and pyrolignite, are easily procured or prepared. The pyrolignite, which seems the most convenient, is worth twenty francs the hundred kilogrammes, and, at a standard of 20° Baumé, contains about seven per cent, of iron. Of this, tannic acid neutralizes twelve
