Page:Popular Science Monthly Volume 5.djvu/241

Rh gradually deposited, as the timber matures, lignine, a hard concretion which makes up the greater part of the volume in hard woods, such as ebony, guaiacum, oak; in knots, the shells of nuts, etc. Wood, further, contains sap, which holds in suspension gummy materials, nitrogenous and albuminous substances, coloring-matters, etc.; these are the elements of decay in wood. Inasmuch as they offer to animal and plant-parasites an abundance of agreeable food, they undergo decomposition more or less rapidly, determining, by their own decay, the decay of all the other elements of the wood.

If we succeed in expelling these essentially putrescible materials, or fixing them in unalterable combinations, we thus prevent their decomposition, and, in consequence, that of the other more resistant organic substances, cellulose and lignine. A certain number of observed facts seem to demonstrate that the action of tannin upon vegetable tissues must be analogous to that which is exercised upon animal tissues—operating in the vegetable tissues a kind of tanning, which will have for result the formation of hard and imputrescible albuminous tannates, quite analogous to the gelatinous tannate products in the tanning of skins.

Thus, the sizing of wines is effected as well with the white of an egg (albuminous matter) as by isinglass (gelatinous matter). The tannic acid contained in the wine forms with either of these materials a solid net-work, which envelops and precipitates the lees to the bottom of the cask. An infusion of oak-bark preserves the skins of animals, and is also employed to protect from rotting the nets of hunters and fishermen. In fine, among exotic or indigenous woods, soft or hard, the most resistant are the richest in tannic acid, as among indigenous woods the oak and the chestnut; the first remarkably hard, the second soft enough, are both preserved during many years, and we cannot doubt that this is owing to the influence of the tannic acid with which they are impregnated, which, after the cutting of the wood, reacts upon the azotic and albuminous materials contained in their capillary vessels. We may therefore conclude that the injection of a solution of tannic acid into the various species of woods will assure their preservation, by putting them, in a chemical point of view, in conditions analogous to those in which we find the oak after it has been felled.

But it is not enough to protect soft wood from rotting; it must also be hardened; and though by the action of tannic acid we in some measure attain this end, the soft materials in the sap-vessels being transformed, still it is important that we should give to woods that are naturally soft a higher degree of hardness, in order to fit them for industrial uses.

I accomplish this object by the intervention of the remarkable property of tannate of iron, which, perfectly soluble, and even colorless, in a state of protoxide, is, under the influence of air, transformed