ulin, by my analysis, is composed of C24 H21 021, and lichenin, by the analysis of Guerin Varry, of C1 H11 010

11

To the physiologist starch is a highly interesting substance as an important element of food, not only of animals, but also, in one sense, of vegetables. "Its ready convertibility," observes M. Liebig, "without change of composition, into soluble forms, such as dextrin and sugar, adapts it admirably for carrying on those changes which occur in the juices of vegetables. It is stored up in the seeds, roots, and pith of plants, and by its decomposition furnishes the materials for many of the most essential vegetable products. It also serves as a most important element of the food of animals, furnishing not indeed the means of increase of mass, but the materials for keeping up respiration and supplying the animal heat. The fats and fixed oils of the vegetable as well as the animal kingdom are in all probability derived principally from the deoxidation of starch."

Formerly, starch was subject to a duty of threepence farthing per pound, and its manufacture was consequently placed under the surveillance of the excise. Before being put into a stove to be dried, each paper of starch was sealed or stamped by the attendant officer, and the infringement of this regulation was attended with the infliction of a penalty on the manufacturer of one hundred pounds. The penalty for knowingly selling any kind of starch with a forged or counterfeit stamp, &c., was five

hundred pounds (Burn's Justice of the Peace, tit. Starch).

The oppressive duty on starch, however, was abolished ten years since, in consequence of the recommendation of the Commissioners of Excise Inquiry. In the year ending 5th January, 1833, the nett produce of this duty amounted to a little more than eighty-five thousand pounds; the quantity of starch on which duty was levied being just eight million pounds.

161

TANNING.

§ I. On the properties of Tannic Acid and the astringent Vegetable Matters employed in Tanning.-II. Preparation of Hides and Skins for Tanning.-III. Modes of applying Tannic Acid to prepared Hides and Skins.—IV. Preparation of light and fancy Leathers.

THE Conversion of the skins of animals into leather, which is the object of the process of tanning, consists in the production of a chemical combination of the skin with the astringent vegetable principle called tannin or tannic acid. If simply dried without being exposed to tannin or any substance which has the power of effecting a change in the nature of the skin, corresponding to that produced by tannin, the skin readily allows the percolation of water through its pores, is unable to resist continued friction without considerable abrasion, and speedily enters into a state of putrefaction if afterwards moistened. But each of these inconveniences may be surmounted by taking advantage of the property which skin possesses in common with some other animal tissues of forming a true chemical combination with tannin. If a piece of prepared skin* is placed in an aqueous solution of pure tannin, or in an infusion of some astringent vegetable substance containing tannin, as oak-bark or gall-nuts, and allowed to remain there for a short time, it separates the whole of the

* Skin which has been cleaned, soaked in lime-water, and freed from its extraneous appendages.

VOL. II.

M

tannin from the liquid, and becomes hard, insoluble in water, almost impenetrable to water, and incapable of putrefying. By such a simple process as this, skin is converted into leather, but subsequent operations of dressing or currying are necessary to bring the leather into that state of pliability and complete impermeability to water which are essential in many of its applications. Similar, but less decided, changes may be produced in skins by impregnating them with a solution of alum or some other preparation of alumina, and also with oil or grease.

Processes for converting skin into leather have been practised in a rude manner from a very early period, and even among the most savage tribes; but it was not until towards the close of the last and the commencement of the present century that the principles of the processes were accurately developed through the researches of Deyeux, Seguin, Macbride, Proust, and Sir H. Davy, and tanners have in general been very tardy even to the present time in adopting the improvements suggested by the researches of these and other experimental philosophers. Until within the last few years, tanning has probably been more stationary than any other manufacturing process; but considerable modifications of the old method of tanning have been recently introduced, consisting chiefly in the employment of new materials. The result has not been, on the whole, an improvement in the quality of the leather, but a reduction in its price.

The leather manufacture of Great Britain is of very great importance, and ranks either third or fourth on the list; being inferior only in point of