separated from each other, and spreading indeed over the whole extent of the globe, through which these varieties are scattered, are new examples of wise contrivance, by which social and commercial intercourse is promoted, and the whole human family are united by mutual ties of interest, which have already produced many happy effects, and seem destined rapidly to produce still The present aspect of the civilized world is, indeed, in this respect, peculiarly encouraging, and seems to hold out the most delightful anticipations to the Christian philanthropist.

more.

FOURTH WEEK.-TUESDAY.

VEGETABLE SUBSTANCES USED IN TANNING.

THE art of tanning the skins of animals is of very remote antiquity. Pliny attributes the invention of leather to Tychius of Boeotia. But though so ancient, this art has only of late been brought under the consideration of scientific men, having been merely pursued by practical persons technically acquainted with its various processes.

In England, the bark of our native oak originally furnished the only substance for the tan-pit; and this material, after it had served its purpose, was then thrown out as useless. It is said that William the Third first turned the attention of English gardeners to its value for horticultural purposes, having imported the practice from Holland. Its useful qualities, however, in producing artificial warmth in hothouses, though now generally recognized and employed, were little understood in this country for many years after the period of the glorious Revolution.

About a century ago, no practical man in this country suspected that the tanning principle resided any where

except in the bark of the oak; and nothing was known of the principle itself, except that it possessed the power of preparing leather for use, and preserving it from de

cay. The growing scarcity of the material, about this period, led to an inquiry into the subject, and it was discovered, that, so far from this useful quality peculiarly belonging to the oak, it was common to many other plants, and that in other places, people availed themselves of this knowledge. Heath pulverized, gall-nuts, and the bark of the birch-tree, were found to be used in Germany; myrtle-leaves, in some parts of Italy; laurelleaves in Corsica; the bark of the willow, in Russia; tormentil-root, in St Kilda; the bark of the red mangrove-tree, in the West Indies.

M. Seguin, an excellent practical chemist, turned his particular attention to this subject, and published his experiments and discoveries in 1756, which threw on the subject the light of science. He found that there was a principle inherent in many vegetables, to which he gave the name of tannin, and on which the preservative quality of the process of tanning depends. The extract which he called by this name, is a brittle substance, of a brown colour; it breaks with vitreous fracture; does not attract moisture from the air; and is extremely astringent to the taste.

The hides of animals, when divested of the hair and epidermis, consist wholly of gelatin, a substance dissoluble in water, and forming, when combined with it, the well known jelly called glue. Tannin is likewise very soluble in water; but, by a chemical change, the analogy of which is by no means uncommon, a union of the two forms an insoluble imputrescent compound; and hence the efficacy of tannin in the preparation of leather.

The attention of scientific men being led, by these discoveries, to this curious and interesting subject, it was speedily found that tannin abounds in the vegetable kingdom. Sir Humphrey Davy, after a careful investigation of the subject, was led to the conclusion, that,

in all substances possessed of the astringent taste, there is reason to suspect the presence of tannin; and that it even exists in substances which contain sugar and vegetable acids. “I have,” he adds, “found it in abundance in the juice of sloes; and my friend Mr Poole of Stowey, has detected it in port wine."

Various experiments have been made with the view of ascertaining the relative quantity of tannin in different vegetable substances. It is intimately connected with the mysterious processes of vegetable secretion; and Mr Biggin found that similar barks, when taken from trees at different seasons, differ greatly as to the quantity of tannin which they contain. More than four and a half times greater quantity of tannin was obtained from oakbark, cut in spring, than from an equal quantity of the same kind of bark cut in winter.* Sir H. Davy observed, that the proportion of the astringent principle in bark varies considerably, according as the age and size of the trees are different. These proportions are, besides, often influenced by accidental circumstances, so that it is extremely difficult to ascertain their distinct relation to each other.

There is another vegetable matter, called by chemists extractive, which is absorbed by skins, together with tannin, and which gives them softness and durability; and it is therefore necessary to take this property into account in the process. The leather produced by means of an infusion of galls, for example, is generally found to be harder, and more liable to crack, than that prepared by an infusion of barks, from the circumstance of a less proportionate quantity of extractive matter having entered into chemical combination with the skin. It is, therefore, a circumstance worthy of remark, as forming another instance of those wise adaptations which we have so continually occasion to remark in all the operations of nature, that extractive is constantly united with tannin in those vegetable substances used in the preparation of

* Philosophical Transactions for 1803.

leather, and situated in such close contact* with it, that, in all ordinary operations, they will always be used together, except intentionally separated. They are substances of very different chemical properties, as well as applied to uses altogether different in the vegetable economy; yet they are both of great importance in the operation of converting hides into leather; and the adaptation consists in this, that they both exist in the bark; by which arrangement they have, from time immemorial, been used together, without the tanner being at all aware of it. It was only late chemical analysis, indeed, which caused them to be distinguished.

In considering this subject, therefore, with reference to a Final Cause, were we even to grant, which I am very far from doing, that the use of bark in tanning hides, is the mere result of human experience, unintended by the Creator, there still remains the remarkable circumstance I have mentioned, of the conjunction of two unallied chemical properties in the bark, both of them of essential importance to the perfection of the process, with which human reason assuredly had nothing to do.

FOURTH WEEK-WEDNESDAY.

VEGETABLE FIXED OILS.

VEGETABLE oils are distinguished into two kinds, fixed or fat oils, from which no vapour is given off at the temperature of boiling water, and volatile or essential oils, which give off vapour at or below that temperature with water, or under 320° by themselves.

* "In every astringent bark," says Sir H. Davy," the interior white bark, that is, the part next to the albumen, contains the largest quantity of tannin. The proportion of extractive is generally greatest in the middle or coloured part; the epidermis seldom furnishes either tannin or extractive matter. The white cortical layers are generally most abundant in young trees, and hence their bark contains, in the same weight, a larger proportion of tannin than the barks of old trees."- Phil. Trans, for 1803.

Those of the first class are obtained by expression, principally, if not entirely, from the fruit or seed of plants. A great variety of seeds are more or less oleaginous, more especially those of the nut kind, from all of which oil may be extracted. Many of the oils of this description are applicable to the arts, or are employed, in combustion, for producing light.

The vegetable oil most known and esteemed, is that expressed from the olive. This tree, which now appears a native of Italy, luxuriating in a genial soil and climate, is not, however, indigenous to that country. Pliny, who largely discourses on the olive tree, and its produce, notices its first introduction; whence Gibbon remarks, "The olive, in the western world, followed the progress of peace, of which it was considered as a symbol. Two centuries after the foundation of Rome, both Italy and Africa, were strangers to that useful plant. It was naturalized in those countries, and at length carried into the heart of Spain and Gaul. The timid error of the ancients, that it required a certain degree of heat, and could only flourish in the neighbourhood of the sea, were insensibly exploded by industry and experience."*

The olive and the cornel are the only trees in which oil is expressed from the pulpy part of the fruit, and not from the seeds or nut alone. The oil obtained from the kernel of the olive, is supposed to become rancid sooner than that obtained from any other part; and, therefore, in producing the best oil, care is taken that the stones are not cracked in the preliminary process of bruising the fruit nor are they subjected to such pressure as would produce this effect at first, when the best oil comes over. The press used for this purpose, is of a very simple construction. The fruit having been bruised by the action of a millstone, is transferred to the trough of a screw-press; and, after as much oil is obtained as can be extracted by the degree of pressure given, hot water is poured on what remains in the trough. The whole is

* Decline and Fall of the Roman Empire, chap. ii.