all ordinary operations, they will always be used together, except when 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 had assuredly nothing to do. FOURTH WEEK-THURSDAY. VEGETABLE FIXED OILS. VEGETABLE oils are distinguished into two kinds, fixed or fat oils, from which no vapor is given off at the temperature of boiling water, and volatile or essential oils, which give off vapor at or below that temperature with water, or under 320° by themselves. 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 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. 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 errors of the ancients, that it required a certain degree of heat, and could only flourish in the neighborhood 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 screwpress; 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 then subjected to a stronger pressure, and a coarser product is procured. When all the oil is entirely expressed, the refuse is used as fuel. The best soap is made of olive oil, mixed with alkalis; but its preparation for this purpose is not so carefully III. * Decline and Fall of the Roman Empire, chap. ii. IX. conducted as when it is intended to form an ingredient of food. Spanish soap, known by us as Castile soap, is made with olive oil, which is also largely used for the same purpose at Marseilles. The heavy duty charged on importing olive oil into England, viz., eight guineas per ton, effectually prevents its application to soap-making there.* Its spe This oil becomes solid at 10° of Fahrenheit. cific gravity is 913. It is never used in the composition of paints, as it does not dry completely. The olive-tree produces oil abundantly; and a plantation of this in a favorable climate, is always a certain source of profit to the industrious. "The young olive plant," says Maria Graham, "bears at two years old, and in six years begins to repay the expense of cultivation, even if the ground is not otherwise cropped. After that period, in good years, the produce is the surest source of wealth to the farmer; and the tree rivals the oak in longevity; so that the common proverb here is, 'If you want to leave a lasting inheritance to your children's children, plant an olive. There is an old olive-tree near Gericomio, which last year yielded two hundred and forty quarts of oil. Yet its trunk is quite hollow, and its empty shell seems to have barely enough hold on the ground to secure it against the mountain storm." More than 4,000,000 gallons of olive oil were imported into Britain in the year 1831, very nearly one half of which was retained for home consumption. Some parts of the south of Italy may be said to be one continued olive-grove. This description particularly applies to what may be called the heel of the boot, which forms an extreme point of the Neapolitan dominions. "In a good year, and at a proper season," says a writer quoted in the Library of Entertaining Knowledge, * [If the duty on olive oil were sufficiently diminished, it would be applied to soap-making, and as more oil would be imported, the amount of duty received would probably be as great as before. At all events, it is to be hoped, that at no distant time those duties will in all countries be reduced or wholly taken off, which only prevent one country from freely enjoying the useful products of another.—Aм. ED.] †Three Months passed in the Mountains East of Rome, in 1819, by Maria Graham.' "I have counted, in the course of an afternoon's ride, as many as a hundred mules returning from Gallipoli, where they had been to deposit their unctuous burdens to different towns and villages in the Terra d'Otranto, or the more distant province of Bari. The quantity of oil required may be conceived when I state, that at one time (in the year 1816) I saw nine English, three American, two French, and six Genoese vessels, (not to mention some small craft from the Adriatic,) all waiting in the port of Gallipoli for entire or partial cargoes of it."* It would be almost endless to state all the seeds from which useful oil may be extracted. Linseed oil, the produce of flax, is of general use in painting, and in the composition of varnishes. Its chief defect is the darkness of its color. Hemp-seed also affords a very useful oil, similar in its qualities to linseed, but strongly impregnated with the peculiar odor of the plant. Next to these I may mention the sesamum, or oil-plant of the East, which is indigenous in the island of Ceylon and on the Malabar coast. It is an annual, growing about two feet high, and producing seeds of the size of those of mustard. It is raised universally throughout Asia and in some parts of Africa, where the seed is valued, not merely for its oil, but as an article of food. The oil is abundant, nine pounds of seed yielding two quarts of this substance. It is perfectly sweet, and is used for the purposes of olive oil, while it has the great advantage of not becoming rancid, though kept for years. From the kernels of walnuts, hazel-nuts, and beech-mast, and from the seeds of the poppy, oils are extracted, which are much esteemed by varnishers, on account of their transparency. The latter is also extensively used in place of olive oil. coa-nut, and some species of palm, likewise yield an abundant and useful oil well known in this country; the latter being chiefly used in the manufacture of a certain kind of soap, and the former being in very general use. The chief defect of the oil extracted from the cocoa-nut, was its congealing at the ordinary temperature of the atmosphere in England. Recently, however, a discovery * Library of Entertaining Knowledge,—Vegetable Substances, p. 201. The co has been made of a method of separating the concrete matter from the liquid part of the oil, by which means a pale, limpid, tasteless fluid is produced, possessing the property of combustibility in an equal degree with the best sperm oil, while the solid unctuous substance is applicable to the manufacture of candles, and to other uses in which fatty matter is employed. This discovery will probably be of considerable importance to the inhabitants of Ceylon, where the cocoa-nut tree is cultivated in such abundance. The utility of its oil adds to the valuable properties of this wonderful tree, which, in a former volume, were shortly described. FOURTH WEEK-FRIDAY. VEGETABLE OILS, ESSENTIAL AND EMPYREUMATIC. WHILE the grosser oils are extracted from the fruit and seed, essential oils are for the most part obtained from the leaves and flowers, or from the most odorous part of the plants. In umbelliferous plants, however, the oil is found in the seeds. In the geum or avens, the root affords it; and in labiated plants, it is contained in the branches and leaves. The essential oils obtained from flowers are generally of a very delicate nature; and the odorous matter of some flowers is so subtile, that it can only be obtained by impregnating other substances with it. Of this description of flowers are the tuberose, jasmine, honeysuckle, sweetbriar, and others having strong scents, but yielding little or no oil by distillation. Ben-oil, extracted from the seeds of a tree growing in the Indies, Ceylon, and Egypt, which is perfectly inodorous, and not liable to rancidity, is an excellent agent for retaining and imparting the perfume of the sweet-smelling flowers.† * [A labiated flower is one which has an upper and lower lip; like the flower of the mint, catnip, Gerardia, &c.—Am. ED.] [The ben-nuts, from which the ben oil is expressed, are the seeds |