taken to render them accessible to the English student. Compared with Dr. Maclaine he will appear to great advantage. That learned person acknowledges he took "considerable liberties with his author; and often added a few sentences." Mr. Vidal seems to have indulged in no such liberties. He has faithfully preserved the sense and character of the original, without any sacrifice of the genius or idiom of the English tongue. We have detected some little blemishes that have escaped Mr. Vidal's diligence, as for instance, transmitting of, propend. Vol. I. pp. 180, 185. Antoninus, Antonine, the which, Vol. II. pp. 61, 371. We hope Mr. Vidal will meet with sufficient encouragement to accomplish his proposed undertakings, namely; a Supplementary volume of Remarks on these Commentaries-a Life of Mosheim--and a Version of some of his Dissertations, as well as of his Notes on Cudworth's Intellectual System of the Universe.

Art. II. Elements of Agricultural Chemistry, in a Course of Lectures for the Board of Agriculture. By Sir Humphrey Davy, LL.D. &c. 4to. pp. 324. Appendix lxiv. Price 21. 2s. Longman,

and Co. 1813.

IF there are many who will submit to hear what they would never be at the trouble to read, there is certainly a greater number who must be content to read, because they never can have an opportunity of hearing. The convenience of the former class is admirably consulted by the numerous popular lectures on every subdivision of the circle of sciences, which contribute to keep certain apartments in our public institutions well aired during the season, and enable the rustic literatus, to whom reading ten pages would be absolute fatigue, by well employing a single winter's campaign in the metropolis, to spout arguments on the merits of the Greek tragedy, to explain the affinities of oxygen and hydrogen, to decide upon the merits of the Huttonian and Wernerian systems, to descant on the Aeolodoric and Myxolydian modes of music, to lay down rules for the establishment of schools and fish-markets,-in a word, to speak like an Encyclopedia, without burthening his shelves with a volume on any of these subjects. Far be it from us to object to these retail shops of wisdom. We bear in mind didicisse fideliter, &c. and wish nothing better than that British Platos might perambulate Smithfield on St. Bartholomew's day, lecturing on Agricultural Chemistry, or even mnemonics, in lieu of the scene which the Right Worshipful the Lord Mayor is wont to open with due ceremony on the anniversary of that Saint.

But we really think it a great pity that these banquets of à la mode learning should benefit no more than the company that can sit round a lecture-room,-should last no longer than the memories of an often inattentive auditory take to digest them, and then leave not a wreck behind, but what, by being converted into succum et sanguinem, helps to fatten their crassa Minerva: numbers, meanwhile, who stand no less in need of instruction, are compelled to ransack libraries, make experiments for themselves, collect the experience of their neighbours, and, in short, to distil their knowledge drop by drop from an alembic full of lumber. To such persons, the appearance of a quarto of the quintessence, ready prepared for use, must be exceedingly acceptable. And since drill-husbandry and sheep-shearing have become objects of fashion, as well as of science, experience, and utility, it cannot but give an incalculable grace to the conversation of such of the plough and harrow cognoscenti, as cannot conveniently come to town and hear Sir Humphrey, to quote Sir Humphrey's opinion on fallowing and burning, and to cite Sir Humphrey's analyses of cow-dung and sheep-dung.

However, though the volume before us certainly will hold out to the superficial and the shallow, an irresistible temptation to plume themselves with learning which would never otherwise have fallen to their share; we by no means imagine that this is the only effect the lectures were calculated to produce when delivered, or that the sole advantage of this publication will be to encourage agricultural pedantry. Whenever a branch of science attracts more than ordinary attention, and is cultivated by a large proportion of the wise or the good of a community; there will be a class of men with memories sufficient to acquire its peculiar phrases, and fortunes sufficient to prevent their being laughed at to their faces, who will strut and swagger, and fancy they have made the world believe, that they are the matadores whose finishing stroke must complete the whole. The highly respectable body of Gentlemen farmers, who by the most strenuous industry, the most patient research, and the most disinterested experiments, have raised the science of agriculture in Britain to a degree of eminence which other countries and other times could neither aim at nor credit, are not without a numerous retinue of this description; and much of the volume before us will unavoidably fall a prey to their rapacity for sound instead of substance: but such as have a better title to the name of agriculturists, will find it instructive and useful; and we congratulate them that the task of treating the subject of Agricultural Chemistry has been undertaken by a

person acknowledged to stand in the foremost rank of science. The opinions advanced are not crude hypotheses, voided upon the public merely to attract their attention to the lecturer, and to gratify his vanity they are the result of patient investigation matured by the experience of ten years, during which they have been in successive courses brought forward by the author, and tried, confirmed, or amended, by the observation of the enlightened auditors.

One objection certainly attaches to popular lectures on an intricate science, (and such the science in question must be termed,) from which Sir Humphrey's work cannot claim an exemption. It arises from the following cause. To discuss the subject with moderate precision, no mean degree of elementary knowledge in auxiliary branches is requisite, which must either be premised in the auditory, or conveyed to them before the matter itself is taken in hand. If the first method he pursued, the discussion will probably be unintelligible to nine out of ten of the hearers; if the latter, the time will be occupied with an imperfect sketch of what does not strictly belong to the subject, and the hearer's mind be stored with a species of knowledge very closely bordering on the superficial. The former fault circumscribes the utility of a work; the latter enhances its price by increasing its bulk, which is the case with the volume before us. The second lecture, on the general powers of matter which influence vegetation, of gravitation, cohesion, chemical attraction, heat, light, and electricity, of ponderable substances, &c. contains an able yet unsatisfactory outline of chemical science and part of the third, on the organization of plants, notwithstanding the matter which it comprizes is judiciously chosen and compressed, will hardly convey sufficient ideas on the subject to those who are not acquainted with the author from whom Sir Humphrey has principally drawn his information: to seek it will be superfluous. But we willingly break off these observations, to present to our readers a slight delineation of the more prominent features of this valuable performance.

The first lecture contains a Prospectus of the course; and shows how extensively Chemistry is capable of illustrating agriculture, by inquiring into the elements of both minerals and vegetables, by analysing the various secretions and productions of plants, by determining the nature of soils, ascertaining the influence of the atmosphere upon germination and vegetation, and exploring the causes of the fecundating power of manures.

The subject of the second lecture we have already indicated: it concludes with an enumeration of all the substances esteemed elementary, according to the present state of che

mistry, with their specific gravities, and numbers representing the quantities in which they enter into combination. Of these however, the major part have little or no connexion with agricultural chemistry.

The elements found in vegetables, are very few. Oxygene, hydrogene, and carbon, constitute the greatest part of their organized matter. Azote, phosphorus, sulphur, manganesum, iron, silicum, calcium, aluminum, and magnesium likewise, in different arrangements, enter into their composition, or are found in the agents to which they are exposed; and these twelve undecompounded substances are the elements, the study of which is of the most importance to the agricultural chemist.

The doctrine of definite combinations, will assist us in gaining just views respecting the composition of plants, and the economy of the vegetable kingdom; but the same accuracy of weight and measure, the same statical results which depend upon the uniformity of the laws that govern dead matter, cannot be expected in operations where the powers of life are concerned, and where a diversity of organs and functions exists. The classes of definite inorganic bodies, even if we include all the crystalline arrange. ments of the mineral kingdom, are few, compared with the forms and substances belonging to animated nature. Life gives a peculiar character to all its productions; the powers of attraction and repulsion, combination and decomposition, are subservient to it; a few elements, by the diversity of their arrangement, are made to form the most different substances; and similar substances are produced from compounds, which, when superficially examined, appear entirely different.' pp. 47, 48.

The third lecture begins with the anatomy of plants, and proceeds to the examination of the compound substances found in vegetables, which Sir Humphrey arranges under 19 heads, viz. Gum or mucilage, starch, sugar, albumen, gluten, gum elastic, extract, tannin, indigo, narcotic principle, bitter principle, wax, resins, camphor, fixed oils, volatile oils, woody fibre, acids and alkalies besides earths, metallic oxides and saline compounds. As a specimen of the manner in which they are treated, we extract part of his account of the substance termed tannin, a vegetable produce of considerable importance, though not very generally understood.

Tannin, or the tanning principle, may be procured by the action of a small quantity of cold water on bruised grape seeds or pounded gall nuts; and by the evaporation of the solution to dryness. It appears as a yellow substance, possessed of a highly astringent taste. It is difficult of combustion, It is very soluble both in water and alcohol, but insoluble in ether. When a solution of glue or isinglass is mixed with an aqueous solution of tannin, the two substances, i. e. the animal and vegetable matters, fall down in combination, and form an insoluble precipitate.

When tannin is distilled in close vessels, the principal products are charcoal, carbonic acid, and inflammable gases with a mi

nute quantity of volatile alkali. Hence its elements seem the same as those of extract, but probably in different proportions. The characteristic property of tannin is its action upon solution of isinglass or jelly; this particularly distinguishes it from extract, with which it agrees in other chemical qualities.

There are many varieties of tannin which probably owe the difference of their properties to combinations with other principles, especially extract, from which it is not easy to free tannin. The purest species of tannin is that obtained from the seeds of the grape; this forms a white precipitate with isinglass.

Tannin is not a nutritive substance, but is of great importance in its application to the art of tanning. Skin consists almost entirely of gelatine or jelly in an organized state, and is soluble by the long continued action of boiling water, When skin is exposed to solution containing tannin, it slowly combines with that principle; its fibrous texture and coherence are preserved; it is rendered perfectly insoluble in water, and is no longer liable to putrefaction; in short it becomes a substance in chemical composition precisely analogous to that furnished by the solution of jelly and the solution of tannin.

In general, in this country, the bark of the oak is used for affording tannin to the manufacturer of leather; but the barks of some other trees, particularly the Spanish chesnut, have lately come into use....The quantity of the tanning principle in bark differs in different seasons; when the spring has been cold the quantity is smallest. On an average, 4 or 5lbs. of good oak barks are required to form 1lb of leather. The inner cortical layers in all barks contain the largest quantity of tannin. Barks contain the greatest proportion of tannin at the time the buds begin to open; the smallest in winter.

In general, skins on being converted into leather increase in weight about one third; and the operation is most perfect when they are tanned slowly. When skins are introduced into very strong infusions of tannin, the exterior parts immediately combine with that principle, and defend the interior parts from the action of the solution: such leather is liable to crack and to decay by the action of water....

A substance very similar to tannin has been formed by Mr. Hatchett, by the action of heated diluted nitric acid on charcoal and evaporation of the mixture to dryness. From 100 grains of charcoal Mr. Hatchett obtained 120 grains of artificial tannin, which, like natural tannin, possessed the property of rendering skin insoluble in water.

Both natural and artificial tannin form compounds with the alkalies and the alkaline earths; and these compounds are not decomposable by skin. The attempts that have been made to render oak bark more efficient as a tanning material by infusion in lime water, are consequently on an erroneous principle. Lime forms with tannin a compound not soluble in water.' pp. 77–82.

VOL. XI.

I i