Blood, the most copious fluid in the animal body, and essentially necessary to the preservation of life: it is generally of a red, but in most insects, and in all worms, of a white colour.

Thehuman body is, by Dr. KEILL, supposed to contain at least one half of its weight in blood ; including in this computation all that exists in the lymphatic ducts, nerves, or any other vessel. This computation, however, is exaggerated 5 and we believe that the greatest quantity in a full-grown adult, seldom exceeds thirty pounds weight. Its most remarkable property is that of incessantly circulating in the cavities of the heart, arteries, and veins, while the animal is alive. Although Hippocrates appears to have possessed a faint idea of this admirable process, when he says, "that all the blood vessels spring from one; and that this one has neither beginning nor end} for where there is a circle, there can be no beginning;" yet as he was not acquainted with the office of the valves, he could neither comprehend, nor demonstrate, the circulation of the blood. This most important of all discoveries in physiology, was reserved for the immortal Harvev. who first ascertained the true nature and uses of the valves, and about the year 1616, • taught, in his Lectures at Cambridge, that justly admired docfrine, the substance of which he published in 1028. He proved that, in most animals, the blood circulates in arteries and veins, and through the medium of one, two, or more hearts (see Animal Kingdom) ; that in arteries it moves from the trunk to the branches ; and that, meeting there with the branches of veins, it returns in a languid state to the heart; that the heart communicates a new impulse, and propels it to the trunk of the arteries ; and that by these, the thickness of their coats, exerting muscular force, again drive it into the veins. - Valves are situated in every part of this circulating course, in order to prevent the return of the blood.

The colour of this fluid in the arteries is of a florid hue ; but somewhat darker in the veins, except in those of the lungs, in which it is of a lighter cast. When exposed to the open air, the blood gradually separates into two parts, namely, the serum, or a yellowish, sometimes greenish fluid, and the cras-samentum, or cake, which resembles a red mass swimming distinct-ly on the top. The latter contracts greatly in its dimensions, and increases in solidity; properties which depend on the state of the individual at the time when the blood is drawn. Hence, in vigorous persons, when attacked with an inflammatory disease, the solid part is so tough that it resembles a piece of flesh, and has therefore been called the buffy coat; whereas, in other diseases, it is very soft and tender, breaking in pieces on the slightest touch. By chemical ana- , lysis, it discovers the same principles with other animal substances yielding in distillation a volatile spirit, a great quantity of phlegm, and fetid oil; lastly, there remains a charred matter, which, when burnt in the open air, leaves a white earth similar to calcined hartshorn. According to some chemists, however, it contains both an acid and an alkali. But the most remarkable circumstance in the blood, is its texture, which consists of millions of red globular particles, or more properly, as Mr.HEWSON calls them, Hat vesicles, each of which has a little solid sphere in its centre. He observes, that they are flat in all animals, of very different sizes in different creatures, and impart to the blood its red colour. In man, they are small, perfectly flat, and appear to have a dark spot in the middle. To see them distinctly, he diluted the blood with fresh serum. Their shape he supposed to be of great importance, but it can be altered with a mixture of different fluids. By a determinate quan-tity of neutral salt contained in the serum, this fluid is adapted to preserve those vesicles in their flat shape ; for, if mixed with water, they become round, and dissolve perfectly, but on adding a little of any neutral salt to the water, they remain in it without dissolving, or any alteration of their form.

The uses of the blood in the animal economy are so various and important, that some have not scrupled to maintain that it is possessed of a vital principle, from which the life of the whole body is derived. This opinion was formerly entertained by Harvey, and has lately been revived and supported, with many ingenious, though inconclusive, arguments, by John Hunter. Yet so much is certain, that the blood stimulates the cavities of the heart and vessels to contract, that its circulation contributes to generate the heat of the body, and propagate it to the remotest parts; in short, that it nourishes every part, and supplies all the secretions, which, without exception, are separated from the blood. Hence it forms the bones, ligaments, tendons, membranes, muscles, nerves, vessels, and the whole organized body.

The blood is of different degrees of viscidity in different animals, and even in the same creature, at different times. It always pos-sesses a considerable degree of te -nacity; which, however, is remarkably greater in strong than in weak animals : thus, the blood of bulls was used by the ancients as a poison, on account of its extreme viscidity, which renders it totally indigestible by the human stomach.

Animal blood was formerly held in great esteem, as a medicine in various diseases ; for instance, the blood of goats and some other creatures was employed by the followers of Galen, and recommended even by the late Dr .Mead in pleuritic attacks. But at present the principal use of blood is confined to the arts, for making Prussian blue ; sometimes for clarifying certain liquors; and very large quantities are used in the manufacture of loaf sugar. In horticulture, it is recommended as an excellentmanure, when poured in spring on the roots of fruit-trees, having previously removed the soil round the trunk : thus employed, it promotes the growth of the tree, and enriches fruit. A mixture of blood with quick-lime, forms an exceedingly strong cement, and has therefore been used in preparing chemical lutes, as well as in making the floors of common farm-houses, and other humble habitations. For the latter purpose, a mixture of clay, ox-blood, and a moderate portion of sharp sand, beaten well together and uniformly spread, produce a neat, firm door, and of a beautiful colour.

Whether blood really affords nourishment, has been doubted by some, and affirmed by others. In our opinion, it contains little or no alimentary matter ; and though it may be digested by very powerful stomachs, it might be more advantageously employed in manuring the soil. In hot climates in particular, it is highly alkalescent, and was therefore wisely prohibited to the Israelites. When blood was used as a common article of food in this country, the scurvy not only prevailed more generally than at present, but it was a more violent and obstinate disease.

Travellers inform us, that in some countries the savage natives are -accustomed to intoxicate themselves by drinking the warm blood of animals. This barbarous practice, with its consequent effect, apparently confirms John Hunter's opinion, that this fluid is the immediate reservoir of the vital principle; and the inebriating quality of the blood certainly deserves the farther researches of the chemical philosopher. Several expressions in Scripture also tend to countenance the conjecture of this acute inquirer.- See Transfusion.