This is the fluid contained in the arteries and veins of the human body, and is generally red; but in some smaller vessels which will not admit the red particles, a fluid apparently similar in every other respect is contained, which should also retain the name. In the vessels of insects also a white fluid circulates, which, from the uniformity of nature, we may suppose to consist of similar component parts, but it is not styled blood; and such insects are generally denominated exsanguineous. Though we sometimes employ the distinguishing epithet red blood, yet this alone deserves the appellation, and to this we shall confine our observations.

From the period when philosophy began to investigate, with particular attention, the nature of the animal fluids, the blood has been a principal object. It has been tortured with all the violence of fire; but only since chemistry enabled us to examine satisfactorily the component parts of bodies, has its real nature been understood. The experiments of MM. Parmentier and Deyeux have illustrated the properties of this my-, sterious fluid more satisfactorily than those of all their predecessors.

The appearance of blood is well known. When drawn it has a peculiar faintish smell, which adheres more tenaciously to the coagulum than the serum. Like the aroma of vegetables its nature is little known. Its specific gravity is about 1090. It unites with cold, but is coagulated by boiling water, and by concentrated acids both vegetable and mineral, which change it to a dark brown. Mild alkalis, neutrals, and lime water, render it more fluid, and of a brighter colour. Vitriolated iron and copper coagulate it. Exposed to oxygen gas its colour is heightened, but the brilliant hue soon disappears, and the blood becomes black: after this change the oxygen has no effect. Exposed to unrespirable gas it becomes black. Vasali has informed us that the electricity of the circulating blood is positive; that of the excrementitious fluids negative. Blood at rest spontaneously separates into two parts, a red coagulum,

-and a yellowish serum; unrespirablc gas impedes, and oxygen accelerates, the coagulation. It coagulates more quickly when it flows slowly; and the coagulation is long protracted when the air is excluded. The blood of the catamenia seems never to coagulate. During the coagulation heat is extricated, seemingly from the coagulated part, as the increased temperature is not found in the serum. The coagulation is prevented by agitation; and when suffered to cool during the agitation, neither alkalis nor acids w ill afterwards coagulate it. In that state, alkalis greatly heighten its colour. In a day or two, at the heat of 50° of Fahrenheit, it becomes putrid, the coagulum softens, and soon disap-mmes the appearance of a dark coloured serum, with a few remains of coagula only; and the smell of ammonia is obvious.

Blood, when distilled] affords a large proportion of hydrogen with carbonic acid gas and azote. The prussic acid also comes over, with insipid phlegm, empy-reumatic oil, and an ammoniacal salt. The coal affords carbonat and muriat of soda, phosphat oflime, and oxid of iron.

When by less violence the different parts into which the blood spontaneously separates are examined, the serum, whose specific gravity is 1.0287, is found to be coagulated by the heat of 160°, into a tender tremulous clot, from which a glutinous fluid may be squeezed, styled the serosity. In fact, the serum appears to be a watery fluid containing albumen; and, when this is coagulated, the remaining water is squeezed out, with a small proportion of animal mucilage or gelatine. That gelatine was contained in the blood was generally doubted till ascertained by Fourcroy, and afterwards by Parmentier and Deyeux. It is confined, however, to the serum. Besides these substances, the serum contains carbonat and muriat of soda, phosphat of soda and of lime. The fixed alkali renders the albumen more soluble, and is apparently combined with it, as oil is in soap. The other salts are dissolved in the aqueous fluid.

There is one substance discovered in the serum, apparently peculiar to it, that we must notice particularly; we mean sulphur. We shall find it of considerable importance in the pathology of the animal fluids, and it is a principle whose source and existence are still obscure. If the albumen, perfectly dry, be heated in a silver vessel to a high temperature, it will be blackened; or, if triturated in a glass mortar with a fully saturated solution of silver, then digested, and afterwards diluted with water, some greyish threads will be deposited, from which sulphur, in the usual way, may be extracted. Again, if fixed alkali be boiled with the albumen and water, by adding distilled vinegar, a substance, whose smell is hepatic, and which discolours silver, will be deposited. Sulphur appears too in the white of an egg; it is found in the substance of the brain; but whether formally existing, or whether produced in the operation from its proximate principles, we cannot yet discover. We must rest, however, on the fact, that sulphur, in one of these ways, exists in the animal fluids. When in a larger proportion, or more copiously evolved, it will be found to press on our notice.

The existence of gelatine, as we have said, has been lately ascertained. On coagulating the serum, and suffering it to remain in the bath, a.substance collects-on its surface, which was found by every chemical test to be jelly. Some portion of jelly is also, with great reason, supposed to remain combined with a part of the soda. The gelatine is, however, confined to the serum; and we have no reason to suppose that it varies in proportion or consistence in any known difference of the state of the constitution.

The coagulum is the next object of our attention, and it is in every view a very important one. The coagulation has been attributed to cold,"to rest, and to the density of the liquor, but it is the effect of neither; and the chief, we believe the only, means of retarding it, is the addition of neutral salts. Various authors have mentioned the effects of Glauber's salts and muriat of soda; and we remember finding the same effects result by letting the blood flow into a solution of nitre without the slightest agitation. This experiment was made under the direction, and under the eye, of Dr. Cullen.