As the immediate causes of the coagulation of the blood were so obscure, Mr. John Hunter supposed it to possess life, and styled it, with Harvey, the 'primum vivens et ultimum moriens.' Since his era, the discovery of the irritability of the fibrin has been adduced as additional evidence of this opinion. We know, however, that this fluid is continually changing, and that by nutriment primarily derived from the vegetable kingdom, it is regularly renewed. We may then ask, whether these fluids possess originally the living principle? Do the carbone and hydrogen which form the nutritious vegetable productions possess life ? Do they assume this distinction only in the grass, or when they form a part of the system of the sheep or ox ? In short, if they do not possess life originally, there is no period in which they can obtain this distinction, consistent with our present knowledge, unless we adopt the system of the Buffonian molecules organiques. Should this opinion be again assumed, we shall find it difficult to escape from the idea of life as connected .with organisation; and of an organised fluid we have no example. One of Mr. Hunter's arguments is drawn from the white and the yolk of the egg, which, by possessing a principle of life, do not putrify during incubation. No profound knowledge is, however, required at this time to show, that in this instance there is a living organised body, and that the yolk is a part of the future chick; at least surrounded with its vessels, and drawn into its abdomen as its first nourishment. Another argument is, the production of vessels in coagulated extravasated blood. But, if the idea was correct, we should find no end to protuberances of this kind, and an ecchymosis must, in time, equal or exceed the bulk of the man; for as the blood is continually adding to the mass, and fresh vessels produced without limitation, this new living excrescence may equal that from which it was primarily drawn. Indeed the determined form of every part, the limits set to extraordinary deviations in bulk or shape, show that some fixed principle regulates our form; and the various facts concur in proving the opinion lately hinted, which will be afterwards more"fully explained, that the additions in the progressive stages, from infancy to age. are of inorga-nised matter only, and that the primordial stamina are unaltered in bulk or length.

Some other arguments adduced by this celebrated physiologist are, that persons killed by lightning, who have died in consequence of violent fatigue or by blows on the stomach; in short, in every instance where the irritability of the vessels is destroyed, the blood no longer coagulates. This, however, proves only that some circumstances will destroy the irritability of both; and we know an agent which will effect this completely, viz. hydrogen gas. We have employed the term irritability indiscriminately, because it will not be now contended that it is the exclusive property of animal life. Another argument is, that vegetable bitters, mixed with blood, did not hinder the coagulation, but that a solution of opium prevented it. This argument is peculiarly weak; for, if we can distinguish any appropriate power in the vegetable bitter, it is that of destroying irritability. The effect of opium in preventing the coagulation of the blood has been repeatedly denied; and some experiments adduced by an anonymous author, whose accuracy and judgment are so generally conspicuous as to demand our concurrence, have shown, that infusions, not only of opium, but of tobacco and bella donna, have not prevented the coagulation. In short, though this system is, on the whole, plausible, it will not admit of a fair examination; and, indeed, the reasoning of Mr. Hunter on almost every point of physiology is so vague and inconclusive, as to leave some doubt whether he has not in this walk more than counterbalanced the advantages which medical science has derived from his dissections and experiments.

Such are the properties of the blood in its natural state. In diseases it admits of little change, though the humoral pathologists copiously declaim on its viscidity and tenuity, its alkaline or acid acrimony. Blood drawn in inflammatory diseases, is at first streaked with purplish lines, and soon a yellow viscid coat covers the red globules; the sides of which often rise around, giving the coagulum the form of a cup. This coat, styled from its colour the buff, is so very dense as to justify, in some measure, the idea of the disease arising from lentor. Mr. Hewson, by an obvious, though a new, remark, taught us, that blood, in these circumstances, was not more but less viscid than usual; and that this crust was owing to the slow coagulation of the blood, by which means the red particles subsided, and left the albumen and fibrin colourless. In confirmation of this opinion, if we find the upper strata of the clot more, the lower are less dense than in the natural state. It is singular that this idea had not more early occurred,and we might then have found the tenuity of the circulating mass accused, for in the blood it was supposed the causes of disease must be. We think it admits of little doubt that the extraordinary agitation, and of course the more intimate mixture, of its component parts, occasion its slow coagulation, and the buffy coat. It does not, however, follow, that this is the only cause of the same appearance. Whatever occasions an increased tenuity of the blood may have a similar effect. We thus find it in scorbutic cases, and in some species of typhus, as Dr. Lind, and MM. Parmentier and De-yeux have shown. In these instances the tenuity of the blood is not occasioned by agitation, but, in the first, by a probable increase of the proportion of neutral salts, with a deficiency of oxygen; and, in the latter, by a diminution of the proportion of fibrin or albumen: a change, however it may be explained, which is the constant consequence of debility.