The coagulum does not soon lose its form or colour; but in a warm temperature is quickly deprived of both. If removed from the serum and placed in a water bath, its consistence is increased, and serum drops from it: if put in warm water, the fluid assumes a milky hue, and a scum arises on the top, both owing to the serum either dissolved or coagulated. In short, a portion of this substance accompanied the albumen with its attendant soda. The albumen, we must add, was found to be the portion most affected by disease. It separated sooner, and was less firm, but in no determined ratio to the violence or the nature of the complaint.
We have observed, that, in various experiments, some remaining thready substance, some unconquerable coagula, remained. In fact, dilution will prevent the coagulation of the albumen, but not of the portion we are next to speak of, the fibrin of the blood. It is thus styled because it concretes in fibres, and is found to be the most animalised portion of the vital fluid; that is, it contains the largest proportion of azote, and is even found to contract on the Galvanic stimulus. In short, when it has assumed the solid form, it resembles in every thing but in colour the muscular fibres; and it is a singular phenomenon to remark this intermediate step between a fluid and a solid, between matter in its common form and an organised body. This subject we must in future consider.
The fibrin is separated by inclosing the coagulum in a bag, by agitating and rubbing it between the hands in a vessel of water; thus separating all the soluble parts. It is obtained also by agitating the blood with the hands, or any instrument, when first drawn from the veins. In this way it adheres to either that is employed. Authors have supposed, that in the agitation of the circulation the fibrin is deposited, and forms the muscular fibres. This, however, is a refinement which will not bear examination. The juxta position of nutritious matter is carried on in the minutest elements; and, as in crystallization, we see only the effects of an infinite number of added molecules. Besides, we shall find reason to think that the embryo contains every organic part of the perfect man; and that the difference consists in the addition of matter not organised, interposed between the truly original organs. We have no instance of a muscle being reproduced. The ends are united by a firm ligamentous substance, and the motion of an injured part is thus preserved But to return:
The coagulation is supposed to depend on the life of the blood, an opinion which we shall afterwards consider, or, in other words, on the irritability of the fibrin depending on its life. This idea of Mr. Hunter is, however, unnecessary; for the aroma of the blood may have, and very probably has, the power of hindering its coagulation in the body; a power, however, which is occasionally lost, since the albuminous and fibrous portions coagulate in many diseases. The basis of the crassamentum, which is the fibrin, appears to be a white solid elastic substance, more heavy than the serum. It is insoluble in water and alcohol; and contains a larger proportion of azote than any other portion of our fluids.
The colouring matter is that portion of the blood which, on the first employment of microscopes, excited the attention of observers. They found that it depended on red particles; but respecting their shape philosophers differed. Lewenhoeck described them as circular; and remarked, that, in passing into a vessel of a somewhat less diameter, they assumed an elliptical form; and that when brought to the orifice of an artery still smaller, they passed it, leaving the transparent fluids only to circulate through it. Haller does not greatly differ from this account. Mr. Hewson, probably misled by an optical illusion, considers them as flat, having a vesicle in the middle, containing a solid central particle. We have no reason to suppose this portion of the blood, from any office it can perform in the animal economy, would require a structure so complicated and almost organised. Dr. Wells, however, in the Philosophical Transactions for 1797, from a difficulty of explaining some of the chemical affinities of the red part of the blood, is inclined to adopt Mr. Hewson's idea; and the microscopical observations of father Torre seem to support it, though other observers confirm Lewenhoeck's description.
Mr. Cavallo, repeating father Torre's observations with similar lenses, saw the same appearances; but, following in his reasoning the laws of optics, drew a different .conclusion. On the whole, he finds them spherical, or nearly so, consisting of double spheres; the light thrown on the internal having seemingly misled Mr. Hewson. Water, he found, dissolved them, though it loses this property if impregnated with common salt or nitre, or by the addition of a small proportion of vitriolic acid. Diluted marine acid did not dissolve them, but deprived them of their colour. Vinegar was a solvent, though inferior to water; and serum or urine would dissolve them after some days. When once dried or dissolved, they never recovered their shape; and when much blood had been lost, these globules were not soon again supplied. The red colour of the blood was long supposed to depend on iron; and M. Parmentier, with his associate, having found that iron, oxygenated by any means to a certain point, was capable of being dissolved by a fixed alkali, and imparting a red colour to a fluid, concluded with great reason that the redness of the blood was owing to this metal. Two scruples of iron were found in a pound of blood; and, if twenty-five pounds of this fluid be allowed to a man of a middle age and size, the whole amount of the iron will amount to nearly three ounces. The quantity of red bipod is probably underrated; and, at least, one half must be added, so as to make the quan-lily of iron more than four ounces. Dr. Wells' objections to the colour being derived from iron, in the vo-lume just mentioned, are strong and almost convincing; yet the arguments taken from chemical phenomena, in fluids less complicated than those of the blood, cannot be allowed to have the same weight as they would have in simpler fluids. They are too long cither for an abstract, or for our consideration in this place; but when we reflect that the red colour is confined to portions found on inspection to be distinct, that we cannot examine these except when in a great measure broken down and mixed with fluids of very different qualities, we should be very cautious of deciding on their nature, from the result of experiments on solutions so essentially different. The red globules, for instance, are said not to be oily because they unite with water; but are we certain that the soda they contain may not occasion their solubility? or, on the other hand, who can say that we have yet ascertained all the aerial contents of this fluid, or the effects of those which it does contain ? We mean not to urge either idea; but these suggestions are sufficient to apologise for our present scepticism, and for our considering the problem as yet requiring further investigation. We omitted to observe, that Abilgaard (Annales de Chimie, N° 106) found a larger proportion of carbone in the venous than in the arterial blood, and that the carbone of the arterial was lighter than that of the venous.