BLOOD is a well known fluid, which circulates in the veins and arteries of the more perfect animals. It is of a red co lour, has a considerable degree of consis tency, and an unctuous feel, as if it con tained a quantity of soap. Its taste is slightly saline, and it has a peculiar smell. The specific gravity of human blood is, at a medium, 1.05. Mr. FOurcroy found the specific gravity of bullock's blood, at the temperature of 60°, to be 1.056. The blood, does not uniformly retain the same consistence in the same animal, and its consistence in different animals is very various. It is easy to see that its specific gravity must be equally various. When blood, after being drawn from an animal, is allowed to remain for some time at rest, it very soon coagulates into a solid mass, of the consistence of curdled milk. This mass gradually separates into two parts, one of which is fluid, and is called serum ; the other, the coagulum, has been called cruor, because it alone retains the red Co lour which distinguishes blood. This se paration is very similar to the separation of curdled milk into curds and whey. The proportion between the cruor and serum of the blood varies much in different animals, and even in the same animal in different circumstances. The most corn. mon proportion is about one part of cruor to three parts of serum. 1. The serum is of a light greenish yellow colour: it has the taste, smell, and feel of the blood, but its consistence is not so great It converts syrup of violets to a green, and therefore contains an alkali. On ex amination, lloulle found that it owes thiS property to a portion of soda. When heated to the temperature of 156°, the serum coagulates. It coagulates also when boiling water is mixed with it, but if serum be mixed with six parts of cold water, it does not coagulate by heat. When coagulated, it has a greenish white colour, and is not unlike the boiled white of an egg. If the coagulum be cut into small pieces, a muddy fluid may be squee z ed from it, which has been termed the serosity. After the separation of this fluid, if the residuum be carefully washed in boiling water and examined, it will be found to possess all the proporties of co. agulated albumen. The serum, therefore, contains a considerable proportion of al.
lumen. Hence its coagulation by heat, and the other phenomena which albumen usually if serum be diluted with six times its weight of water, and then boiled to coagulate the albumen, the li quid which remains after the separation of the coagulum, if it be gently evaporat ed till it becomes concentrated, and then he allowed to cool, assumes the form of a jelly. Consequently it contains gelatine. If the coagulated serum be heated in a silver vessel, the surfa.ce of the silver b- comes black, being converted into a sul phuret. Hence it is evident that it con tains sulphur : and Proust has ascertain ed that it is combined with ammonia in the state of a hydrosulphuret. If serum be mixed with twice its weight of water, and, after coagulation by heat, the albu men be separated by filtration, and the liquid be slowly evaporated till it is con siderably concentrated, a number of crys tals are deposited when the liquid is left standing hi a cool place. These crystals
consist of carbonate of soda, muriate of soda, besides phosphate of soda and phos phate of lime. The soda exists in the blood in a caustic state, and seems to be combined with the gelatine and albumen. The carbonic acid combines with it dur ing evaporation. Thus it appears that the serum of the blood contains albumen, gelatine, hydrosulphuret of ammonia, so da, muriate of soda, phosphate of soda, and phosphate of lime. These compo nent parts account for the coagulation oc casioned in the serum by acids and alco hol, and the precipitation produced by tannin; acetate of lead, and other metallic salts. The cruor, or clot, as it is some times called, is of a red colour, and pos considerable consistence. Its mean specific gravity is about 1.245. If this cruor be washed carefully by letting a small jet of water fall upon it, till the water runs off colourless, it is partly dis solved, and partly remains upon the scarce. Thus it is separated into two por tions : namely, 1. A white, solid, elastic substance, which has all the properties of fibrin ; 2. The portion held in solution by the water, which consists of the colouring matter, not, however, in a state of purity, for it is impossible to separate the cruor 'completely from the serum. We are in debted to Bucquet for the first precise set of experiments on this last watery solu tion. It is of a red colour. Bucquet proved that it contained albumen and Iron. litIengliini had ascertained, that if blood be evaporated to dryness by a gen kle heat, a quantity of iron may be sepa rated from it by the magnet. The quan tity which he obtained was considerable; according to him the blood of a healthy man contains about two ounces of it Now, as neither the serum nor the fibrin extracted from the cruor contains iron, it follows of course, that the water holding the colouring matter in solution must contain the whole of that metal. This watery solution gives a green colour to syrup of violets. When exposed to the air, it gradually deposits flakes, which. have the properties of albumen. When heated, a brown coloured scum gathers on its surface. If it be evaporated to dry ness, and then mixed with alcohol, a por tion is dissolved, and the alcoholic solution yields, by evaporation, a residuum, which lathers like soap in water, and tinges ve getable blues green : the acids occasion a precipitate from its solution. This sub stance is a compound of albumen and so da. Thus we see that the watery solu tion contains albumen, iron, and soda. When new-drawn blood is stirred briskly round with a stick, or the hand, the whole of the fibrin collects together upon the stick, and in this manner may be sepa rated altogether from the rest of the blood. The red globules, in this case, remain behind in the serum. It is in this manner that the blood is prepared for the different purposes to which it is put ; as clarifying sugar, making puddings, &c. After the fibrin is thus separated, the blood no longer coagulates when allow. ed to remain at rest, but a spongy, flaky matter separates from it, and swims on the surface.