The discovery of iron in the blood is generally attribut ed to Menghini ; the fact has been very fully confirmed by later observers, although they have differed very much as to the amount of the quantity of the iron. Menghini, and most of the earlier writers, appear to have very much over-rated the quantity. Berzelius informs us that the colouring matter of the blood, separated from the other parts, leaves about 1-80th of incombustible matter, of which rather more than one half is the oxide of iron. We are still quite uncertain in what state the oxide of iron exists in the blood. Fourcroy brought forward an opinion, professing to be the result of experiment, that the iron was in combination with phosphoric acid, but some late expe riments, especially those of Mr. Brande, show that this opinion is untenable. Various saline bodies, when added to the blood, produce considerable effects upon the red particles, some dissolving them, others causing them to shrink up into a small bulk, some brightening their co lour, and others rendering it deeper. Many experiments of this kind were performed by liewson, and it is probable that he. has accurately detailed the results ; but as they lead to no important conclusion, we shall not enter into any detail of them.
Perhaps the most interesting fact with which we are ac quainted respecting these particles, is the change which is produced in their colour by the contact of the atmos phere. It had been long observed that the upper part of the clot was of a bright scarlet colour, while the lower part was of a purplish red, but no one seems to have been aware of the true cause of this appearance, until Cigna of Turin announced it as depending upon the action of the air. The idea was immediately embraced by Priestley, who, after verifying the statement of Cigna, discovered that the action of the air depended solely on its oxygennus portion, while the azote, as well as the unrespirahle gases generally, had no effect of this kind upon the colour of the blood, but even, on the contrary, tended to deepen the colour of scarlet blood. Upon examining the air which had been employed in these experiments, he found that it bad lost a part of its oxygen, and was no longer capable of supporting life, the same change having been induced upon it as upon air that had passed through the lungs in the natural process of respiration. It was generally con cluded, although perhaps without any very sufficient evi dence, that the action of the air in these cases was upon the iron in the blood ; there arc, indeed, many reasons for supposing that the red globules are the part of the clot upon which the air more particularly acts ; but how this action is effected, whether the iron has any share in it, or what change it undergoes, are questions that at present we are unable to answer.
The liquid which separates from the clot, when the spontaneous coagulation of the blood takes place, is called the serum. It is a viscid fluid, of a dull straw colour, of considerable specific gravity, a saltish taste, and when warm of an odour resembling milk. It consists of water, holding in solution different animal and saline substances, the principal of which is albumen, a body which possesses the singular property of becoming solid at a temperature of about 160 degrees. This process, like the concretion of the fibrin, has obtained the name of coagulation, yet it is in fact a very different kind of operation, not indeed exactly similar to any other with which we are acquainted. If the serum, after being coagulated, be cut into small pieces, a quantity of a brownish fluid gradually drains from it, which has been named the scrosity ; this consists of wa ter, holding in solution an animal substance, different from albumen, together with a quantity of saline matter. The albumen after coagulation being no longer soluble in wa ter, by sufficient ablution the other constituents of the se rum may be removed from it. In this state, however, a large quantity of water still remains attached to it ; and if this be slowly evaporated, the albumen is converted into a hard, semi-transparent, brittle substance, which resembles membrane in all its physical and chemical properties, ex cept that it is destitute of any appearance of organization.
Albumen is coagulated by various chemical re-agents as well as by the application of heat, particularly by the mineral acids, by alcohol, by various metallic salts, and, according to the curious discovery of Mr. Brande, by the action of the galvanic pile. It is probable, however, that, in these different cases, neither the substances procured are similar, nor is the operation the same by which they are produced. Many conjectures have been formed to ac count for the action of heat in coagulating albumen, but, as we think, without affording any just explanation of it ; and, upon the whole, we are disposed to attribute it to a specific action, which is not referable to any general prin ciple.
The substance which drains from the albumen after it has been coagulated, called the serosity, exists in small quantity only, in comparison with the other ingredients of the blond, and there has been much controversy respect ing the nature of the animal matter contained in it. Cul len, who was one of the first chemists that paid much at tention to it, speaks of the scrosity as a solution of fibrin in water; and Hewson conceived it to be similar to the mucus of the lungs. Parmentier and Deyeux, in the year 1790, published an elaborate set of experiments upon the subject, from which they concluded that the animal mat ter in scrosity is jelly. Their account of it had so much the appearance of correctness, that their opinion was ge nerally adopted, until, in 1805, Dr. Bostock found that it was without foundation, and that the blood contains no jelly, a discovery which has been since confirmed by Pro fessor Berzelius, Dr. Matcet, and Mr. Braude. Dr. Bo stock named this substance the uncoagulable matter of the blood ; while Dr. Marcet, from a reference to its chemical properties, calls it muco-extractive matter.
All the fluids of body are furnished with a quantity of salts ; and, as they are soluble in water, they remain, at least for the most part, in the serosity. Gug lielmini appears to have been one of the first who distinct ly announced the presence of salts in the blood ; they have been since examined by different chemists, but by none with so much accuracy as by Berzelius and Marcet. From their experiments we learn, that they consist principally of soda, muriate of soda, and some of the phosphates. Since we find that a certain quantity of saline matter is al ways present in the animal fluids, it is natural to conclude that they perform some important office in the animal eco nomy ; it has been supposed that they may stimulate the muscular fibres to contraction, that they may contribute to the operation of the secreting organs, or that they may aid in the process of digestion ; but all these are mere conjectures, for which we have no certain foundation.
We have now described the blood, as it exists in its or dinary state, without regarding the changes which it oc casionally experiences from the operation of peculiar cir cumstances. Of these the most important is the difference between the venous and the arterial blood. The most ob vious difference is the colour, which, in the larger trunks of the systemic arteries is a bright scarlet ; and, in the systemic veins, of a purplish red. We have very different statements of the comparative temperature of the two kinds of blood ; for, simple as the experiment may appear, writers of equal authority give us very opposite statements. But the most important circumstance in which the arterial and the venous blood have been supposed to differ from each other, is the one which was announced by Crawford, that arterial has a greater capacity for heat than venous blood : but this point will be considered more fully in the next chapter, on respiration and animal temperature.