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the lead is separated in the form of carbonate, and the liberated albumin becomes dissolved. The solution is filtered through paper freed from saline matter by washing with a weak acid. It however retains traces of lead, which may be removed by transmitting sulphuretted hydrogen and heating the liquid to 140°, when the first flocculi of coagulated albumin which are thus formed carry with them the whole of the sulphide of lead, and may be removed by filtration. The liquid is finally evaporated at a temperature below 120°. Graham obtains albumin free from saline impurities by acidulating white of egg with acetic acid, and submitting it to the process of dialysis.

No method is known by which the absolute purity of albumin can be ascertained. Its combining proportion is also uncertain, since it forms no definite compound upon the purity of which a decided opinion can be formed.

(1652) Sources of Albumin.-Albumin is a very abundant constituent of the animal body: it constitutes about 7 per cent. of the entire mass of the blood; it is an essential component both of lymph and of chyle; and it is abundant in all serous exudations, although it is found in these fluids in smaller proportion than in the serum of the blood. It is also contained in large proportion in many of the soft solids of the body, as for example, in the brain, the kidneys, the liver, and the pancreas, in all of which it exists in the uncoagulated form. The liquid of the pancreas appears to be the only secretion in which it occurs in health. It is never found in the excreta except under the influence of disease: thus it is occasionally poured out from the mucous membrane of the bowels in cases of intestinal catarrh, and has been repeatedly found in the alvine dejections in cases of dysentery, as well as in those of Asiatic cholera. It is likewise often met with in the urine in acute febrile disorders, as well as in some chronic affections of the heart, liver, and kidneys, especially in cases of fatty degeneration of the kidneys. Its occurrence in the ovum is general, and is familiarly illustrated in the case of the eggs of birds.

The juice of many plants, such as that of carrots, turnips, and cabbages, when heated, becomes turbid, from the coagulation of a substance which analysis shows to have the same composition as albumin, and with which it coincides in all important particulars. Wheat flour likewise contains an azotised substance which is soluble in cold water, forming a solution which yields greyish flocculi of albumin on evaporation.

Paralbumin is the name given by Scherer to a substance

838

GLOBULIN-VITELLIN.

obtained from the liquid of ovarian dropsy. Its solution is precipitated by acetic acid, which redissolves it if added in excess; with alcohol it also gives a precipitate, but the flocculi are redissolved by water at 100° F., and it is only imperfectly coagulated by heat. Many other varieties of albumin which have been described probably arise from the modifying effects of salts upon albumin.

(1653) Globulin, or Crystallin, is a substance which closely resembles albumin. It occurs mixed with albumin in the cells of the crystalline lens in a very concentrated solution, forming, according to Simon, from 10 to 14 per cent. of the dry lens.* It also enters largely into the composition of the red globules of the blood, and has hence received its name. Globulin has not been obtained in the soluble form free from salts. It may be extracted from the crystalline lens by rubbing up the lens in a mortar to break up the cells, evaporating to dryness at 212°, washing with ether and dilute alcohol to remove the fat, and then dissolving the crystallin out with hot alcohol, of sp. gr. o'915. A large portion of crystallin separates on cooling; after it has been dried it is insoluble in water, and it is precipitated from its alcoholic solution by diluting it with water.

Globulin is distinguished from the soluble protein-compounds by the circumstance that it is precipitated both from acid and alkaline solutions by exactly neutralizing them. It putrefies rapidly, and its solution when boiled emits ammonia. Its reactions with the mineral acids and with kreasote resemble those of albumin.

(1654) Vitellin is the name which has been given to the albuminoid constituent of the yolk of the egg; the yellow colour of the yolk is, however, due to an oil containing phosphoric acid, which is dissolved readily by ether, whilst the vitellin is coagulated. This reaction with ether, and the fact that its aqueous solution is not precipitated by salts of lead and of copper, are the principal characters by which vitellin is distinguished from albumin. By this ultimate analysis it is found to contain more hydrogen and oxygen than albumin (see Table, p. 831). The yolk of the egg contains 16 or 17 per cent. of this substance. According to the experiments of Valenciennes and Fremy, the composition of the azotised constituent of the yolk of the egg varies in different classes of animals. These chemists have termed that obtained from the eggs of cartilaginous fishes, such as the ray, Icthin, from

Lehmann, in his experiments upon this substance, appears not to have separated the albumin, and hence the description which he gives of it differs from that of Simon, who removed this impurity.

ἰχθὺς,

FIBRIN PREPARATION.

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a fish), and that obtained from the eggs of the turtle, Emydin.

(1655) FIBRIN.-This substance, like albumin, occurs in two distinct states, the solid and the liquid, during the life of the plant or animal, but on removing it from the living organism it undergoes speedy, and so-called spontaneous, coagulation—a circumstance by which fibrin is at once distinguished from all other analogous substances. It is contained in the blood in the liquid state, in a proportion not generally exceeding 25 parts in 1000, though it is liable to considerable variation in quantity under the influence of disease, being greatly increased in febrile and acute inflammatory affections, especially in rheumatism and pneumonia, where it has been found as high as 118 parts in 1000. anæmic diseases, in typhus, chlorosis, &c., it is frequently, but not uniformly, slightly diminished, though by long fasting it is somewhat increased in quantity. Lymph does not usually contain more than o'4 or o'5 parts per 1000. In the chyle of animals it has been found to vary from 07 to 70 per 1000.

In

Fibrin, in the form of muscular tissue, constitutes a large proportion of the soft parts of animals. It occurs in muscle arranged in bundles of fibres, and from this circumstance the name of fibrin derives its origin. Muscular fibrin, however, presents several points of difference from the fibrin obtained from the blood. According to Liebig, fibrin may also be obtained from the juice of plants, and it exists in the gluten of wheat.

Preparation.-Blood-fibrin may be obtained in a state of tolerable purity by whipping up freshly-drawn blood with a bundle of twigs in the act of coagulation the fibrin attaches itself in soft, white, opaque, elastic strings to the twigs, and may be afterwards washed clean by maceration and kneading in water till it is free from colour. When dry, fibrin forms a horny, yellowish, or grey solid.

The reactions of blood-fibrin in the solid form are similar to those of coagulated albumin, except that fibrin decomposes peroxide of hydrogen with effervescence, whilst albumin produces no such effect; but fibrin by long boiling in water is also rendered equally inert. When fibrin is heated to 300° with a small quantity of water, in a sealed tube, it is redissolved, and forms a solution which becomes coagulated by acids, and resembles that formed by albumin under similar circumstances. The coagulation of fibrin is prevented by allowing the blood, at the moment of its efflux from the body, to mix with solutions of certain salts, such as carbonate or nitrate of potassium, and acetate, sulphate, or

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chloride of sodium. The fibrin of venous blood, before exposure to the oxygen of the air, is freely soluble in a solution containing about one-sixth of its weight of nitre, as well as in similar solutions of the alkaline sulphates, acetates, phosphates, and chlorides. This solution becomes coagulated like albumin when heated, but is precipitated by acetic acid; when exposed to the air, the solution absorbs oxygen and deposits insoluble flocculi. Fibrin is freely soluble in dilute solutions of potash or of soda, and the alkaline liquid presents many of the characters of sodic albuminate, but on neutralizing the solution with acetic or phosphoric acid, the fibrin is re-precipitated: if the alkaline solution be mixed with metallic salts, it yields precipitates similar to those formed with albumin. The solutions both of fibrin and of albumin in acetic acid are precipitated on the addition of potassic ferrocyanide, a character by which they are at once distinguished from gelatin, which gives no precipitate under similar circumstances. If the acetic solution of fibrin be neutralized by potash, it is precipitated by the same reagents as albumin, but it does not become coagulated when heated. There is a marked difference between the action of oxygen upon albumin and fibrin. Scherer found that when freshly-drawn serum, or white of egg, was exposed to the action of oxygen gas, confined over mercury, scarcely any absorption of the gas occurs, and little or no carbonic anhydride is evolved; coagulated fibrin, on the other hand, when exposed in a moist state to the air, gradually absorbs oxygen, and emits carbonic anhydride, and in a few days becomes putrid. By long boiling in water it is gradually dissolved, and becomes oxidized. Albumin is similarly altered. Analysis appeared to prove that fibrin is a body which is more highly oxidized than albumin (see Table, p. 831); and coagulated fibrin, as it exists in muscle, is also more fully oxidized than that which separates from the blood on standing. Lehmann's view, that blood-fibrin is a state of transition between albuminous matter and the animal tissues, is thus rendered highly probable. It appears also, from the analyses of Dumas and others, to contain about 1 per cent. more of nitrogen than albumin.

(1656) Varieties of Fibrin.-The observations of Liebig on the different forms of fibrin are interesting. He finds that moist blood-fibrin, if left under water in a warm place in a vessel loosely covered, putrefies, and gradually becomes dissolved; in one of his experiments, after the fibrin had been macerating for a fortnight, a liquid was obtained which coagulated when heated, and presented many of the characters of a solution of albumin. The solution

VARIETIES OF FIBRIN-PREPARATION OF CASEIN.

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likewise contained leucine, and acetic and valeric acids, besides some other acid substances, which have been but incompletely examined. Sulphide of ammonium was also formed. If the water in which the fibrin is digested be previously mixed with one-tenth of its bulk of hydrochloric acid, blood-fibrin swells up slowly into a gelatinous mass, which, on the addition of a stronger acid, shrinks to nearly its original volume, and again swells up when put into water, but it does not form a true solution.

The fibrin of muscle, on the contrary, after it has been well washed and pressed, to free it from soluble matters, is dissolved more or less completely by dilute hydrochloric acid. If the acid contained in this solution be neutralized by an alkali, a coagulum occurs which is readily dissolved by an excess of alkali, or even by lime water; this latter solution becomes coagulated, on boiling, like white of egg. On the addition of ammonia to the solution of fibrin in hydrochloric acid, the muscle fibrin is precipitated, forming syntonin, and may be purified by successive washings in water, alcohol, and ether. The muscle of different animals varies in solubility in dilute acid; that of the fowl and of beef was found to be almost wholly soluble, that of mutton was less so, whilst in the muscular fibrin of veal the insoluble portion amounted to nearly one-half; this residue contained the fat and the vascular portions, besides a quantity of fibrin which resembled blood-fibrin in characters. The soluble portion was found to contain less nitrogen than the blood-fibrin. When fibrin is treated with acetic acid, and viewed by the microscope, it is found to consist of two portions, one of which is granular, and soluble in acetic acid, while the other is fibrous and insoluble. The proportion of these two components differs widely in different specimens. The ultimate composition of these two constituents appears to be very nearly the same.

The portion of the gluten of wheat flour which is insoluble in hot alcohol and in ether is regarded by Liebig and by Dumas as coagulated fibrin. It is soluble in very dilute hydrochloric acid, like the fibrin of muscle.

(1657) CASEIN is found abundantly in milk, of which it constitutes nearly 3 per cent. Its solutions do not become coagulated by boiling; they, however, present a reaction nearly as characteristic: the hot liquid gradually absorbs oxygen, and in consequence, a pellicle, which is insoluble in water, is gradually formed upon the surface.

Preparation.-Casein, in the soluble form, as found in milk,

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