THE albuminoids, or, as they are frequently called, the proteids, are the most important of the proximate principles, constituting as they do the nutritive element, and the basis of all the tissues and fluids of the body. Thus they are found in white of egg, in the blood, in lymph, in chyle, in milk, and in the

juices of the parenchymatous tissues. The albumin of the economy is derived from the albuminoid constituents of the food; as, casein from milk, gluten from bread, myosin from flesh, and ovo-albumin from white of egg. These substances, though differing from each other in many respects, are by the action of the acid gastric juice and alkaline pancreatic juice speedily reduced and converted into peptones, in which form the distinctive characters of the different substances are lost. The peptones are absorbed into the circulation, and again undergo metamorphosis into fibrin, globulin, myosin, casein, &c.

These substances in fulfilling their purpose in the economy break up and become oxidised; on the one hand, into certain saccharine and fatty bodies, which are ultimately eliminated in the form of carbonic acid and water; and on the other, into nitrogenous substances, leucin, tyrosin, uric acid, &c., which are finally eliminated by the kidneys in the form of urea.

All nitrogenous substances found in the body belong to this group of proximate principles, or are products derived from their decomposition.

They all contain Carbon, Hydrogen, Nitrogen, and Oxygen, and most of them Sulphur and Phosphorus. The proportions of these elements vary but slightly in the different bodies; the exact constitution of the albuminoids has not however been determined.

A. CHEMICAL AND PHYSICAL CHARACTERS OF THE

GROUP.

The albuminoids are amorphous bodies, and have never been obtained in a crystalline form. Prepare a solution of

albumin by carefully separating the white of egg from the yolk. Break up the white by snipping it through several times with scissors. Agitate briskly with four ounces of distilled water, and filter.

Test 1. Solutions of albumin possess extremely low diffusive powers.--Notice that in filtering, the solution passes very slowly through the filter-papers; so that several filters have to be used to obtain in a short space of time a quantity sufficient for testing.

Test 2. Xanthoproteic reaction.-To a few drops of solution of albumin add a drop of concentrated nitric acid, heat; a yellow colour is developed. A drop or two of ammonia added by means of a stirring-rod, and the solution acquires an orange colour.

Test 3. Millons reaction.-Add a few drops of mercuric nitrate to a small quantity of solution of albumin. On boiling, the coagulated albumin acquires a red colour and the fluid becomes pinkish.

Test 4. Alkaline solution of copper reaction.—To a small quantity of solution of albumin add an equal bulk of liquor potasse and two or three drops of cupric sulphate solution, the mixture will assume a violet colour.

Test 5. Reaction with ferrocyanide of potassium and acetic acid.-Add a few drops of strong acetic acid to a solution of albumin, a drop of solution of ferrocyanide of potassium will cause a white precipitate. (N.B.--This test distinguishes the albuminoid principles from the gelatinous.)

B.-DISTINCTIVE CHEMICAL AND PHYSICAL CHARACTERS OF THE ALBUMINOIDS.

I. ALBUMINS.

(II) SOLUBLE ALBUMIN, as obtained from blood serum or white of egg, is a neutral, viscid, glairy substance. It is met with in all the animal tissues and fluids.

Test 1. It is soluble in water.-Shake up a little white of egg, which has been well broken up, with distilled water; notice that it is dissolved. Its solubility distinguishes it from all the other members of the group except the peptones.

Test 2. It is insoluble in alcohol and ether.

Test 3. It is coagulated by heat.--Place a small thermometer in a test tube and fill the test-tube half full with solution of albumin; then plunge it in a water bath and gradually raise the temperature. A white film will appear in the solution when the temperature reaches 70° C. (158° F.). (N.B.-The presence of extremely small quantities of dilute acids will cause coagulation to take place at a lower temperature; so also will neutral salts such as sodium chloide and sodium sulphate. On the other hand, an extremely minute quantity of sodium carbonate retards coagulation till a higher temperature is reached.) 1

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Test 4. It is coagulated by strong mineral acids.-Place a small quantity of albumin in a test-tube and run a drop of strong nitric acid down the side of the tube to the bottom; a white zone of coagulated albumin will appear at the point of junction of the two fluids.

Test 5. It is not coagulated by acetic, tartaric, carbonic, or nomal phosphoric acids.

1 Albumin, even after dialysis, always contains certain saline substances, which enter intimately into its composition; these substances probably are of use in keeping albumin in a state of solution in the body. The point at which the various soluble albumins coagulate seems to depend on the quantity of saline ingredients that enter into their composition.

Ovo-albumin is distinguished from sero-albumin by the following characteristic. The specific rotatory power of ovoalbumin for yellow light is -35°5; that of sero-albumin, -56°. Ovo-albumin is coagulated by ether; sero-albumin is not. Strong hydrochloric acid readily coagulates ovo-albumin; and the coagulum is not easily redissolved; sero-albumin does not coagulate so easily, and the coagulum is more readily redissolved.

Met-albumin; a modification of albumin met with in dropsical fluids. It is not coagulable by heat; gives no precipitate with acetic acid or acetic acid and potassium ferrocyanide; heated with acetic acid, a slight cloudiness is given to the solution; alcohol precipitates it, but does not coagulate it.

Par-albumin. This substance was obtained by Scherer from the fluid of certain ovarian cysts; and is usually associated with a body resembling glycogen, and which is capable of conversion into sugar. Its solutions are extremely viscid; it is precipitated from its warm solutions by acetic acid and carbonic acid gas; it also gives precipitates with lead acetate, mercuric chloride,

Practical application of tests for albumin (See Urine).

(12) GLOBULIN is obtained from blood serum, from milk, from chyle, from the aqueous and vitreous humours of the eye, and from connective tissue.1 In union with hæmatin it forms the colouring matter of the blood, "hæmoglobin." (See Blood.)

Preparation.-Rub up the fresh lenses of some bullocks' eyes with fine sand. Agitate with distilled water and filter. Collect the filtrate in a tall measuring-glass, and pass a current of carbonic acid gas through it, the globulin will be deposited. Allow the precipitate to settle, decant off supernatant fluid, and collect deposit on a watch-glass.

Chemical and physical characters

Test 1. It is insoluble in water.-Place a small portion of the precipitate on a glass slide, add by means of a stirring-rod a few drops of distilled water; it is not dissolved.

Test 2. It is soluble in dilute saline solution. -Make I per cent. solution of sodium chloride. Add a few drops of this solution to the precipitate on the glass slide; it is dissolved.

Test 3. It is precipitated by carbonic acid gas.—This was shown in its preparation.

Test 4. It becomes opalescent when heated to 73° C. and coagulates at 93° C.-Try the experiment in the manner described for coagulation of albumin. (§ 11, Test 3.)

Para-globulin. The researches of Hoppe-Seyler and C. Schmidt have shown, that the globulin obtained from blood serum differs from that of the crystalline lens in not being precipitated from its solutions by heat or alcohol, and also by the property it possesses

potas ium, ferrocyanide, and tannic acid; but it is not precipitated by magnesium sulphate.

1 Globulin in its coagulated form is insoluble in water, but if the water be saturated with oxygen it is dissolved; it is also soluble in neutral saline solutions.