mercuric chloride. Treat the residue on the filter with dilute hydrochloric acid (o'2 per cent.), and set aside for twenty-four hours; neutralize with sodic carbonate, and filter off the resulting precipitate, which is to be well washed with cold water.

2. Artificially.-Place a small quantity of solution of egg albumin in a beaker; add an equal quantity of dilute hydrochloric acid (made by diluting 6*25 cc of strong hydrochloric acid with 1 litre of water). Heat the mixture very gently at first, gradually raising to boiling point; then set aside to cool.

Chemical and physical properties.--The reaction of syntonin can best be studied with the artificially-prepared solution.

Test 1. It is not coagulated by heat.

Test 2. Neutralized with liquor potassæ it is precipitated. This precipitate is soluble with excess of liquor potassæ.

Test 3. Solutions of syntonin are precipitated by neutralization independently of the presence of sodium or potassium phosphate. Proceed as with Casein (§ 16, Test 4). —This distinguishes acid from alkali albumin (§ 16, Test 3).

(18) COAGULATED ALBUMIN. When moist, it is a white opaque substance, which becomes yellow and brittle when dried. It is insoluble in water and dilute acids and alkalis; but soluble in strong acids and alkalis, which change it into acid and alkali albumin respectively.

(19) PEPTONES. These bodies are formed by the action of the gastric juice, or by weak acid solutions of pepsin, or by the alkaline pancreatic juice, on albuminous substances, at a temperature from 40°-50° C. The consideration of these bodies may be conveniently deferred till the demonstration on Digestion.

1 The conversion of albumin into acid albumin does not take place immediately, therefore if too strong a heat were applied at first, coagulation would result.

III. GELATINS.

These principles are distinguished from the albumins by not being precipitated by potassium ferrocyanide with acetic acid, and in containing a smaller proportion of carbon and a larger quantity of nitrogen in their composition.

(20) MUCIN is obtained from mucous fluids, such as bile and saliva. From the connective tissue of the embryo, from the submaxillary gland, and from tissues which have undergone mucoid and colloid degeneration.

Preparation. *1. From bile.-By precipitating with alcohol, washing precipitate with distilled water, redissolving in lime-water, filtering through animal charcoal, reprecipitating with acetic acid.

*2. From the submaxillary gland.-The submaxillary gland of an ox is rubbed down with pounded glass, and the mass placed in water for a night, then filtering and again treating the residue with water. The filtrate is precipitated by acetic acid, and the precipitate washed with water, acetic acid, warm alcohol, and then dried.

Chemical and physical properties—

Test I. Gives a stringy precipitate with alcohol.Place a few drops of saliva on a glass slide, pass a stirringrod moistened with alcohol across, a white stringy precipitate of mucin will form. Try the same experiment with bile.

Test 2. Gives a stringy precipitate with acetic acid. Proceed as above, only moisten the stirring-rod with acetic acid.

Test 3. Soluble in lime-water.-Add lime-water to the precipitate on the glass slide, then dissolve.

(21) GELATIN is obtained from bones, tendons, and areolar tissue, by long boiling in water. It is some

times found in the blood of leucocythæmic patients, and in the juice of certain carcinomatous tumours.

Preparation.

From bones.-Bones that have been thoroughly cleaned and dried are digested with dilute hydrochloric acid (1-20) till all the earthy matter is dissolved ; the residue, which is called ossein, is then boiled for many hours, and dried at a temperature of 100o.

Chemical and physical properties.-Dry and pure gelatin is an amorphous transparent substance, hard and brittle, with no taste, or, if any, feebly sweet. Insoluble in ether and alcohol. In cold water gelatin swells up without dissolving. Warm water dissolves it, and the solution on cooling gelatinizes, even if the solution contains only I per cent. of gelatin; prolonged boiling destroys this property of gelatinizing.

Test 1. Its solutions are precipitated by mercuric chloride and tannic acid.

Test 2. It is not precipitated by either acetic acid or alum (distinguishes it from chondrin).

(22) CHONDRIN is obtained from cartilaginous tissue. Young bones prior to ossification, and adults' bones in certain diseased conditions, yield considerable quantities.

Preparation. The costal cartilages of the calf are cut in thin slices, boiled for twenty-four hours, and the solution evaporated to a gelatinous consistence; the fatty matters are removed by digestion with boiling ether, and dried at a temperature of 100o.

Chemical and physical properties.-Chondrin is a diaphanous, horny substance, insoluble in alcohol and ether; on the addition of cold water it swells up (to about 12 times its original bulk), but is not dissolved; it dissolves freely in boiling water and forms a jelly on cooling.

Test 1. Gives only a slight precipitate with tannic acid. Test 2. Gives a precipitate with acetic acid.

Test 3. Gives a precipitate of alum soluble in excess.

(23) ELASTICIN. This substance is the special principle of yellow elastic tissue, and is consequently obtained from those textures in which this tissue is most abundant; as the yellow elastic ligaments of the vertebræ, the ligamentum nuchæ, the middle coat of arteries and veins, the areolar tissue, and the lower vocal cords.

*Preparation. The ligamentum nuchæ, or the middle coat of the arteries or veins, are boiled with alcohol and ether to remove the fatty matter; then treated with water at a temperature of 100° for twenty-four hours; and afterwards for one hour with water at 120°, this removes the other gelatinous principles; the residue is then boiled with strong acetic acid, and washed with water; again boiled with strong soda ley and treated with strong acetic acid and washed with water; finally the residue is digested with hydrochloric acid, washed with water, and dried.

Chemical and physical properties.-Elasticin forms a yellow, fibrous, brittle mass, soluble only in strong caustic alkalis; it is insoluble in water below the temperature of 120°. Its hot solution does not gelatinize on cooling. Test 1. Precipitates with tannic acid. Test 2. No precipitate with acetic acid.

Requisites for Demonstration III.

MATERIALS.-White of egg.

Bullocks' eyes.

Fresh

fibrin or dried fibrin. Gelatin. Chondrin. REAGENTS.-Distilled water. Alcohol. Ether. Nitric acid. Hydrochloric acid. Acetic acid. Tartaric acid. Tannic acid. Solutions of Ammonia. Sodium chloride. Sodium sulphate. Sodium carbonate. Sodium phosphate. Potassium hydrate. Potassium phosphate. Potassium ferrocyanide. Mercuric nitrate. Mercuric chloride. Cupric sulphate. Iodine. Methyl anilin. APPARATUS.-Thermometer. Water bath.

Carbonic

acid gas generator. Fine sand. Filtering paper. Beaken. Test-tubes. Red and blue litmus-paper.

PART II.

DEMONSTRATION IV.

PRODUCTS OF DECOMPOSITION.

THE highly complex organic substances we have been considering, entering into the composition of the animal tissues and fluids, are decomposed in the body by the action of oxygen introduced into the economy by the process of respiration, into less complex bodies.

These products of decomposition are divided into two distinct groups, viz. 1, the non-nitrogenous organic acids derived directly from the oxidation of the saccharine and oleaginous principles, and indirectly from the albuminous; and, 2, the nitrogenous bases which are obtained, together with the non-nitrogenous organic acids, from the oxidation of albuminoid principles. This oxidation is seldom, if ever, accomplished at one stage. Intermediate compounds of simpler constitution are commonly produced, but ultimately the final oxidation is reached, and thus the greater part of the elements of the food are