said to contain it free. Fats are insoluble in water, on which they float, but soluble in ether, boiling alcohol, volatile oils; they fuse by heat, unless so great as to decompose or inflame them, and they solidify by cold, becoming crystalline. They render paper transparent, and become rancid when exposed to air; volatile acids being formed by oxidation. Their great heating and lighting power is due to their abundance of carbon and hydrogen in proportion to oxygen. On adding an alkali, they saponify, or form a soap, which is therefore a compound of the fatty acid and soda in hard soap, potash in soft soap. The fat of bacon and cod liver oil are the only ones which saponify by the carbonates of alkali. The following are the fats contained in the human body:-Stearin, a white, hard mass, soluble in boiling alcohol, from which it crystallizes in scales nearly square; it melts at 143°. It is contained in very minute proportion, indeed some analysts deny its presence. Margarin is softer, and crystallizes in beautiful radiating needles; it melts at 118°. The acid of margarin, margaric acid, has been found free in the fæces, in pus, and the fluid of ovarian dropsy. Olein is fluid to 25°, before which, however, it gives up the margarin, which it holds in solution, and which then becomes solid. The mixture of these bodies which form human fat is fluid at all temperatures above 40°. The great amount contained in the human body proves that it cannot be derived from the fat of food alone, and by feeding cows, bees, &c., on starch or sugar, and other food carefully deprived of fat, it was found to be produced from these aliments.

Lipoids are substances which differ from fats in being non-saponifiable. They are cholesterin and serolin. Cholesterin (C28 H24 O) crystallizes in white rhombic scales. It is tasteless and inodorous, and requires a heat of 293° to fuse it. It exists sparingly in the bile dissolved by choleic acid, and abundantly in gall-stones. It is found

in blood, nervous matter, fæces, and meconium, and in the following products of disease :-Pus, tubercle, many tumours, especially encysted, hydrocele fluid, atheroma of arteries, and cataract.

Serolin, once regarded as peculiar to the serum of blood, is probably but a mixture of ordinary fat and albumen.

III. We now have to consider the nitrogenized substances, which include most of the proximate principles, as they are called, rather vaguely, in contradistinction. to the "secondary organic compounds," such as urea, which are mostly discharged with the excretions. They do not crystallize, which Prout regarded as characteristic, as that condition is produced by inorganic forces. They are complex, having four elements, hence called quarternary, and of these many equivalents. From the weak affinities of their nitrogen, they yield easily to decomposition, which can be prevented, as in the preserving of meat, by exclusion of oxygen by air-tight cases, by salt, which abstracts water, by sugar, creasote, &c. The tissues which they make in the living body are always undergoing a destructive oxidation, somewhat like eremacausis, or slow combustion, a more active change being resisted by the nitrogen they contain and by the conditions of life. Thus they may be called "noble" in the same sense as the term is applied to gold. Their chemical inertness may depend on their strong resistance to diffusion. By putrefaction, which is hastened by the contact of putrid matter, they evolve carbonic acid, nitrogen, ammonia, and sulphuretted, phosphuretted, and carburetted hydrogen. All the nitrogenized bodies give an intense red colour on being heated with a solution of mercury in nitric acid. This is Millon's test. The first group are the albuminoids, or protein bodies, so called from their chief importance by Mulder, who described them as compounds of a body he named protein, and varying amount of sulphur and phosphorus. He obtained protein

by boiling any of these bodies with caustic potash, which removed the sulphur, and phosphorus. He assigned its formula as C36 H25 N4 O10+2 HO. His view has not now many supporters, and protein is rather a product of decomposition than a base of organic bodies.

Albumen is found in serum of blood, lymph, chyle, the fluids of the cavities, and in pancreatic juice alone, of the secretions; also in the substance of brain and solid organs. Its most familiar form is white of egg. It varies much in these situations from the amount of alkali, salts, or fat combined with it. It is always fluid, and is kept so by an alkali, especially soda, which forms 1.58 per cent. of white of egg. Pure albumen consists in the 100 parts, of carbon 52-97, hydrogen 6-81, nitrogen 15.11, oxygen 23.54, sulphur 1.57, phosphorus 0.4 The tests are its coagulation by a heat of 160°, or more, according to the amount in solution; nitric, hydrochloric, or sulphuric acids; the latter stains albumen a deep purple; also by bichloride of mercury (hence its antidotal powers, as discovered by Orfila, in poisoning by that salt), ferrocyanide of potassium, in an acid solution, and some other reagents. It coagulates in flakes if the solution is neutral, as a milky cloud if slightly alkaline; and if strongly acid or alkaline, the solution forms, on evaporating, a colourless skin on the surface which was formerly thought to be peculiar to casein. Albumen is remarkable for its very low diffusive power, especially in its usual alkaline state, and thus it takes forty-nine times as long to pass through an animal membrane as hydrochloric acid, and in a given time over twenty times as much chloride of sodium will diffuse. This property is of vast advantage in retaining the blood and serous fluids within their proper vessels. Albumen has been at one time considered allied to the acids, at another to the bases, and thus its precipitation has been accounted for; but this occurrence is due undoubtedly to the acid or salt added being a crystalloid, and thus, being more

soluble, it displaces the albumen. Its chemical inertness is one of its most useful physiological properties.

Fibrin is found in blood, lymph, chyle, and in many products of disease, as dropsical effusions, tubercle, and false membrane. It is known readily by its spontaneously becoming solid when it is found to be fibrillated, or arranged in interlacing threads. The membrana putaminis from the egg-shell exhibits this microscopic appearance best. When gelatin is precipitated by metaphosphoric acid, a substance exactly like fibrin results. In weak solutions, as hydrocele fluid, it does not separate spontaneously, but will do so when a piece of solidified fibrin is placed in the fluid. It may be obtained by whipping blood, washing the clot, or by filtering frog's blood, as the cells, being large, are held back.-(Müller.) Fibrin may be distinguished from albumen by not dissolving, but swelling and becoming transparent and jellylike when digested in water containing a little hydrochloric acid, by dissolving in a weak solution of nitre, by decomposing the deutoxide of hydrogen, and by more rapidly putrifying. Its composition is not exactly known, but it is said to contain more oxygen than albumen, from which it is produced by oxidation. Whether it is a material for the formation of tissue, or a product of their destruction, as also the effect of ammonia in keeping it fluid, will be noticed in the chapter on "Blood."

Syntonin, or muscle-fibrin, was shown by Liebig to be different from blood-fibrin, for it does not dissolve in solution of nitre, but does so in that of hydrochloric acid. From albumen it differs in being precipitated from an alkaline solution, by common salt. It is found in voluntary and involuntary fibres, and in muscle cells, as those of the spleen and arteries.

Casein occurs in milk, fluid of thymus and allantois, yolk of egg, and in the blood, especially of suckling woIt differs from albumen in not being coagulable by heat, when, however, it forms a tough skin on the

men.

surface of the fluid if freely exposed to oxygen, while the weaker acids (acetic, lactic, &c.) and rennet precipitate it.

Globulin from the blood cells, and crystallin from the lens, differ but slightly from other protein bodies.

Salivin, pepsin, pancreatin, and other animal ferments, are supposed to be modifications of albumen, probably undergoing decomposition. They will be noticed hereafter.

Pyin, obtained by Güterbock from pus, is according to Mulder, a tritoxide of protein.

Gluten is abundant in the food of man derived from the cereal grains. It was discovered by Beccaria, who, from its ready putrefaction, its odour when undergoing that process, and its importance in food, regarded it as identical with albumen. It may be obtained by washing the starch out of dough, when it remains as a tough, elastic mass.

Legumin is another protein body contained in peas, beans, &c. It has the greatest proportion of phosphate and is most analogous to casein. From peas the Chinese make a food, taofoo, hardly distinguishable from cheese. The nitrogenized substances which follow are supposed to be derived from the protein bodies by some change, probably regressive.

Gelatin is obtained by boiling bones (ossein), cartilages, (chondrin), skin, fibrous tissue, and other structures whose use is mechanical, as the passive organs of locomotion. Isinglass, from the swimming-bladder of fish, is the purest example. It is soluble in boiling water, and "sets" into a jelly on cooling, if it be in proportion of 1 per cent. This jelly, when heated, becomes fluid, and sets again, but this cannot be repeated more than four or five times. Tannic acid, alcohol, bichloride of mercury, &c., precipitate gelatin. Chondrin, or that from cartilage, is thrown down in addition by hydrochloric and acetic acids, by acetate of lead, alum, and