intense green colour. On supersaturating with hydrochloric acid, biliverdin is precipitated as a fine green mass; it is insoluble in chloroform, but soluble in alcohol. The formation of biliverdin is probably represented by the equation:

C16H17KN2O3 + H2O + 0 = C16H19KN205.

3. Bilifuscin, C16H20N2O4, and biliprasin, C16H22N2O5, are described as nearly black, shining, brittle masses, insoluble in ether and chloroform, soluble in alcohol and alkalies.

PROTEIN SUBSTANCES.

1292. Protein bertatos are wildly disseminated in the azima' and vegetable kingdoms; they are the e mpounds taking the princital part in the physiological changes occurring in the organism In the animal body, especially in that of the higher animals, they form the main part of the tissues thence the name, from re, bat occur also in nearly ali animal liquids. They are without exception amorphous, and all contain nitrogen in addition to carbon, hy drogen. and oxygen, in most cases sulphur also. Accurate formule have nos yet been obtained for any of the bodies, but from the results obtained it is certain that their molecules are very large and of very complicated structure. In agreement with this they do not diff through membranes, and are really decomposed by putrefaction or by heating with aqueous acids or alkalies. Amongst the products of these decompositions there occur regularly ammonia, and amide de rivatives of organic nuclei, such as glycocine, leucine, aspartic acid. glutamic acid, and tyrosine. On dry distillation they yield methylamine and pryridine bases in addition to ammonia. On heating with manganic oxide and sulphuric acid they yield very numerous oxidation products, amongst which there occur several volatile fatty acids, from formic acid upwards, together with their aldehydes and nitrils, benzaldehyde, benzoic acid, &c. Those protein substances soluble in water have without exception an action on polarised light; they are all precipitated by tannic acid, nearly all by alcohol.

GELATINOUS TISSUES AND GELATINES.

1293. The gelatinous tissues occur only in the animal organism and are invariably organised; they are, as such, insoluble in cold or warm water, but swell up slightly and then putrefy readily. Solutions of tannins render them incapable of putrefaction by converting them into leather. On long boiling with water they are converted into gelatines, whose hot solutions on cooling become converted into a homogeneous jelly. This jelly dries on exposure to air into an amorphous transparent or semitransparent brittle mass, which only swells up in cold water; they are not dissolved by alcohol. On long boiling of the aqueous solutions, more quickly by heating to temperatures of 150° and above, they lose the property of gelatinising on cooling. On evaporation they then yield a mass resembling gelatine in most properties, but readily dissolved by cold water.

From their properties two varieties of gelatine may be distinguished, chondrine and glutine, the tissues from which they are derived being respectively designated as chondrogen and collagen.

Chondrogen and Chondrin.

1294. Bone cartilage previous to ossification, and therefore also the bony structure of the animal fœtus, consists essentially of chondrogen. It is also found in the cornea of the eye, and in all permanent cartilage, &c. It is purified by treating the cartilage with cold water, dilute acids, alkalies, alcohol, and ether, and then appears, after drying, as a structureless semitransparent mass.

On boiling the cartilage of the ribs for 12-24 hours with much water, chondrin goes into solution and gelatinises on cooling, and is precipitated by addition of alcohol. It is lævorotary. Alkalies or ammonia readily and completely dissolve chondrin jelly; the solution then possesses an enormous optical activity : [a] = == 552°.

Addition of acetic acid or of traces of mineral acids precipitates dissolved chondrin, but a very slight excess of the latter redissolves it. Chondrin behaves similarly with alum solution. It is precipitated completely by chlorine water, plumbic acetate, and argentic nitrate, only incompletely by mercuric chloride.

Boiling hydrochloric acid or gastric juice at blood heat decompose chondrin into a nitrogenous substance, which still requires examination, and a lævorotary substance, which reduces alkaline copper solutions. On boiling with sulphuric acid much leucine is formed, but no glycine.

After subtraction of the ash left on combustion, chondrin contains 50 % C, 6.6 % H, 14.4 % N, 0.4 % S, and 28.6 % 0.

The gelatinous substance from the vertebræ of fish behaves similarly to chondrin except that it does not gelatinise.

Collagen and Glutine or Gelatine.

1295. Collagen, the substance yielding gelatine, forms the hyaline intracellular mass of the connective tissues, of the skin, sinews, &c., and also the organic portion of bones. On boiling with water, ossein dissolves more readily than chondrogen, being converted into glutine, which in the dry state forms glue, when in a state of greater purity is termed gelatine. The latter is chiefly prepared from the swimming bladder of fishes (sturgeon). A hot aqueous solution containing only one per cent. of glutine still gelatinises on cooling.

Glutine is readily dissolved by acids, including acetic, forming liquid glue; aqueous alkalies also dissolve it. Unlike chondrin, glutine solutions are not precipitated by normal or basic plumbic acetate, but are completely by mercuric chloride and also by tannic acid.

Glutine is also lavorotary: [a] = 130° at 30° temperature. On decomposition with dilute acids it yields glycocine and leucine. Whilst containing nearly an equal amount of carbon and hydrogen to chondrin, glutine contains essentially more nitrogen (18-3 %), the oxygen being correspondingly less.

1296. Sericine, or silk gelatine, may be included amongst these bodies, as it possesses the power of gelatinising in a very high degree; it is obtained from raw silk by long boiling with water. It is decomposed by boiling dilute sulphuric acid, with formation of a little leucine, about 5% tyrosine and 10 % serine (§ 803).

ALBUMINOID SUBSTANCES.

1297. The albuminoids differ but little from one another in composition. After deducting ash they contain

In

Their formulæ must therefore be very complex, as from the percentages given above there must be to 1 atom of sulphur about 70 to over 300 atoms of carbon and about 110 to over 600 atoms of hydrogen. Only the true albumins are soluble as such in water, the others only being rendered soluble by aid of small quantities of acids, bases, or salts, with which they form transient compounds. addition to these soluble modifications they can be converted in different ways into insoluble or 'coagulated' modifications. These latter are insoluble in water, and as a rule in salt solutions also; alkalies and acids redissolve them with difficulty. The conversion into the insoluble modification ensues either by heating, action of ferments, or long standing under strong alcohol, and appears to depend on the formation of more complicated molecules, accompanied by elimination of water. The action is therefore probably comparable to the formation of anhydride molecules from several simpler hydrated molecules. These anhydride coagulated molecules will then offer greater resistance to the solvent action of acids and alkalies, as they must be first hydrated and decomposed into the simpler molecules (hydrolysis).

Every albuminoid can be brought into solution by the action of alkalies or acids, especially of hydrochloric acid. The resulting alkali compounds are termed albuminates; the acid derivatives, acid albumins or syntonins. At first there is no permanent alteration effected by this means on the non-coagulated modifications, inasmuch as on neutralisation both the albuminates and acid albumins again yield the original albuminoid substance with unaltered properties.

On long continuance of the action, greater concentration of the reagents, and simultaneous action of heat (especially in the case of the coagulated albumins) decompositions readily ensue, in which all albumins yield leucine, tyrosine, and aspartic acid (the vegetable albumins also yielding glutamic acid), and invariably larger quantities of uncrystallisable compounds of unknown nature.

1298. These decompositions occur with especial ease on boiling with moderately dilute sulphuric acid, and also with solution of potassic hydrate. The relative yield of products, roughly determined, shows considerable variations. For instance, on heating with sulphuric acid (one part H2SO, and four parts OH2) there are obtained

On heating albuminoids with an equal quantity of bromine and ten times as much water in closed vessels, the formation of carbonic anhydride, bromoform, brom-acetic acid, oxalic acid, aspartic acid, leucine, and bromanil (§ 1006) is observed, there being obtained—

The decomposition of albuminoids by solutions of alkalies yields similar products to those obtained with sulphuric acid. The formation of an alkaline sulphide is peculiar to them, so that the liquid evolves sulphuretted hydrogen on acidulation, and yields a black precipitate of plumbic sulphide on addition of a lead salt.

When moist the albuminoids putrefy with very great ease, probably in consequence of the presence of ferment-like organisms, with which as a matter of fact the putrefying mass swarms. There are then formed hydrogen, sulphuretted hydrogen, carbonic acid, ammonia, trimethylamine, ethylamine and its homologues, and in union with these members of the series of fatty acids, from acetic up to valeric acid, and also leucine and tyrosine.

On heating with much water to 150° all albuminoids, even the coagulated modifications, are converted into readily soluble, noncoagulable substances, without, however, suffering such decompositions as attend the action of acids and bases.

1299. The albuminoids show the following reactions, which are used for their identification :

The aqueous saline, or very weakly acid or alkaline, solutions are precipitated in flocks by tannic acid and by salts of the heavy metals (e.g. lead, copper, mercury, &c.), and in most cases also by addition of

alcohol.

Concentrated acetic acid and tribasic phosphoric acid dissolve all the albuminoids. The acetic solution after dilution is precipitated by potassic ferro- or ferricyanide or by potassic platino-cyanide.

Concentrated hydrochloric acid dissolves all albuminoids. On exposure to air, especially on boiling, this solution acquires a blue or bluish violet colour.

Strong nitric acid, especially that containing nitrous acid, colours them intensely yellow. The resulting nitro compounds dissolve in alkalies and ammonia with intensely reddish brown colour, and on addition of mineral acids to these salts are reprecipitated in flocks. These nitro products are known as xanthoproteic acids.

A solution of nitrate of mercury containing nitrous acid (Millon's reagent, formed by dissolving 1 part of mercury in 1 part of cold nitric acid, and afterwards diluting with 2 parts of water) gives a fine red colour with albuminoids, probably from the formation of mercury xanthoproteïnate.

On passing chlorine gas into solutions of the albuminoids, white flocculent precipitates are formed of chlor substitution products, containing from 6.5% to 14% of chlorine, according to the duration of the action. These dissolve in aqueous ammonia with formation of am