In order to determine the amount of fat present in a tissue, a portion of the tissue is carefully dried and weighed, and then thoroughly extracted with hot ether; the residue from the ethereal extract is then dried and weighed. From this the percentage amount of fat present can be calculated.

The process of extraction with ether may be most conveniently carried out by means of Drechsel's apparatus. This consists of two flasks; the upper, which is fitted to the lower one, is in connection

FIG. 76.-A few Fat-cells, one showing so-called margarin crystals, highly magnified
(E. A. Schäfer).

with a Liebig's condenser, and contains on a fluted filter the finely divided solid from which the fat is to be extracted. In the lower flask is the ether. This is heated on a water-bath; the ether vapour passes up and is condensed in the condenser, and on its way back passes through the upper flask. A constant circulation is thus maintained, and the dissolved fats gradually accumulate in the lower flask.

The following methods may be employed in the separation of the fats-Stearin is insoluble in cold ether; fat is repeatedly extracted with cold ether; the residue consists of stearin. Olein is obtained from a mixture of the fats by cooling them to 0° C.; olein alone remains liquid, and may be separated from the others by pressure. By combining these two methods palmitin may be obtained as a residue.

The melting points of the fats may be determined by much the same kind of apparatus as that used in the determination of the heat-coagula

tion temperature of proteids (see p. 119). After having been melted and then allowed to cool, the fat solidifies again, but always a few degrees below that at which it melted.

The fatty acids may be obtained by dissolving the fats in alcoholic potash, evaporating to dryness, dissolving the residue, which consists of compounds of potash with fatty acids (potash soaps) in water, and finally adding hydrochloric acid; the fatty acids are thus liberated : these are solid and may be collected on a filter.

The fatty acids may be separated from one another by dissolving them in hot alcohol; this solution is treated with lead acetate, and insoluble lead soaps are thus obtained. Lead oleate is extracted from the mixture by boiling ether; and lead stearate and palmitate remain undissolved. By adding hydrochoric acid to the ethereal solution of lead oleate, oleic acid is liberated and remains dissolved in the ether. The mixture of lead stearate and palmitate may be also decomposed by hydrochloric acid, and the fatty acids so liberated dissolved in alcohol, and separated by fractional precipitation with barium chloride or acetate; the stearic acid is precipitated as barium stearate first. On adding more of the barium salt, a mixture of the stearate and palmitate of barium is obtained; and finally the precipitate consists wholly of barium palmitate; the successive precipitates are collected on separate

An approximate estimation of the composition of a mixture of palmitic and stearic acid (the acids obtained from palmitin and stearin respectively) may be made by means of determining the melting and solidifying points of the mixture. Tables have been constructed which give these particulars with regard to various proportions of these substances when mixed together.'

General properties of fats. They are all soluble in hot alcohol, ether, benzol, carbon disulphide, and chloroform. They all have the physical characteristic known as greasiness. Each is solid below a certain temperature. Above this temperature, known as the melting point, they are fluid. The melting point varies somewhat according to the treatment to which the fat has been subjected.

When mixed with colloid substances in an alkaline solution, fats are broken up into microscopic globules, so that the fluid becomes white like milk, and the suspended fat does not readily rise to the top or separate from the fluid; such a mixture is known as an emulsion. The change known as saponification is a chemical change. It occurs when a fat is mixed with certain metallic compounds; the fat splits into its two components, glycerin and a fatty acid, the latter combining with the metallic base to form what is called a soap. Thus when palmitin is

1 Heintz, Poggendorff's Annalen, xcii. 588.

boiled with potash the result is glycerin, and potassium palmitate. The potash and soda soaps are soluble in water; the lead soap is insoluble.

Chemical constitution of the fats.-The fats found in adipose tissue are compounds of glycerin or glycerol with fatty acids, and they may be termed glycerides.1

The fatty acids form a series of acids derived from the monatomic alcohols by oxidation. Thus to take ordinary ethyl alcohol, C2HO; the first stage in oxidation is the removal of two atoms of hydrogen to form aldehyde C2H,O; then on further oxidation these are replaced by one of oxygen to form acetic acid, CH,O2.

A similar acid can be obtained from all the other alcohols. Thus :

The term hydrocarbon applied by some authors to the fats is wholly incorrect; a hydrocarbon is a compound like marsh gas (CH4) or olefiant gas (CH) consisting of hydrogen and carbon only. In spermaceti the fats are not glycerides, but derivatives of cetyl alcohol, C1H33.OH, the chief being cetyl palmitate. In Chinese wax (produced by the Coccus ceriferus) and in bees' wax there are also no glycerides. The wax is a C27H55

mixture of ceryl-cerotate CHOO, free cerotic acid, C7H540, and melicyl palmitate, a derivative of melicyl alcohol, CoHe1.HO (Schorlemmer, Org. Chem. p. 174).

From methyl alcohol, CH,.HO, formic acid, CHO.HO, is obtained

From the alcohol with formula CnH2n+1.HO, the acid with formula CnH2n-10.HO is obtained.

This is the series of acids known as the fatty acid series; the sixteenth member in the series has the formula C16H31O.OH, and is called palmitic acid; the eighteenth has the formula C18H35O.OH, and is called stearic acid. Each acid, as will be seen, consists of a radicle, CnH2n-10, united to hydroxyl; and it is these radicles that unite with glycerin to form fats.

Oleic acid, however, is not a member of the fatty acid series proper, but belongs to a somewhat similar series of acids known as the acrylic series, of which the general formula is CnH2n-30.OH. It is the eighteenth term in the series, and its formula is C18H33O.OH (see p. 69).

Glycerin, or glycerol, is a triatomic alcohol, C3H5(OH), i.e. three atoms of hydroxyl united to the radicle glyceryl, C3H5; or three atoms of water in which half the hydrogen is replaced by the triatomic radicle, C3H5. The hydrogen in the hydroxyl atoms is replaceable by other organic radicles. As an example take the radicle of acetic acid, acetyl (C2H2O). The following formulæ represent the derivatives (ethers) that can be obtained by replacing one, two, or all three hydroxyl hydrogen atoms in this way:

Triacetin is the type of a neutral fat; stearin, palmitin, and olein ought more properly to be called tristearin, tripalmitin, and triolein respectively. Each consists of glycerol in which the three atoms of hydrogen in the hydroxyl are replaced by radicles of the acid. The following formulæ represent their constitution :

C,H,(O.C,H,O)

18

Olein, CH,(O.C1HO)

33

16

Stearic acid, CHO.OH Stearyl, CHO CH ̧(OH), Stearin,C,H,(O.C1,H,O),

Decomposition products of the fats.-The fats split up into the substances which we have seen go to build them up.

Under the influence of superheated steam, and in the body under the influence of certain ferments (for instance, the fat-splitting ferment steapsin of the pancreatic juice), a fat combines with water, and splits into glycerol and the fatty acid. Take, as an example, tripalmitin or palmitin, as it is more generally called. The following equation represents what occurs :

C3H5(O.C16H310)3+3H2O=C3H5(OH)3+3C16H310.OH

[palmitin]

[glycerol]

[palmitic acid]

In the process of saponification, we have much the same sort of reaction; the final products are, however, glycerol and the palmitate of the base employed. As an instance take what occurs when tripalmitin is heated with potassium hydroxide; mutatis mutandis similar reactions occur with other fats and other bases; the equation representing the reaction is as follows:

C3H5(OC16H310)3+3KHO=C3H5(OH)+3C16H31O.OK

[palmitin]

[glycerol]

[potassium palmitate]

When fats decompose, certain volatile acids are liberated, and these it is which give the characteristic smell to rancid fats. When strongly heated, fats give off a characteristic penetrating odour. This is due to the formation of acrolein (C,H,O, the aldehyde of allyl alcohol, C3H5OH) from the glycerin.

Glycerin is obtained commercially from fats by decomposing them with superheated steam. It may also be obtained by precipitating the fatty acids as insoluble soaps from a solution of fat by litharge and water; glycerin remains in solution in the water, and may be freed from lead by a stream of sulphuretted hydrogen.

BONE

Bone, like all the other tissues of the body, consists of water, organic substances, and mineral salts; it differs from most of the other tissues in the large amount of mineral matter present. The mean percentage of water as estimated by Volkmann is 48; Lukjanow gives approximately the same number (46-7 per cent.); this is a mean of twenty determinations of pigeons' bones. The general composition

1 Lukjanow, Zeit, physiol. Chem. xiii. 339. Aeby (Centralbl. f. d. med. Wissensch. 1871, No. 14) considers that 11-12 per cent. of the water present is in a state of loose chemical combination, analogous to water of crystallisation.