Fat is a deposition in the cellular membrane of certain parts of the body, especially under the skin, in the omentum, in the region of the kidneys, and within the cylindrical bones : it also occurs here and there among the muscles, and sometimes is accumulated to an extent so unnatural as to form a species of disease. In birds it is chiefly seated immediately below the skin, and in water-fowl it is largely secreted by the glands of the rump : in the whale and other warm-blooded inhabitants of the deep, it is chiefly contained in the head and jaw-bones, and abundantly interposed between the skin and the flesh ; in fish it abounds in the liver, as in the shark, cod, and ling, or is distributed over the whole body, as in the pilchard, herring, and sprat.
Various opinions have been entertained re specting the formation of fat, and its insolu bility in water has led to the idea of its produc tion in the places in which it occurs; but as it is found in the blood and in some other of the fluids of the body, it is probably partly received with the food, and partly formed by the process of secretion. Its remarkable absorption in cer tain cases of disease of the chylopoietic viscera, and of deficiency of proper food, seems to point it out as a source of nutriment of which the ani mal economy may avail itself on emergency ; and accordingly in cases of emaciation or atro phy, it is the first substance which disappears. It varies in consistency and characters in the diffe rent tribes of animals, and in the greater num ber of amphibia and fishes it is usually liquid at ordinary temperatures. (See ADIPOSETIssuE.) The general chemical characters of fat have been long known, as well as its important pro perty of saponification by means of the alkalis; but the real nature of the changes which it un dergoes in this process, and the essential dis tinctive characters of its varieties, were first satisfactorily investigated by Chevreul,* whose essay upon the subject has been justly cited as a model of chemical research. It is chiefly from this source, and from the abstract of its contents given by Berzeliusit that we have taken the following details.
All the varieties of fat are resolvable into mixtures of stearin and chain, (from (mete, suet, and INatov, oil,) that is, into a solid and liquid ; but there are peculiar differences be longing to these products in each individual species, which sometimes seem to depend upon very trifling causes, and at others to be con nected with distinct ultimate composition.
There are two modes by which the stearin and Blain of fat may be separated : the one consists in subjecting it to pressure, (having previously softened it by heat, if necessary;) and the other, by the action of boiling alcohol, which, on cooling, deposits the stearin, and retains the Blain in solution ; the latter separates on the addition of water, still however retaining a little stearin ; they may be ultimately separated by digestion in cold alcohol, sp. gr..835, which
takes up the elain, and leaves it after careful distillation ; the stearin remains undissolved.
Fat may be separated from its associated cellular texture, by cutting it into small pieces and melting it in boiling water ; it collects upon the surface, and when cold is removed, and again fused in a water-bath, and strained through fine cambric. Many varieties of fat, when dissolved in boiling alcohol and precipi tated by water, leave a peculiar and slightly acid and saline extract in solution, apparently derived from the enveloping membranes.
1. The softer kinds of fat are termed lord, of which hog's-lard furnishes a good example: it is white, fusible at a temperature between 75° and and of a specific gravity= about 0.938. When cooled to 32°, and pressed between folds of bibulous paper, it gives out 62 per cent. of colourless elain, which remains fluid at very low temperatures, has a sp. gr. = .915, and is soluble in less than its weight of boiling alco hol, the solution becoming turbid when cooled to about The residuary stearin is ino dorous, hard, and granular : when fused, it remains liquid at the temperature of 100°, but, on congealing, it rises to 130°, and assumes a crystalline appearance.
When hog's-lard becomes rancid, a pecu liar volatile acid forms in it, which has not been examined. 100 parts of hog's-lard yield, when saponified, 94.65 margaric and oleic acid, which when fused concrete at and 9. of glycerine. According to Chevreul's analysis, the ultimate elements of hog's-lard are— Carbon 79.098 Ilydrogen 11.146 Oxygen 9.756 100.000 2. Human fat is another species of lard ; but it differs in diflbrent parts of the body. The fat from the kidney, when melted, is yellow, inodorous, begins to concrete at 77°, and is solid at about 60°. It requires 40 parts of boiling alcohol of 0.841 for solution, and this deposits stearin as it cools, which, when puri fied by pressure between folds of filtering paper at 77°, is colourless, fusible at and may then be cooled down to 105°, before it concretes; in the act of concreting its tempera ture rises to and it becomes crystalline, and soluble in about four parts of boiling alco hol, the greater part being deposited in acicular crystals as the solution cools. The elain of human fat, obtained by the action of hot water upon the paper by which it had been absorbed, is colourless, remains fluid at and con cretes at a lower temperature. Its specific gravity at 60° is .913 ; it is iuodorous, and has a sweetish taste. It is soluble in less than its weight of boiling alcohol, and the solution be conies turbid when cooled to about 62°. 100 parts of human fat yield, when saponified, about 96 of margaric and oleic acids fusible at about 90°, and from 9 to 10 of glycerin.
According to Chevreul, human fat and its Blain are composed as follows :—