232 



FAT. 



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 Berzelius,f that we have 

 taken the following details. 



All the varieties of fat are resolvable into 

 mixtures of stearin and elain, (from <rrx%, suet, 

 and ehcuov, 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 

 elain 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 

 elain 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 



* Recherches chimiques sur les corps gras d'ori- 

 gine animate. Paris, 1823. 



t Lehrbuch der Chemie. B. 3 and 4. Dresden, 

 1827. 



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 lard, of 

 which hog's-lard furnishes a good example: it is 

 white, fusible at a temperature between 75 

 and 85, 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 140. 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 150; and 9. of 

 glycerine. According to Chevreul's analysis, the 

 ultimate elements of hog's-lard are 



Carbon 79.098 



Hydrogen 11. 146 



Oxygen 9.756 



100.000 



2. Human fat is another species of lard ; 

 but it differs in different 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 122, and 

 may then be cooled down to 105, before it 

 concretes; in the act of concreting its tempera- 

 ture rises to 120, 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 40, and con- 

 cretes at a lower temperature. Its specific 

 gravity at 60 is .913; it is inodorous, and has 

 a sweetish taste. It is soluble in less than its 

 weight of boiling alcohol, and the solution be- 

 comes 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 aud its 

 elain are composed as follows : 



