Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/800

Rh 740 OILS Chinese tallow, &c., are solid fats. Animal fats are for the most part solid, the oils of marine animals and neat s-foot oil being important exceptions. The various solid fats differ greatly in consistency, and the hardness of individual samples is largely affected by the nature of the food and by the health of the animal yielding them, and by some other circumstances. The relative fluidity or solidity of the various oils and fats depends on the proportions of the three principal constituents of all oils olein, stearin, and palmitin. The fluid oils contain olein in larger proportion, that body being itself liquid at ordinary temperatures, while solid stearin and palmitin predominate in the hard fats. The viscosity of the fluid oils also ranges between wide limits. The rate of flow of an oil, which is a matter of considerable importance in several industrial applica tions, is estimated by comparison with the standard rate at which water of the same temperature flows through an a,perture of fixed dimensions. The most viscid of the fluid oils is castor oil, which at a temperature of 15 C. is more than two hundred times thicker or more slow-flowing than water. Olive oil at the same temperature is more than twenty times thicker, and linseed and hemp oils, though among the most limpid of fixed oils, still flow about ten times as slowly as water. Oils communicate to paper and like substances a stain which remains an irremovable translucent grease spot. They are almost entirely insoluble in water, and, except ing croton and castor oils, in cold alcohol, but in boiling alcohol they dissolve more freely, and they are perfectly .soluble in ether, bisulphide of carbon, chloroform, benzol, and light petroleum spirit. In their pure condition they are neutral bodies, but, on their becoming rancid, free fatty acids are developed which give them an acid reaction. Exposed to air they absorb oxygen freely, and the class containing linoleic acid, known as drying oils, of which linseed oil is the type, thereby harden into a solid translucent semi- elastic caoutchouc-like body, a property of the utmost value in the arts. When they are exposed in thin layers over a great surface the absorption of oxygen proceeds with such energy that heat is evolved sufficient to produce spontane ous combustion, a circumstance frequently exemplified in the heating and igniting of heaps of oily cotton waste. The non-drying oils also on exposure to air thicken and become greasy ; they acquire the peculiar disagreeable smell and acrid taste known as rancidity, owing to a kind of fer mentation being set up in them through the agency of impurities, whereby the fixed fatty acids they contain are decomposed, and odorous volatile fatty acids formed by oxidation at their expense. The specific gravity of all oils is lower than that of water, ranging from 900 in the case of cocoa butter to 970, the specific gravity of castor oil. Most fluid oils have a specific gravity between -915 and -930. The specific gravity of oils varies with the temperature far more than is the case with water. It is found that for each degree Centigrade rise of temperature whale oil increases in volume 1 per 1000, rape oil 89, and olive oil S3. When a solid fat is heated slowly till it melts and is allowed gradually to cool, it remains fluid till it falls con siderably under the temperature at which it melted, and at the moment of solidification there is a sensible increase in its temperature. Butter, for example, melts at 30 to 31-5 C., but does not resolidify till it falls to 19 or 20 C. These phenomena have been investigated in the case of the pure fats stearin and palmitin by Duffy (Chem. Soc. Qu. &amp;lt;/., v. 197), who finds that these bodies undergo with great readiness three isomeric modifications, each having a dis tinct melting point widely apart from each other (stearin from beef giving 51, 63, and 67), the solidifying point being slightly under the lowest of the three. The freezing point of the ordinary fluid oils ranges down to from - 27 to - 28 C. for hemp oil, nut oil, and linseed oil, while olive oil solidifies at + 2 to 4 C. Fluid oils heated to from 280 to 300 C., and solid fats to from 300 to 325 C., undergo destructive distillation, resolving into a mixture of rich inflammable gases and a peculiarly irritating acrid vapour, acrolein. Oils and fats are compounds of carbon, hydrogen, and oxygen, in proportions ranging, as a rule, for carbon be tween 76 and 80 per cent., hydrogen from 11 to 13 per cent., and oxygen from 10 to 12 per cent. Their proximate constitution was first demonstrated by Chevreul, who in deed, in the great series of classical researches embodied in his Recherches sur les corps gras d origine animale (1823), established the modern chemistry of oils. The phenomena of saponification, as exemplified on a great scale in the important industry of soap-making, furnished the key for discovering the intimate constitution of oils. Oils and fats treated with alkalis, alkaline earths, and basic metallic oxides in presence of water undergo decomposition and enter new combinations. A soap is formed by the union of the alkaline body with acid con stituents of the oil, known as fatty acids, ajid the sweet body, glycerin, is liberated. The saponirication of stearin with sodic hydrate, for example, may be thus represented : Stearin. Sodic Stearate of Glycerin. hydrate, sodium (soap). By heating oil with steam under a pressure of from 10 to 12 atmospheres, or with water superheated to about 220 C., the oil is decomposed into free fatty acid and glycerin. Thus, again taking the simple fat stearin, we have : Stearin. Water. Stearic acid. Glycerin. In the above reactions it will be observed that three molecules of water are required for the formation of free stearic acid and glycerin from one molecule of stearin, and to that extent the resulting pro ducts are heavier than the original. Reading the eq nation in the inverse manner we find the formation of stearin takes place by the substitution of the three acid residues of stearic acid C 18 H 35 for the three hydroxyls H. in the molecule of glycerin. JS T o fat or oil is found in nature consisting of a single chemical fat such as stearin alone. All are mixtures of at least two and for the most part three or more simple fats or glycerides of fatty acids closely allied in nature and constitution. These glycerides or combinations of glycerin and fatty acids are in their chemical relations ethers. Glycerin itself is a triatomic alcohol, and bears to the fatty acids and re sulting ethers the same relation which a basic substance bears to an acid and to the salt which results from their combination. In all natural fats glycerin combines, as in stearin, by having substi tuted for its three replaceable hydrogen atoms three equivalents of fatty acids, whence the natural simple fats are all triacid com 3iO)3 1 pounds tristearin ( C i8 H 35)3 1 3, tripalmitin i63i3 o 3 , ^35) ^S&quot;5J &c., though commonly called stearin, TT triolein v 18 / ^ palmitin, and olein, &c. (For further information as to the con stitution of glycerides see GLYCERIN, vol. x. p. 697.) The three simple fats above named form by far the largest and most important constituents of all oils and fats, the only others which bulk largely being the glycerides of linoleic acid in drying oils, and of physetoleic acid characteristic of marine oils. The number of fatty acids found combined with glycerin in oils is, however, very considerable. They constitute members of homologous series, the first or stearic series of which possess the common formula C, ( H.,, t 2 . Belonging to it are the following : Boiling and Melting point. Substances in which chiefly found. Acetic OH 4 O 2 118 C Butter fat. Butyric .... 156 C Butter fat. Caproic Cg H^Oa 194 C. Cocoa-nut oil, butter fat. Caprylic Cg HigC2 232 Cocoa-nut oil, butter fat. Capric Melt. pt. 39 Cocoa-nut oil, butter fat. Laurie CloH2 4 O-&amp;gt; Cocoa-nut oil, bay berry oil. * Myristic .... Ci 4 H 2 8Oo 53 8 Oil of mace, &c. Palmitic Ci 6 H 3 .,O.&quot;, 62 Lard, palm oil, &c. Stearic dgH 3 gO.i 09 2 Tallow, &c. Arachidic Behenic C 20 H 40 o;; C.&amp;gt;&amp;lt;&amp;gt;H 44 O2 75 75 Ground nut oil. Cerotic C.~&amp;gt;7H 54 O2 78 Beeswax. 88
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