8

Each of these condensed glycols acts as a diatomic alcohol, and furnishes an oxide; diethylene oxide, H.,, being, for example, the oxide corresponding to diethylene glycol. They also form acids by oxidation with elimination of water, e.g. :

and with acids they furnish two sets of ethers analogous to those of ordinary glycol.

It appears to be not improbable that there are diatomic alcohols. in the isologous groups to which the benzoic and salicylic series belong; for instance, it has been suggested that saligenin (1461) may be the dibasic alcohol, which by its oxidation would furnish salicylic acid:

This subject awaits further elucidation from experiment.

Wurtz found that just as there are pseudo-alcohols (1201) which are isomeric with the monatomic alcohols, so there is a class of pseudo-glycols isomeric with the true glycols.

CHAPTER IV.

FATS AND FIXED OILS.-TRIATOMIC ALCOHOLS, AND THEIR

COMPOUNDS.

§ I. NATURAL FATS AND OILS.

(1217) BEFORE proceeding to the examination of the important triatomic alcohol glycerin, it will be convenient to examine the general nature of its most abundant natural compounds. The fats and fixed oils constitute an important and well-defined natural group of organic compounds, which are obtained abundantly both

NATURAL FATS AND OILS.

293

from the animal and the vegetable kingdom. In their chemical relations they present a close analogy with the compound ethers, and are indeed now regarded as the compound ethers of a tribasic alcohol, glycerin, formed by the action of this alcohol upon certain monobasic acids, as will be fully shown hereafter (1240). The predominating elements in their composition are carbon and hydrogen, and as a general rule, the greater the proportion of carbon and the less that of oxygen which they contain, the higher is their melting point. The vegetable fats and oils occur in various parts of the plant, but they are most abundant in the seed. The seeds of the crucifere are particularly remarkable for the large quantity of oil which they furnish; rape-seed contains from 35 to 40 per cent. of oil, and linseed furnishes more than a fifth of its weight. The fleshy fruit of the olive also yields oil in great abundance, and the same is the case with the Elais guineensis or palm-oil fruit. In animals, fat is particularly liable to accumulate immediately beneath the cutis, also in the omentum and around the kidneys. The fatty bodies obtained from warm-blooded animals are generally solid at ordinary temperatures; whilst those from fish and from cold-blooded animals are liquid. In all cases, both in the animal and in the plant, a certain quantity of some albuminoid substance occurs associated with the oily body, and this substance exerts an important chemical influence upon the fat when kept. In the extraction of the oil, simple pressure is generally sufficient to separate it from the cells of the tissues in which it is contained. If it be solid at ordinary temperatures, it may often be procured by boiling the tissue with water it then rises to the surface and forms a layer which solidifies on cooling.

The fats and oils are lighter than water, having a specific gravity varying from about o'91 to o'94; they have different degrees of solidity, and do not consist of any single proximate principle in a state of purity, but are for the most part mixtures, in varying proportions, of at least four different closely allied bodies; three of which, viz., stearin (from oréap, suet) and palmitin, so called from palm oil, in which it is abundant, and margarin (from μápуapov, a pearl, owing to its pearly lustre), are solid at ordinary temperatures; whilst the third, olein, is liquid. The larger the proportion of olein which is present, the softer is the fat, and the lower is its fusing point.

All fats are soluble in ether, and to a certain extent also in alcohol, Oil of turpentine and benzol likewise dissolve them with facility, and the different fats and oils may be mixed with each

294

GENERAL NATURE OF THE NEUTRAL FATS.

other in all proportions. They possess the property of rendering paper semitransparent, producing what is well known as a greasy stain. The fats and oils may be heated to nearly 500° (260° C.) without undergoing any important change, but they cannot be distilled without experiencing decomposition. Hence the term, fixed oils, is applied to these bodies in contradistinction to that of volatile oils, which is given to the fragrant essences obtained from the vegetable kingdom, which may be distilled without alteration. At about 500° the fats and oils begin to give off acrid and offensive vapours, and at a little above 600° (316° C.), they are rapidly decomposed with evolution of gaseous hydrocarbons, the escape of which gives to the oil the appearance of ebullition; as the result of the distillation, a mixture of solid and liquid hydrocarbons, of water and of various fatty acids, accompanied by a peculiarly irritating substance termed acrolein (1242), is formed, and may be condensed in the receiver.

(1218) General Nature of the Neutral Fats.-The fatty bodies when heated with the caustic alkalies, experience a peculiar change, long known under the title of saponification, or conversion into soap (1233), during which process all fats yield up a viscid liquid, which, owing to its sweet taste, has been termed glycerin (from γλυκύς, sweet). The nature of this change may be ascertained by dissolving the soap in water, and then adding some acid, such as the tartaric or the hydrochloric, which combines with the alkali, and forms a soluble compound with it. Unctuous flocculi are thus separated, and on the application of heat they melt, and form an oily layer on the surface of the fluid. This substance when cold is found to possess properties very different from those of the original fat. It is crystalline, freely soluble in alcohol, and the spirituous solution reddens litmus paper strongly. It possesses distinctly acid characters, and it is soluble at once, and without the appearance of milkiness, in hot alkaline liquids.

This unctuous matter varies in consistence and quality with the nature of the fat from which it is procured, and is for the most part a mixture of acids (derived from the bodies already mentioned as forming the greater number of the fats), and termed respectively, oleic, stearic, and palmitic acids. It is found that the united weight of the glycerin and of these fatty acids, always exceeds by 3 or 4 per cent. that of the fat originally employed. In fact, the olein, stearin, and palmitin are each compounds of the radicle of their respective fatty acid, with the basis of glycerin, which has been displaced by the alkali in the act of forming soap; the alkali, in displacing the radicle of the glycerin,gives up a portion

GENERAL NATURE OF THE NEUTRAL FATS.

295

of water to it, and hence increases its weight; while the radicle of the acid, on being separated from the soap, also combines with a portion of hydrogen. Simple mixture of the glycerin and of the fatty acid together, therefore, does not again produce the oil or fat from which they were obtained.

The masterly analytical researches of Chevreul upon the fats, have recently received complete explanation in the parallel remarkable synthetic experiments of Berthelot, which have shown that the ordinary varieties of natural fats and oils are the tribasic ethers of the triatomic alcohol glycerin. We have already seen that compound ethers of the monobasic alcohols, by the action of water, become converted into the corresponding acid and alcohol; in a similar manner, the fats, by saponification, or by the action of water at an elevated temperature, are converted into the corresponding fatty acid, and glycerin. Oxalic ether, for instance, undergoes the following decomposition :—

and the formation of stearic acid and glycerin from stearin, may be represented by an analogous equation; thus ::

When the oils are mixed with an aqueous solution of the alkalies, saponification takes place slowly, but if the oil be dissolved in alcohol, and then mixed with an alcoholic solution of the caustic alkali, both at a boiling temperature, the saponification is instantaneous and complete.

The saponification of the fats and oils may be effected by heating them with the anhydrous bases, such as caustic lime and oxide of lead but a large proportion of the glycerin is always destroyed in the operation, for want of water upon which the glycyl (H) can react at the moment of its liberation from the fat; and unless the heat be carefully regulated, the fatty acid itself is liable to become decomposed.

When the oily seeds, such as almonds, walnuts, or poppy seeds, are crushed or pulverized, so as to break up the cellular tissue, and bring their several components into mixture with each other, the neutral fatty bodies which the seeds naturally contain are gradually converted into fatty acids, and glycerin is liberated. This change

296

RANCIDITY-DRYING AND NON-DRYING OILS.

has been shown by Pelouze to depend upon the action of an albuminous ferment contained in the pulp, and to be independent of any absorption of oxygen.

The fats are more fusible than the acids which they furnish on saponification, but, when exposed to a low temperature, they become much harder than the fatty acids. Generally they are destitute of odour and taste; the peculiar scent emitted by some of them depending upon the presence of a small proportion of some glycerin compound of one of the volatile oily acids, such as the butyric, valeric, or rutic. It is worthy of remark, that although the fats themselves are but sparingly soluble in alcohol, their solubility is greatly increased by the presence of a free fatty acid. Ammonia appears to combine with many of the oils, but it converts them into amides, and not into true soaps (Rowney). The compounds of ammonia with almond oil and with castor oil are crystalline.

(1219) Rancidity of Oils.-Fats and oils are subject to a peculiar change in their properties, popularly distinguished by the term rancidity.' This change is attended with a slight absorption of oxygen, and it appears to be due to the decomposition of certain mucilaginous and albuminous matters contained in the oil, which during their decay react on the fat, setting free the fatty acids, and decomposing the glycerin. Perfectly pure fats and oils do not become rancid; and rancidity may be completely removed by melting or washing them first with boiling water, and subsequently with a cold weak alkaline ley. When the vegetable oils are employed for purposes of illumination, it is necessary to purify them from these mucilaginous matters, which otherwise become charred and encrust the wick, and thus prevent the oil from burning freely. To effect this purification, the oil may be mixed with one-fiftieth of its weight of oil of vitriol, which is to be added in small portions at a time, keeping the mixture in continual agitation; dark flocculi are thus formed, which subside when the mixture is left at rest. The supernatant oil is then to be drawn off, and the adhering acid is to be removed by agitating it with onefourth of its volume of water, mingled with a small proportion of lime. Lastly, the oil is rendered fit for use by filtration through charcoal.

(1220) Drying and Non-drying Oils.-Oils may be further arranged under two distinct classes, according as they possess or do not possess the property of absorbing oxygen, by which they are gradually converted into a solid mass; those which become solid in this manner, are termed drying oils. Some oils, such as