Chap. III. An elaborate set of experiments has been made by M. de Saussure, to determine the elementary constitution of several of the fixed oils; but no very important conclusions could be drawn from such analyses, unless the different substances, of which almost all these bodies are composed, had been previously insulated, and analyzed separately. The following table exhibits the results of Saussure's analyses. I have introduced likewise a few other analyses of fixed oils made by other chemists. Constituents of fixed oils. Carbon. Linseed oil Castor oil Olive oil Hog's lard Hydrogen. Oxygen. 0 100 Saussure. 0 Azote. Total. Experimenters. When the fixed oils are saponified they are converted into the various fatty acids described in the first chapter of this volume, and into glycerine. Chevreul considers it as probable that they are compounds of glycerine and these acid bodies, having in this respect some resemblance to the acid ethers. The alkalies, by combining with the acid, set the glycerine at liberty. It would not be easy to verify this conjecture by experiment. Our mode of purifying the different constituents of the fixed oils, such as they have been described in this section, and obtaining each in a separate state, is too imperfect to warrant much confidence in the accuracy of the analyses that have been made, or to enable us to deduce from them conclusions of much importance. The fixed oils are all insoluble in water. When agitated Sect. VII. with that liquid, the mixture becomes milky, but the oily par- Properties. ticles gradually separate and swim upon the surface. The presence of a mucilaginous substance, as gum arabic, prevents the oil from separating, and occasions, of course, a permanent milkiness. Such mixtures are called emulsions. They are often formed by triturating oily seeds, as almonds, with water; the two ingredients necessary to form an emulsion, oil and mucilage, being present in the seed. Most of the fixed oils are but sparingly soluble in alcohol. Mr. Brande found that very little olive or almond oil was taken up by alcohol of the specific gravity 0-820. Linseed oil is more soluble, and alcohol dissolves any quantity whatever of castor oil. On that account this oil is sometimes employed to adulterate volatile oils of high value, especially oil of cloves.* In general the fixed oils are somewhat more soluble in sulphuric ether than in alcohol. Four measures of sulphuric ether of the specific gravity 0-7563 were found by Mr. Brande to dissolve 1 measure of almond oil, 1 measure of olive oil, 2 measures of linseed oil, and any quantity whatever of castor oil.† The fixed oils unite readily with each other, with volatile oils, and likewise with bituminous and resinous substances. The alkalies unite readily with the fixed oils, and form the important compounds called soaps. The fat oils enter into these combinations more readily than the drying oils. The earths likewise combine with these oily bodies, and form a kind of soap insoluble in water, and are therefore not capable of being applied to the same uses as common soap. The combustible acids, as far as is known at present, do not unite with oils; neither has the muriatic acid any remarkable effect. Phosphoric acid, when concentrated, deepens their colour, and gives them a peculiar smell, especially if assisted by heat: a proof that it acts upon them. The sulphuric acid acts with much greater energy. The oils become immediately black, and assume gradually the properties of bitumen in proportion to the continuance of the action. If the acid be allowed to remain long enough, they are completely decomposed; water is formed, charcoal precipitated, and an acid evolved.§ Other * Phil. Trans. 1811, p. 264. Ibid. Equal volumes of sulphuric ether and castor oil united together has been found a useful external application in rheumatism. Elemens de Chimie of the Dijon Academy, iii. 142. § Fourcroy, vii. 330. Chap. III. products doubtless also make their appearance, though the action of this acid on oils has not yet been examined with sufficient care. Nitric acid acts with still greater energy. When poured suddenly upon the drying oils, it sets them on fire. The same effect is produced upon the fat oils, provided the acid be mixed previously with a portion of the sulphuric. When the nitric acid is sufficiently diluted, it converts the drying oils into a yellow resinous-like mass, and the fat oils to a substance very like tallow. But the action of this acid upon fixed oils has not yet been sufficiently examined. Many attempts have been made by chemists to form permanent compounds of the concentrated acids and oils, under the name of acid soaps. The only acid which was found to answer was the sulphuric. Achard published a number of experiments on these compounds. They dissolve in water, and lather like common soap; but as they are not in reality permanent compounds, they cannot be used with much prospect of advantage. The fixed oils acts (though feebly) upon some of the metals when the atmospheric air is not excluded. Copper is soon corroded by them, and a dark green solution obtained. The only other metal upon which they are known to act in the same manner is mercury. By triturating mercury with fixed oils, it gradually disappears, and a bluish coloured unguent is formed; consisting, in part at least, of the black oxide of that metal united to the oil. But this experiment scarcely succeeds, except with those oils which are in a state approaching to solidity. But upon the metallic oxides the fixed oils act with greater energy. They dissolve the white oxide of arsenic with great facility, as was shown long ago by Brandt. When boiled with the oxides of mercury, lead, or bismuth, they form very tough solid compounds, called plasters. Fixed oils are liable, by keeping, to undergo a change well known by the name of rancidity. They become thick, acquire a brown colour, an acrid taste, and a disagreeable smell. The oil thus altered converts vegetable blues into red, and of course contains an acid. It is believed at present that this change is owing to the alteration of the foreign substances present in oils, or to the action of those foreign bodies upon the oily matter itself. Several of the fixed oils, when newly extracted, let fall on standing a quantity of mucilaginous matter; and from the experiments of Scheele, it appears probable that they always retain less or more of a similar principle. He boiled together one part of litharge, two parts of olive oil, and a little water. When the oil had acquired the consistence of an ointment, it Sect. VIII. SECTION VIII.-OF BITUMENS. The term bitumen has often been applied by chemists to all the inflammable substances that occur in the earth; but this use of the word is now so far limited, that sulphur and mellite are most commonly excluded. It would be proper to exclude amber likewise, and to apply the term to those fossil bodies only which have a certain resemblance to oily and resinous substances. In this restricted sense the word is used in the present section. Bituminous substances may be subdivided into two classes: namely, bituminous oils and bitumens, properly so called. The first set possess nearly the properties of volatile oils, and ought in strict propriety to be classed with these bodies; but as the chemical properties of bitumens have not yet been investigated with much precision, it was deemed rather premature to separate them from each other. The second set possess properties peculiar to themselves. Let us endeavour to describe the substances belonging to these two classes as far as possible. 1. Bituminous Oils. Only two species of bituminous oils have been hitherto examined by chemists. Others indeed have been mentioned, but Scheele's Opusc. ii. 189. + Nicholson's Jour. v. 5. Chap. III. their existence has not been sufficiently authenticated. These two species are called petroleum, and maltha, or sea-wax; the first is liquid, the second solid. How ob. tained. Naphtha. 1. Petroleum is an oil of a brownish yellow colour. When pure, it is as fluid as water, and very volatile.* Its specific gravity varies from 0-730 to 0.878. It has a peculiar smell. When heated, it may be distilled over without alteration. Petroleum is found in the earth in various states of purity; sometimes without any mixture of foreign substances. In this state it is usually distinguished by the name of naphtha, and is said to occur in great abundance on the shores of the Caspian and in Persia. It occurs also in different parts of Europe, especially Italy and Germany. When less fluid and darker coloured, it is commonly called petroleum. It is supposed to owe this increased spissitude and deepened colour to the action of the air. When distilled, it yields a quantity of pure petroleum, while a portion of bitumen of the consistence of tar or pitch remains behind. When long exposed to the air, petroleum becomes black, and acquires the semifluidity of tar. In this state the greatest part of it is insoluble in alcohol; so that it has assumed the state of true bitumen. Petroleum is used in those countries, where it abounds, as an oil for lamps. It is employed also as a solvent of resinous bodies, and of the proper bitumens, which it dissolves with facility. When petroleum is distilled at a low heat, the liquid which comes first over is distinguished by the name of naphtha. It is colourless, perfectly fluid; is very volatile, and has a peculiar smell. Coal tar, or the oily matter which comes over when coal is distilled at a red heat, yields, when repeatedly rectified, the very same kind of liquid. Naphtha is observed also in different places rising spontaneously from the earth nearly in a state of purity. As, for example, in different parts of Persia, at Amiano in Italy, and in many other places. This natural production of naphtha is observed in many places where no coal has been ever discovered. Yet I think it not improbable, from the exact similarity between the properties of natural naphtha and coal naphtha, that they have in reality the same origin. *The volatility seems to have been over-rated by older chemical writers, On mixing well rectified petroleum with water in a retort, I found that the water could be readily distilled over in a moderate heat, while the whole petroleum remained behind. + See Kirwan's Mineralogy, ii. 42. |