SECTION I.
The Analysis of Vegetable Substances.
A vast many vegetable substances, such as kernels and seeds, yield, by strong compression, great quantities of mild, fat, unctuous Oils, which are not soluble in ardent spirits: these are what we called Expressed Oils. They are also sometimes called Fat Oils, on account of their unctuousness, in which they exceed all other sorts of Oil. As these oils are obtained without the aid of fire, it is certain that they existed in the mixt just as we see them, and that they are not in the least altered: which could not have been the case had they been obtained by distillation; for that never produces any Oils but such as are acrid and soluble in spirit of wine.
Some vegetable matters, such as the rind of citrons, lemons, oranges, &c. also yield, only by being squeezed between the fingers, a great deal of Oil. This spirts out in fine small jets, which being received upon any polished surface, such as a looking glass, run together and form a liquor that is a real Oil.
But it must be carefully noted, that this sort of Oil, though obtained by expression only, is nevertheless very different from the Oils mentioned before, to which the title of Expressed Oils peculiarly belongs: for this is far lighter and thinner; moreover, it retains the perfect odour of the fruit which yields it, and is soluble in spirit of wine; in a word, it is a true essential Oil, but abounds so in the fruits which produce it, and is lodged therein in such a manner, occupying a vast number of little cells provided in the peel for its reception, that a very slight pressure discharges it; which is not the case with many other vegetables that contain an essential Oil.
Succulent and green plants yield by compression a great deal of liquor or juice, which consists of most of the phlegm, of the salts, and a small portion of the oil and earth of the plant. These juices, being set in a cool place for some time, deposite saline crystals, which are a combination of the acid of the plant with part of its oil and earth, wherein the acid is always predominant. These salts, as is evident from the description here given, bear a great resemblance to the tartar of wine treated of above. They are called Essential Salts; so that Tartar might likewise be called the Essential Salt of Wine.
Dried plants, and such as are of a ligneous, or acid nature, require to be long triturated with water, before they will yield their essential salts. Trituration with water is an excellent way to get out of them all their saline and saponaceous contents.
A vegetable matter that is very oily yields its essential salt with much difficulty, if at all; because the excessive quantity of oil entangles the salt so that it cannot extricate itself or shoot into crystals. Mr. Gerike, in his Principles of Chymistry, says, that if part of the oil of a plant be extracted by spirit of wine, its essential salt may be afterwards obtained with more ease and in greater quantity. This must be a very good method for such plants as have an excessive proportion of essential oil; but will not succeed if the essential salt be hindered from crystallizing by a redundancy of fat oil, because fat oils are not soluble in spirit of wine.
Essential Salts are among those substances which cannot be extracted from mixts by distillation: for the first impression of fire decomposes them.
Though the acid which predominates in the Essential Salts of plants, be most commonly analogous to the vegetable acid, properly so called, that is, to the acid of vinegar and tartar, which is probably no other than the vitriolic acid disguised; yet it sometimes differs therefrom, and somewhat resembles the nitrous or the marine acid. This depends on the places where the plants grow which produce these salts: if they be maritime plants, their acid is akin to the acid of sea-salt; if on the contrary they grow upon walls, or in nitrous grounds, their acid is like that of nitre. Sometimes one and the same plant contains salts analogous to all the three mineral acids; which shews that the vegetable acids are no other than the mineral acids variously changed by circulating through plants.