Mr. Hellot remarks, on this occasion, that part of the Acid, by the intervention of which this Oil is separated, unites therewith; and, to the greater or smaller quantity of the Acid thus combined with the Oil, he imputes its being more or less ponderous: which is the more probable, as the heaviest Oil is always obtained from a mixture in which the Acid bears the greatest proportion, and vice versa. Perhaps the different specific gravity of Essential Oils is wholly owing to the greater or smaller quantity of Acid they contain.
Mr. Hoffman hath made several observations on this Oil, which evidently prove that it contains much Acid. He says, that, if it be kept for some time in a bottle, it grows red, and loses its transparency; that its agreeable aromatic taste becomes acid and corrosive; and that if you hold it over the fire in a silver spoon, it corrodes it, and leaves a black spot on it; and that it also corrodes Mercury, when heated therewith in a matrass. To this Mr. Pott adds, that it makes a very perceptible effervescence with Fixed Alkalis; and that being rectified by those salts it loses all the acid properties observed by Mr. Hoffman.
Mr. Hellot obtained a still more considerable quantity of this Oil, by adding three or four ounces of a Fat Oil to the mixture of Spirit of Wine with the Vitriolic Acid. Now, as the Oil we are speaking of hath the properties of Essential Oils, and is soluble in Spirit of Wine, Mr. Hellot observes, that Oil of Vitriol by uniting with Fat Oils converts them into Essential Oils: which agrees very well with our opinion concerning the cause of the solubility of Oils in Spirit of Wine; which, in the Memoir already referred to on other occasions, we attribute to an Acid superficially and slightly united with Oils.
The Oil which thus rises, in distilling Spirit of Wine mixed with the Vitriolic Acid, is known by the name of the Sweet Spirit of Vitriol. This name is very improper, because it may suggest a notion that this Oil derives its origin from the Vitriolic Acid, as some Chymists have erroneously thought; whereas it comes entirely from the Spirit of Wine, as we have shewn. If any reason can be assigned for keeping up the name, it must be because of the considerable quantity of the Vitriolic Acid that remains in the combination, and is dulcified by its union with the Spirit of Wine.
This Oil is an ingredient in Hoffman's famous Anodyne Mineral Liquor. That liquor is thought to be nothing but this very Oil dissolved, and combined with the two liquors that rise first in the distillation, and immediately before the sulphureous acid phlegm. It dissolves very easily and quickly in those spirituous menstrua; so that, if you intend to have it by itself, and to prevent its recombining with the liquors that come off before, (which should be prevented, because it hinders the separation of the Æther), you must take great care to change the receiver as soon as the acid phlegm with which it rises begins to appear.
We have seen that, by the methods which Mr. Hellot hath pointed out, this Sweet Oil of Vitriol may be increased, both in weight and quantity. In that ingenious Chymist's Memoir we also find some methods of preventing it from rising in the distillation. They consist wholly in the addition of some Absorbent bodies, which, he tells us, divert the action of the Vitriolic Acid, at least in some measure, from the inflammable part of the Spirit of Wine. One of these methods is as follows.
"Put into Spirit of Wine as much soft Soap as it can dissolve: filter it, and pour on it some of the heaviest and most concentrated Oil of Vitriol: shake the mixture. The Soap will be instantly decompounded, and its Oil will float on the surface; because the Vitriolic Acid robs it of the Alkali, which renders it miscible with Spirit of Wine. Distil it, and you will obtain but a very little of Rabel's water; which, moreover, will have the disagreeable smell of a most rancid Oil. There will afterwards ascend a great quantity of Spirit of Wine having the same smell; then an aqueous, acid, and sulphureous liquor; but not a drop of yellow Oil. Mean time there forms a bituminous fungus, of some confidence, rising above the Oil of the Soap which floats on the rest of the liquid."
Most of the Vitriolic Acid having been absorbed by the Alkali of the Soap, in this experiment, as Mr. Hellot observes, it is not surprising that it should not act upon the Spirit of Wine with so much efficacy as to decompose it, and separate its Oil. For the same reason but a little of Rabel's Water comes over, and almost all the Spirit of Wine rises without undergoing any sensible alteration. The disagreeable smell of those liquors comes from the Oil of the Soap, which, being naturally heavy, remains behind in the retort, where it grows rancid and is partly burnt.
The last experiment in Mr. Hellot's Memoir, of which we shall take notice, is a peculiar process for preparing Æther; by means whereof, with the help of an earthy medium, it is easy to distil the vinous acid Spirit containing the Æther, without any sensible change of smell from the beginning to the end of the operation; without its being succeeded by an acid sulphureous liquor, oil, black scum, resin, or bitumen; and without the necessity of taking any great care about the management of the fire, as the liquor may always be kept boiling in the retort, and distilled to dryness without any danger. This medium is common potter's earth. Mr. Hellot puts six ounces thereof, well dried and pulverized, into a large retort, with one pound of Spirit of Wine and eight ounces of Oil of Vitriol. These he digests together three or four days. The mixture acquires no sensible colour. He sets the retort in a sand-bath, and continues the distillation to dryness with a moderate charcoal fire. Excepting a few drops that rise first, and which are pure Spirit of Wine, all the rest of the liquor that distils hath constantly the smell of Æther: which is even somewhat more penetrating than that of the vinous acid Spirit obtained without the intervention of this earthy medium.