The hyduret of benzoyle has the ordinary characters of an essential oil. When pure it is a colorless, transparent liquid, the specific gravity of which is 1043. It possesses the peculiar almond flavor, and is not poisonous. This, which is the true oil of bitter almonds, ought to constitute about eighty-five or ninety per cent. of the crude oil. When oil of vitriol is added to pure hyduret of benzoyle the mixture acquires a dark reddish brown color, but no other visible change takes place.
If the hyduret of benzoyle be exposed to the air it speedily becomes oxidized, and by the substitution of an atom of oxygen for one of hydrogen it is converted into benzoic acid. The benzoic acid present in oil of bitter almonds is the result of this transformation, and sometimes it occurs to such an extent that it is deposited from the oil in crystals. Benzoic acid is not colored by the action of oil of vitriol.
Benzoine is also a product of a remarkable change which hyduret of benzoyle, when mixed with hydrocyanic acid, is liable to undergo. Like benzoic acid, it is a solid crystalline body, but unlike benzoic acid, when mixed with oil of vitriol, it forms a violet colored compound.
The characters and properties of hydrocyanic acid are too well known to require notice. It is this constituent, which is sometimes present to the extent of eight or ten per cent., that gives to oil of bitter almonds its poisonous properties.
In examining oil of bitter almonds, with the view of determining whether it be pure or not, it is necessary to consider the influence on the action of the reagents employed, of variations in the number and proportions of the several constituents present. This is especially the case with reference to the use of oil of vitriol as a test.
On adding oil of vitriol to the samples of oil under notice, it was found that it formed with all of them a clear but very dark colored mature, from which no separation took place. The color of the mixture thus produced, however, differed to a {208} greater or less extent in each case. The lightest of the oils produced a reddish-brown color, similar to that afforded by pure hyduret of benzoyle, while the heaviest oil formed a bright red mixture, having a shade of violet, and those of intermediate density gave intermediate shades of color.
These results, viewed in connection with the differences of density in the different specimens, were at first thought to indicate that the light specimens had some admixture foreign to the oil, but on examining the action of the test on pure hyduret of benzoyle and the other legitimate constituents of the crude oil, it was evident that such an inference could not be justly drawn, and, indeed, suspicion now seemed rather to attach to the heavy oil. Subsequent experiments, however, showed that the light oil distils at a lower temperature than the heavy, and that if the heaviest specimens were distilled with water, the first portions that passed over produced precisely the same reaction as the light specimens above referred to, while the last portions that passed over, and especially the oil obtained from the water by distilling it, after saturating it with common salt, produced with oil of vitriol a splendid crimson color, the purity and intensity of which could hardly be surpassed.
It thus became pretty evident that the differences in the reaction of oil of vitriol with the different specimens of oil under notice, arose from variations in the circumstances under which the oils were distilled, and it seemed probable that the heavy oil had been obtained by distilling the almond cake with water, to which a large quantity of salt had been added, so as to raise the point of ebullition, while the light oil either was the product of a process in which less salt had been added to the water, or consisted of the first portions distilled.
In order to obtain more satisfactory evidence of the absence of spirit, or other foreign substance, from these samples of oil, nitric acid was used as a test. If oil of bitter almonds be mixed with about twice its volume of nitric acid, of specific gravity 1.420, no immediate action occurs. The greater part of the oil floats over the surface of the acid, and, if the former be free {209} from adulteration, no change of color takes place within several hours in either; but after the lapse of three or four days crystals of benzoic acid will begin to be formed from the oxidation of the hyduret of benzoyle by the nitric acid, and these will increase in quantity until the whole becomes a solid mass of crystals, which will gradually assume a bright emerald green color. This reaction is very characteristic. If spirit be present in the oil to the extent of eight or ten per cent., the acid, after a few minutes, will begin to react upon this, and a violent effervescence will shortly ensue, accompanied by the disengagement of nitrous vapors.
By using strong nitric acid, of specific gravity not less than 1.5, the presence of a very minute quantity of spirit may be detected. The pure oil, when mixed with an equal volume of this strong acid, forms a clear and uniform mixture, from which nothing separates, and which undergoes but a very slight change of color and no other visible alteration. The presence of two or three per cent. of spirit, however, is sufficient to cause a violent reaction and the disengagement of nitrous vapors.