Experiment 67. If gallic acid be placed on a red-hot iron, it burns with flame, and emits an aromatic smell, similar to that of benzoic acid; but, if mixed with chlorate of potassa and struck with a hot hammer, a detonation will ensue. Various vegetable acids, as the benzoic, which is highly inflammable, produce similar effects.
[CHAPTER IV.]
There is a variety of scented fires, all partaking, in a greater or lesser degree, of a peculiar flavour, according to the substances, which enter into their composition. It is a fact, that, in the ordinary odoriferous fire, into which, either the so called scented gums, or essential oils, enter as a component part, these substances are not only decomposed in the act of combustion, but evolve, during that process, a part of their respective odours, to which we attribute the scent imparted to the atmosphere. In those instances, in which gunpowder forms a part of the composition, it is to be remarked, that the peculiar smell of fired gunpowder is scarcely recognized, owing to the preponderance of the scent in the composition. Hence it is, that scented fire-works are more calculated for confined places than for the open air.
Scented fires are various both in their nature and composition, and may always be so modified, as, in their effect, to produce, not only the particular flame, or appearance of the fire, but the extrication, along with the gaseous products, of the odour of the essential oil, or other substance made use of.
Linnæus, in a dissertation on the odours of different substances, endeavoured to classify them. M. Lorrey (Mémoires de la Société Royale de Medicine 1784) divided them into five classes; viz. camphors, narcotics, ethers, volatile acids, and alkalies; but it is obvious, that it is an impossibility to class all the odours which exist, and may be formed by the mixture, or combination of various substances. We may consider them either pleasant, or unpleasant to the sense of smelling. But as we recognize bodies very frequently by their odour, with which we become familiar, as camphor and assafœtida, for instance; so the olfactories may be affected by other odours. Aromatic and fetid odours are opposite to each other. Some of the gases, as the olefiant, have a fragrant smell, and others, as hydrogen, and sulphuretted hydrogen, either alone or mixed, are extremely unpleasant. The intestinal gas (gas intestinaux of the French) is a particular instance of the odour of a compound gas, or mixture of gaseous fluids. The experiments of M. Jurine of Geneva, of MM. Chevreul and Magendie, (Ann. de Chim. et de Phys. t. ii, 294), of M. Vauquelin, (Journal de Pharmacie, t. iii, p. 205), and of MM. Lameyran and Fremy, (Bulletin de Pharmacie, t. 1, p. 358), are interesting on this subject. Intestinal gas differs in its composition. It always contains carbonic acid gas, and azotic gas, and hydrogen gas, either pure, or combined with carbon and sulphur. Thenard (Traité de Chimie, iii, p. 576) contains some observations on this subject.
In the camphor odour, Lorrey includes not only camphor itself, but various species of laurel, myrrh, and turpentine. In the narcotic odour, he embraces opium, various gum-resins, roses, lillies, jessamine, &c. and musk, amber, and castor. In the ethereal odour, different kinds of ether. Under the odour of volatile acids, he considers that of fruits, aromatic barks, citron; and under the alkaline odour, the acrid, and, in general, the antiscorbutic plants. Fourcroy, in treating of the aroma of plants, or the spiritus rector of Boerhaave, (Bulletin de la Société Philomatique, an. 6, p. 52,) has some interesting facts on this subject.
It is evident, that perfumes, so called, owe their peculiar fragrance to an essential oil, which characterizes each kind; for the essential oil obtained by distillation, partakes of the odour of the plant. Hence the oils of mint, roses, thyme, cinnamon, cloves, &c. &c. all of which are peculiar in this respect. Odoriferous fire-works owe their particular properties to the presence of some gum, resin, or oil. As to the expansibility, or rather the divisibility of odour, several interesting facts are known. In a work, entitled l'Existence de Dieu, par les merveilles de la Nature, we are informed, that, if we take the one-fourth of a drachm of benzoin, and place it in the four corners of a room, the odour will be recognised in an instant. The chamber in which the experiment was made, the author states, was 24 feet by 16, and contained 9212 cubic feet of air, which, multiplied by 1000, would give 9216000 inches, and 1000000 parts of an inch were rendered appreciable. Therefore, he infers, that 9216000000000 are equally perceptible in the chamber. Prevot (Bulletin de la Société Philomatique, an. 6) has some observations of the same nature, respecting camphor. If such are the effects with benzoin, what, we may ask, would be those of the more powerful perfumes, such as musk? One grain, or perhaps the tenth part of a grain of musk, would scent the atmosphere of a room very perfectly.
De Laval (Description of the Maldiva Islands) mentions the use of scented fire by the inhabitants, in the celebration of their festivals. On the day of every new moon, they place at the entrance of the churches, and the gates of their houses, cocoa shells cut in the middle, and filled with white sand and burning coals, upon which they burn, almost all night, sweet scented gums and woods; and at the nocturnal festival, called maulude, the night on which Mahomet died, their halls are illuminated with a multitude of lamps, and the air is filled with the smoke of perfumes. The use of scented fire appears to form a principal part of their devotional exercises. Perfumes are even burnt on the graves of deceased persons.