Since aromatic substances during evaporation become mixed with air, it appears probable that they act upon the olfactory nerves only at the moment when they become oxidized.
The entire process of resinification of oil of turpentine can be followed very clearly on the pitch pine (Pinus austriaca, or other species of Pinus), just as oil of turpentine in general can be taken as an example of an essential oil on which the peculiarities of the non-oxygenated essential oils may be easily studied. In many localities the pitch pine is partly deprived of its bark when it has reached a certain age. From the trunk exudes oil of turpentine which in the air becomes more and more viscid by the absorption of oxygen and changes into balsam, called turpentine. The latter is collected and distilled with water, when the unchanged oil of turpentine passes over with the steam, while the odorless resin (rosin or colophony) remains behind in the stills.
The above-mentioned qualities of the essential oils indicate naturally how those used in perfumery, which are often very costly, are to be preserved. For this purpose small strong bottles should be chosen which are closed with well-fitting glass stoppers, over which is applied a glass capsule ground to fit tightly over the neck of the bottle. These bottles should always be completely filled (hence small bottles should be selected), and kept tightly closed, in the dark. As the action of oxygen is retarded by low temperatures, it is advisable to keep bottles containing essential oils in a cool cellar. But care must be had never to pour out an essential oil in the cellar near an open candle light. The vapors are very apt to take fire, as they are quite inflammable.
As there are a great many aromatic vegetable substances, so there are numerous odors, or, to retain the customary though incorrect appellation, numerous essential oils. All of these, however, cannot be used in the art of perfumery, as some of them do not possess a pleasant odor, as is the case, for instance, with oil of turpentine. (We may state here, however, that very pure oil of turpentine, distilled from certain Coniferæ, has an agreeable, refreshing odor which at present has found application in perfumery under the title of forest perfume or pine-needle essence.) Besides, there are numerous essential oils which, while possessing a very pleasant odor, still cannot be used in perfumery except for very cheap preparations, though they are employed in much larger quantities in the manufacture of liqueurs. Such oils are: oil of cumin, fennel, juniper, absinth, etc.
As we shall return to this subject in connection with the essential oils which are used in perfumery in general, we will now consider at greater length the aromatic vegetable substances which are employed for the manufacture of fragrant odors.
[CHAPTER IV.]
THE AROMATIC VEGETABLE SUBSTANCES EMPLOYED IN PERFUMERY.
Every fragrant portion of a plant can be used for the preparation of an aromatic substance, and therefore for the manufacture of a perfume. Hence we are unable, in the following enumeration of the aromatic vegetable substances, to make any claim to absolute completeness; for every new scientific expedition may acquaint us with hitherto unknown plants from which the finest odors may be obtained. We have said above that we have not yet even fixed in our perfumes all the odors of the known aromatic plants, and therefore there is still a large field open to the progressive manufacturer.
In the following pages we must restrict ourselves to the description of those aromatic vegetable substances which are used in the laboratories of the most advanced and scientific perfumers for the manufacture of odors. At the same time we lay particular stress on the fact that the knowledge of these raw materials is a matter of the greatest importance to the manufacturer of perfumes because it enables him to appreciate the differences, often very minute, between fine and inferior qualities. Every manufacturer who aims at the production of fine goods must make it the rule to use nothing but the best raw materials.
The price of the latter is apparently disproportionately high; for all that, only the most expensive materials should be bought, for it is the only kind that can be used. Let us give but two instances in illustration. We find in the market, grades of vanilla the prices of which are as one to four; the latter is fresh and contains the aromatic substance in large amount; the former is old, dry, and worthless, with an artificial glossy surface and little odor. The differences in the price are still greater in an aromatic substance of animal origin, musk, the cheapest grades of which are altogether artificial and perfumed with a mere trace of genuine musk.