that some may be found among them the odors of which can be utilized.
Syntheses of this kind only rarely lead to natural principles; more frequently what perfumers call chemical products are obtained, or perfumes which betray their origin to a greater or less extent, and can not be used in the preparation of the finest products, but have cheapness and great strength in their favor. We should observe that chemical synthesis takes to the odorous principle itself, while essences contain only a very slight proportion of the active substance.
Finally, it sometimes happens that no new material is discovered, but some laboratory reaction already known is turned to industrial use. Such is the case with heliotropine, formerly known as piperonylic aldehyde or piperonal; terpineol, or white lilac; anisic aldehyde, or hawthorn, etc.
Twenty-five or thirty years ago it was believed that with a few well-known exceptions, such as those of bitter almonds, anise, mustard, and some others, the essences were constituted of hydrocarbons, C10H16 in indefinite numbers, all isomeric and similar to spirits of turpentine. Our views on this subject have been considerably modified. It has been found that the hydrocarbons or terpenes contained in essences may be referred to well-defined species possessing characteristic reactions and derivatives, some of them crystallizable, by which they may be distinguished. Oxidized principles have been isolated in essences related to the fatty series, capable of facile transformation into cyclic derivatives, which may be regarded as connecting links between the fatty and the aromatic series. Frequently a natural essence represents a harmonious mixture of various combinations. Oil of bergamot, having a composition of this character, possesses an odor vastly more characteristic than any of its components taken separately. And it sometimes happens that compounds suitable for fine perfumery, if they were pure, are spoiled by the presence of disagreeably smelling substances. The elimination of such principles, or the refining of such perfumes, has given rise to a second branch of our chemical industry.
The discoveries that have so far been made are quite insufficient to explain the composition and odor of essences. We find certain substances, like linalool and geraniol, common constituents in essences of the most different characters, and are hence forced to recognize that in a great number of cases they are only the vehicle, the substratum, of the really characteristic perfume; and we begin to suspect the presence of still rarer principles corresponding probably with a more differentiated, more specialized organism, and related to the specific characteristics of the vegetable cell.—Translated for the Popular Science Monthly from the Revue Scientiftque.