direction of a chemico-vibratory theory, as pointed out by Southerden (Nature, March 26, 1903), the olfactory cells being directly stimulated, not by the ordinary vibrations of the molecules, but by the agitations accompanying chemical changes.

The vibratory hypothesis of the action of odors has had some influence on the recent physiologists who have chiefly occupied themselves with olfaction.  “It is probable,” Zwaardemaker writes (L’Annee Psychologique, 1898), “that aroma is a physico-chemical attribute of the molecules”; he points out that there is an intimate analogy between color and odor, and remarks that this analogy leads us to suppose in an aroma ether vibrations of which the period is determined by the structure of the molecule.

Since the physiology of olfaction is yet so obscure it is not surprising that we have no thoroughly scientific classification of smells, notwithstanding various ambitious attempts to reach a classification.  The classification adopted by Zwaardemaker is founded on the ancient scheme of Linnaeus, and may here be reproduced:—­

    I. Ethereal odors (chiefly esters; Rimmel’s fruity series).

II.  Aromatic odors (terpenes, camphors, and the spicy, herbaceous, rosaceous, and almond series; the chemical types are well determined:  cineol, eugenol, anethol, geraniol, benzaldehyde).

    III.  The balsamic odors (chiefly aldehydes, Rimmel’s jasmin,
    violet, and balsamic series, with the chemical types:  terpineol,
    ionone, vanillin).

    IV.  The ambrosiacal odors (ambergris and musk).

    V. The alliaceous odors, with the cacodylic group (asafoetida,
    ichthyol, etc.).

    VI.  Empyreumatic odors.

    VII.  Valerianaceous odors (Linnaeus’s Odores hircini, the capryl
    group, largely composed of sexual odors).

    VIII.  Narcotic odors (Linnaeus’s Odores tetri).

    IX.  Stenches.

A valuable and interesting memoir, “Revue Generale sur les Sensations Olfactives,” by J. Passy, the chief French authority on this subject, will be found in the second volume of L’Annee Psychologique, 1895.  In the fifth issue of the same year-book (for 1898) Zwaardemaker presents a full summary of his work and views, “Les Sensations Olfactives, leurs Combinaisons et leurs Compensations.”  A convenient, but less authoritative, summary of the facts of normal and pathological olfaction will be found in a little volume of the “Actualites Medicales” series by Dr. Collet, L’Odorat et ses Troubles, 1904.  In a little book entitled Wegweiser zu einer Psychologie des Geruches (1894) Giessler has sought to outline a psychology of smell, but his sketch can only be regarded as tentative and provisional.

At the outset, nevertheless, it seems desirable that we should at least have some conception of the special