frequency and form of the vibrations initiated by the action of the chemical molecules on these olfactory cells, though he admitted that such a conception involved a very subtle conception of molecular vibration. Vaschide and Van Melle (Paris Academy of Sciences, December 26, 1899) have, again, argued that smell is produced by rays of short wave-lengths, analogous to light-rays, Röntgen rays, etc. Chemical action is however, a very important factor in the production of odors; this has been well shown by Ayrton (_Nature_, September 8, 1898). We seem to be forced in the 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'Année 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 Linnæus, 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 (Linnæus's _Odores hircini_, the capryl group, largely composed of sexual odors). VIII. Narcotic odors (Linnæus's _Odores tetri_). IX. Stenches.