en_ and
elsewhere. The writer who more than any other has in recent years restored
the study of the sense of smell from a by-path to its proper position as a
highway for investigation is without doubt Professor Zwaardemaker, of
Utrecht. The invention of his first olfactometer in 1888 and the
appearance in 1895 of his great work _Die Physiologie des Geruchs_ have
served to give the physiology of the sense of smell an assured status and
to open the way anew for much fruitful investigation, while a number of
inquirers in many countries have had their attention directed to the
elucidation of this sense.

Notwithstanding, however, the amount of work which has been done in this
field during recent years, it cannot be said that the body of assured
conclusions so far reached is large. The most fundamental principles of
olfactory physiology and psychology are still somewhat vague and
uncertain. Although sensations of smell are numerous and varied, in this
respect approaching the sensations of vision and hearing, smell still
remains close to touch in the vagueness of its messages (while the most
sensitive of the senses, remarks Passy, it is the least precise), the
difficulty of classifying them, the impossibility of so controlling them
as to found upon them any art. It seems better, therefore, not to attempt
to force the present study of a special aspect of olfaction into any
general scheme which may possibly not be really valid.

The earliest and most general tendency in regard to the theory of
smell was to regard it as a kind of chemical sense directly
stimulated by minute particles of solid substance. A vibratory
theory of smell, however, making it somewhat analogous to
hearing, easily presents itself. When I first began the study of
physiology in 1881, a speculation of this kind presented itself
to my mind. Long before Philipp von Walther, a professor at
Landshut, had put forward a dynamic theory of olfaction
(_Physiologie des Menschen_, 1807-8, vol. ii, p. 278). "It is a
purely dynamic operation of the odorous substance in the
olfactory organ," he stated. Odor is conveyed by the air, he
believed, in the same way as heat. It must be added that his
reasons for this theory will not always bear examination. More
recently a similar theory has been seriously put forward in
various quarters. Sir William Ramsay tentatively suggested such a
theory (_Nature_, vol. xxv,