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, p. 187) in analogy with light and
sound. Haycraft (_Proceedings of the Royal Society of Edinburgh_,
1883-87, and _Brain_, 1887-88), largely starting from
Mendelieff's law of periodicity, similarly sought to bring smell
into line with the higher senses, arguing that molecules with the
same vibration have the same smell. Rutherford (_Nature_, August
11, 1892, p. 343), attaching importance to the evidence brought
forward by von Brunn showing that the olfactory cells terminate
in very delicate short hairs, also stated his belief that the
different qualities of smell result from differences in the
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