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,
Roentgen 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'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