as indefinite as Hodge's "piece of chalk" as regards size. The
professor defined a poison as "any substance which otherwise than by
the agency of heat or electricity is capable of destroying life,
either by chemical action on the tissues of the living body or by
physiological action by absorption into the living system." This
definition excepted from the list of poisons all agencies that
destroyed life by a simple mechanical action, thus drawing a
distinction between a "poison" and a "destructive thing." It explains
why nitrogen is not a poison and why carbonic acid is, although
neither can support life. This point the lecturer illustrated. A
poison must be capable of destroying life. It was nonsense to talk of
a "deadly poison." If a body be a poison, it is deadly; if it be not
deadly, it is not a poison. Three illustrations of the chemical
actions of poisons were selected. The first was sulphuric acid. Here
the molecular death of the part to which the acid was applied was due
to the tendency of sulphuric acid to combine with water. The stomach
became charred. The molecular death of certain tissues destroyed the
general functional rhythmicity of the system until the disturbance
became general, somatic death (that is, the death of the entire body)
resulting. The second illustration was poisoning by carbonic oxide.
The professor gave an illustrated description of the origin and
properties of the coloring matter of the blood, known as _hæmoglobin_,
drawing attention to its remarkable formation by a higher synthetical
act from the albumenoids in the animal body, and to the circumstance
that, contrary to general rule, both its oxidation and reduction may
be easily effected. It was explained that on this rhythmic action of
oxidizing and reducing _hæmoglobin_ life depended.

Carbonic oxide, like oxygen, combined with _hæmoglobin_, produced a
comparatively stable compound; at any rate, a compound so stable that