g in dispute.

Each of the main disputants was biased by his own particular views as
to the moot points of chemistry. Lavoisier, for example, believed oxygen
gas to be composed of a metal oxygen combined with the alleged element
heat; Dr. Priestley thought it a compound of positive electricity and
phlogiston; and Humphry Davy, when he entered the lists a little later,
supposed it to be a compound of oxygen and light. Such mistaken notions
naturally complicated matters and delayed a complete understanding of
the chemical processes of respiration. It was some time, too, before the
idea gained acceptance that the most important chemical changes do not
occur in the lungs themselves, but in the ultimate tissues. Indeed,
the matter was not clearly settled at the close of the century.
Nevertheless, the problem of respiration had been solved in its
essentials. Moreover, the vastly important fact had been established
that a process essentially identical with respiration is necessary to
the existence not only of all creatures supplied with lungs, but to
fishes, insects, and even vegetables--in short, to every kind of living
organism.

ERASMUS DARWIN AND VEGETABLE PHYSIOLOGY

Some interesting experiments regarding vegetable respiration were made
just at the close of the century by Erasmus Darwin, and recorded in his
Botanic Garden as a foot-note to the verse:

"While spread in air the leaves respiring play."

These notes are worth quoting at some length, as they give a clear idea
of the physiological doctrines of the time (1799), while taking advance
ground as to the specific matter in question:

"There have been various opinions," Darwin says, "concerning the use of
the leaves of plants in the vegetable economy. Some have contended
that they are perspiratory organs. This does not seem probable from an
experiment of Dr. Hales, Vegetable Statics, p. 30. He, found, by cutting
off branches of trees with apples on them and taking off the leaves,
that an apple exhaled about as much as two leaves the surfaces of which
were nearly equal to the apple; whence it would appear that apples have
as good a claim to be termed perspiratory organs as leaves. Others have
believed them excretory organs of excrementitious juices, but as
the vapor exhaled from vegetables has no taste, this idea is no more
probable than the other; add to this that in most weathers they do not
appear to perspire or exhale at all.

"The internal surface of