o things, oxygen and
nitrogen,[3] mingled together. They know, again, that water is not a
single thing, but the product of two things, oxygen and hydrogen, joined
together. They know that when the air makes the fire burn and gives the
animal life, it is the oxygen in it which does the work. They know that
all round them things are undergoing that union with oxygen which we call
oxidation, and that oxidation is the ordinary source of heat and light.
Let me ask you to picture to yourselves what confusion there would be
to-morrow, not only in the discussions at the sectional meetings of our
association, but in the world at large, if it should happen that in the
coming night some destroying touch should wither up certain tender
structures in all our brains and wipe out from our memories all traces of
the ideas which cluster in our minds around the verbal tokens, oxygen and
oxidation. How could any of us--not the so-called man of science alone,
but even the man of business and the man of pleasure--go about his ways
lacking those ideas? Yet those ideas were, in 1799, lacking to all but a
few.

Although in the third quarter of the seventeenth century the light of
truth about oxidation and combustion had flashed out in the writings of
John Mayow, it came as a flash only, and died away as soon as it had come.
For the rest of that century, and for the greater part of the next,
philosophers stumbled about in darkness, misled for most of the time by
the phantom conception which they called phlogiston. It was not until the
end of the third quarter of the eighteenth century that the new light,
which has burned steadily ever since, lit up the minds of the men of
science. The light came at nearly the same time from England and from
France. Rounding off the sharp corners of controversy, and joining, as we
may fitly do to-day, the two countries as twin bearers of a common crown,
we may say that we owe the truth to Priestley, to Lavoisier, and to
Cavendish. If it was Priestley who was the first to demonstrate the
existence of what we now call oxygen, it is to Lavoisier that we owe the
true conception of the nature of oxidation and the clear exposition of the
full meaning of Priestley's discovery; while the knowledge of the
composition of water, the necessity complement of the knowledge of oxygen,
came to us through Cavendish and, we may perhaps add, through Watt.

The date of Priestley's discovery of oxygen is 1774; Lavoisier's classic
m