468 
To-day the children in our schools know 
that the air which wraps round the globe is 
is not a single thing, but is made up of two 
things, oxygen and nitrogen,* mingled to- 
gether. They know, again, that water is 
not a single thing, but the product of two 
things, oxygen and hydrogen, joined to- 
gether. 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 your- 
selves 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, phi- 
losophers stumbled about in darkness, mis- 
led for the 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 
*Some may already know that there is at least a 
third thing, argon. 
SOIENCE. 
[N. 8. Von. X. No. 249. 
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 Cavendish, to 
Lavoisier, and Priestley. Ifit was Priestley 
who was the first to demonstrate the exis- 
tence of what we now call oxygen, it is to 
Lavoisier 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 necessary 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 memoir 
‘on the nature of the principle which enters 
into combination with metals during cal- 
cination’ appeared in 1775, and Caven- 
dish’s paper on the composition of water 
did not see the light until 1784. 
During the last quarter of the eighteenth 
century this new idea of oxygen and oxida- 
tion was struggling into existence. How 
new was the idea is illustrated by the fact 
that Lavoisier himself at first spoke of that. 
which he was afterwards, namely, in 1778, 
led to call oxygen, the name by which it 
has since been known, as ‘the principle 
which enters into combination.’ What diffi- 
culties its acceptance met with is illustrated 
by the fact that Priestley himself refused to 
the end of his life to grasp the true bear- 
ings of the discovery which he had made. 
In the year 1799 the knowledge of oxygen, 
of the nature of water and of air, and in- 
deed the true conception of chemical com- 
position and chemical change, was hardly 
more than beginning to be, and the century 
had to pass wholly away before the next 
great chemical idea, which we know by the 
name of the Atomic Theory of John Dal- 
ton, was made known. We have only to 
