6 REPORT—1899. 
these ‘elements of old means a knowledge of the composition of the 
atmosphere, of water, and of all the other things which we call matter, as 
well as a knowledge of the general properties of gases, liquids, and solids, 
and of the nature and effects of combustion. Of all these things our 
knowledge to-day is large and exact, and, though ever enlarging, in some 
respects complete. When did that knowledge begin to become exact ? 
To-day the children in our schools know that the air which 
wraps round the globe is not a single thing, but is made up of two 
things, oxygen and nitrogen,! 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 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 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 exist- 
ence 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 composi- 
tion 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 
* Some may already know that there is at least a third thing, argon. 
