But as we know of no such being living in space, who can tell us
what takes place in our invisible air, and we cannot see it
ourselves, we must try by experiments to see it with our
imagination, though we cannot with our eyes.

First, then, can we discover what air is? At one time it was
thought that it was a simple gas and could not be separated into
more than one kind. But we are now going to make an experiment
by which it has been shown that air is made of two gases mingled
together, and that one of these gases, called oxygen, is used up
when anything burns, while the other nitrogen is not used, and
only serves to dilute the minute atoms of oxygen. I have here a
glass bell-jar, with a cork fixed tightly in the neck, and I
place the jar over a pan of water, while on the water floats a
plate with a small piece of phosphorus upon it. You will see
that by putting the bell-jar over the water, I have shut in a
certain quantity of air, and my object now is to use up the
oxygen out of this air and leave only nitrogen behind. To do
this I must light the piece of phosphorus, for you will remember
it is in burning that oxygen is used up. I will take the cork
out, light the phosphorus, and cork up the jar again. See! as
the phosphorus burns white fumes fill the jar. These fumes are
phosphoric acid which is a substance made of phosphorous and the
oxygen of the air together.

Now, phosphoric acid melts in water just as sugar does, and in a
few minutes these fumes will disappear. They are beginning to
melt already, and the water from the pan is rising up in the
bell-jar. Why is this? Consider for a moment what we have done.
First, the jar was full of air, that is, of mixed oxygen and