tle are flowing into the jar of air, and you
see I obtain exactly the same kind of action as before, and that shews me
that there is oxygen in the air--the very same substance that has been
already obtained by us from the water produced by the candle. But then,
beyond that, how is it that the candle does not burn in air as well as in
oxygen? We will come to that point at once. I have here two jars; they are
filled to the same height with gas, and the appearance to the eye is alike
in both, and I really do not know at present which of these jars contains
oxygen and which contains air, although I know they have previously been
filled with these gases. But here is our test-gas, and I am going to work
with the two jars, in order to examine whether there is any difference
between them in the quality of reddening this gas. I am now going to turn
this test-gas into one of the jars, and observe what happens. There is
reddening, you see; there is then oxygen present. We will now test the
other jar; but you see this is not so distinctly red as the first: and,
further, this curious thing happens,--if I take these two gases and shake
them well together with water, we shall absorb the red gas; and then, if I
put in more of this test-gas and shake again, we shall absorb more; and I
can go on as long as there be any oxygen present to produce that effect.
If I let in air, it will not matter; but the moment I introduce water, the
red gas disappears; and I may go on in this way, putting in more and more
of the test-gas, until I come to something left behind which will not
redden any longer by the use of that particular body that rendered the air
and the oxygen red. Why is that? You see in a moment it is because there
is, besides oxygen, something else present which is left behind. I will
let a little more air into the jar, and if it turns red you will know that
some of that reddening gas is still present, and that consequently it was
not for the want of this producing body that that air was left behind.

Now, you will begin to understand what I am about to say. You saw that
when I burnt phosphorus in a jar, as the smoke produced by the phosphorus
and the oxygen of the air condensed, it left a good deal of gas unburnt,
just as this red gas left something untouched,--there was, in fact, this
gas left behind, which the phosphorus cannot touch, which the reddening
gas cannot touch, and this something is not oxygen, and yet is part of the
atmos