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.
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