[Illustration: Fig. 22.]
You may easily see that although we obtained from water twice as much in
volume of the hydrogen as of oxygen, it does not follow that we have twice
as much in weight--because one is heavy, and the other a very light gas.
We have means of weighing gases or air; but without stopping to explain,
that, let me just tell you what their respective weights are. The weight
of a pint of hydrogen is three-quarters of a grain; the weight of the same
quantity of oxygen is nearly twelve grains. This is a very great
difference. The weight of a cubit foot of hydrogen is one-twelfth of an
ounce; and the weight of a cubit foot of oxygen is one ounce and a third.
And so on we might come to masses of matter which may be weighed in the
balance, and which we can take account of as to hundredweights and as to
tons, as you will see almost immediately.
Now, as regards this very property of oxygen supporting combustion, which
we may compare to air, I will take a piece of candle to shew it you in a
rough way, and the result will be rough. There is our candle burning in
the air: how will it burn in oxygen? I have here a jar of this gas, and I
am about to put it over the candle for you to compare the action of this
gas with that of the air.
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