You have asked me several times, and I am very glad you have, "How do you
weigh gases?" I will shew you; it is very simple, and easily done. Here is
a balance, and here a copper bottle, made as light as we can consistent
with due strength, turned very nicely in the lathe, and made perfectly
air-tight, with a stop-cock, which we can open and shut, which at present
is open, and therefore allows the bottle to be full of air. I have here a
nicely-adjusted balance, in which I think the bottle, in its present
condition, will be balanced by the weight on the other side. And here is a
pump by which we can force the air into this bottle, and with it we will
force in a certain number of volumes of air, as measured by the pump.
[Twenty measures were pumped in.] We will shut that in and put it in the
balance. See how it sinks: it is much heavier than it was. By what? By
the air that we have forced into it by the pump. There is not a greater
_bulk_ of air, but there is the same bulk of _heavier_ air, because we
have forced in air upon it. And that you may have a fair notion in your
mind as to how much this air measures, here is a jar full of water. We
will open that copper vessel into this jar, and let the air return to its
former state. All I have to do now is to screw them tightly together, and
to turn the taps, when there, you see, is the bulk of the twenty pumps of
air which I forced into the bottle; and to make sure that we have been
quite correct in what we have been doing, we will take the bottle again to
the balance, and, if it is now counterpoised by the original weight, we
shall be quite sure we have made our experiment correctly.

[Illustration: Fig. 26.]

It is balanced; so, you see, we can find out the weight of the extra
volumes of air forced in, in that way, and by that means we are able to
ascertain that a cubic foot of air weighs 1-1/5 ounce. But that small