t infinitely small degree, can cause the nitrogen to combine
directly with the other element of the atmosphere, or with other things
round about it; it is a perfectly indifferent, and therefore to say, a
safe substance.

But before I take you to that result, I must tell you about the atmosphere
itself. I have written on this diagram the composition of one hundred
parts of atmospheric air:--

Bulk. Weight.
Oxygen, . . . . . 20 22.3
Nitrogen, . . . . 80 77.7
---- -----
l00 100.0

It is a true analysis of the atmosphere, so far as regards the quantity of
oxygen and the quantity of nitrogen present. By our analysis, we find that
5 pints of the atmosphere contain only 1 pint of oxygen, and 4 pints, or 4
parts, of nitrogen by bulk. That is our analysis of the atmosphere. It
requires all that quantity of nitrogen to reduce the oxygen down, so as to
be able to supply the candle properly with fuel, so as to supply us with
an atmosphere which our lungs can healthily and safely breathe; for it is
just as important to make the oxygen right for us to breathe, as it is to
make the atmosphere right for the burning of the fire and the candle.

But now for this atmosphere. First of all, let me tell you the weight of
these gases. A pint of nitrogen weighs 10-4/10 grains, or a cubic foot
weighs 1-1/6 ounce. That is the weight of the nitrogen. The oxygen is
heavier: a pint of it weighs 11-9/10 grains, and a cubic foot weighs 1-3/4
ounce. A pint of air weighs about 10-7/10 grains, and a cubic foot 1-1/5
ounce.

[Illustration: Fig. 25.]

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 heavi