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.]