he increase in the weight of the copper equals the weight of the
oxygen in the volume of air taken.

(3) A more accurate method is the following. A eudiometer tube is filled
with mercury and inverted in a vessel of the same liquid. A convenient
amount of air is then introduced into the tube and its volume accurately
noted. There is then introduced more than sufficient hydrogen to combine
with the oxygen present in the inclosed air, and the volume is again
accurately noted. The mixture is then exploded by an electric spark, and
the volume is once more taken. By subtracting this volume from the total
volume of the air and hydrogen there is obtained the contraction in
volume due to the union of the oxygen and hydrogen. The volume occupied
by the water formed by the union of the two gases is so small that it
may be disregarded in the calculation. Since oxygen and hydrogen combine
in the ratio 1: 2 by volume, it is evident that the contraction in
volume due to the combination is equal to the volume occupied by the
oxygen in the air contained in the tube, plus twice this volume of
hydrogen. In other words, one third of the total contraction is equal to
the volume occupied by the oxygen in the inclosed air. The following
example will make this clear:

Volume of air in tube 50.0 cc.
Volume after introducing hydrogen 80.0
Volume after combination of oxygen and hydrogen 48.5
Contraction in volume due to combination (80 cc.-48.5 cc.) 31.5
Volume of oxygen in 50 cc. of air (1/3 of 31.5) 10.5

All these methods agree in showing that 100 volumes of dry air contain
approximately 21 volumes of oxygen.

2. _Determination of nitrogen._ If the gas left after the removal of
oxygen from a portion of air is passed over heated magnesium, the
nitrogen is withdrawn, argon and the other rare elements being left. It
may thus be shown that of the 79 volumes of gas left after the removal
of the oxygen from 100 volumes of air, approximately 78 are nitrogen and
0.93 argon. The other elements are present in such small quantities that
they may be neglected.

3. _Determination of carbon dioxide._ The percentage of carbon dioxide
in any given volume of air may be determined by passing the air over
calcium hydroxide or some other compound which will combine with the
carbon dioxide. The increase in the weight of the hydroxide equals the
weight of the carbon d