Fig. 38.—Dumas and Boussingault's apparatus for the analysis of air by weight. The globe B contains
10–15 litres. The air is first pumped out of it, and it is weighed empty. The tube T connected
with it is filled with copper, and is weighed empty of air. It is heated in a charcoal furnace.
When the copper has become red-hot, the stopcock r (near R) is slightly opened, and the air
passes through the vessels L, containing a solution of potash, f , containing solutions and pieces
of caustic potash, which remove the carbonic anhydride from the air, and then through o and t ,
containing sulphuric acid (which has been previously boiled to expel dissolved air) and pumice-stone,
which removes the moisture from the air. The pure air then gives up its oxygen to the
copper in T. When the air passes into T the stopcock R of the globe B is opened, and it becomes
filled with nitrogen. When the air ceases to flow in, the stopcocks are closed, and the globe B
and tube T weighed. The nitrogen is then pumped out of the tube and it is weighed again.
The increase in weight of the tube shows the amount of oxygen, and the difference of the second
and third weighings of the tube, with the increase in weight of the globe, gives the weight of
the nitrogen.
Air free from moisture and carbonic anhydride28 contains 20·95 to
20·8829 parts by volume of oxygen; the mean amount of oxygen will
therefore be 20·92 ± 0·05 per cent. Taking the density of air = 1 and
of oxygen = 1·105 and nitrogen 0·972 the composition of air by weight
will be 23·12 per cent. of oxygen and 76·88 per cent. of nitrogen.30
