ent care is
taken to introduce an excess of one of the gases.

~A more convenient form of eudiometer.~ A form of eudiometer (Fig. 21)
different from that shown on page 43 is sometimes used to avoid the
calculations necessary in reducing the volumes of the gases to the same
conditions of temperature and pressure in order to make comparisons.
With this apparatus it is possible to take the readings of the volumes
under the same conditions of temperature and pressure, and thus compare
them directly. The apparatus (Fig. 21) is filled with mercury and the
gases introduced into the tube A. The experiment is carried out as in
the preceding one, except that before taking the reading of the gas
volumes, mercury is either added to the tube B or withdrawn from it by
means of the stopcock C, until it stands at exactly the same height in
both tubes. The gas inclosed in tube A is then under atmospheric
pressure; and since but a few minutes are required for performing the
experiment, the conditions of temperature and pressure may be regarded
as constant. Hence the volumes of the hydrogen and oxygen and of the
residual gas may be read off from the tube and directly compared.

[Illustration: Fig. 21]

~Method used by Berzelius and Dumas.~ The method used by these
investigators enables us to determine directly the proportion by weight
in which the hydrogen and oxygen combine. Fig. 22 illustrates the
apparatus used in making this determination. B is a glass tube
containing copper oxide. C and D are glass tubes filled with calcium
chloride, a substance which has great affinity for water. The tubes B
and C, including their contents, are carefully weighed, and the
apparatus connected as shown in the figure. A slow current of pure
hydrogen is then passed through A, and that part of the tube B which
contains copper oxide is carefully heated. The hydrogen combines with
the oxygen present in the copper oxide to form water, which is absorbed
by the calcium chloride in tube C. The calcium chloride in tube D
prevents any moisture entering tube C from the air. The operation is
continued until an appreciable amount of water has been formed. The
tubes B and C are then weighed once more. The loss of weight in the
tube B will exactly equal the weight of oxygen taken up from the
copper oxide in the formation of the water. The gain in weight in the
tube C will exactly equal the weight of the water formed. The
difference in these weights will of course e