air, and it soon settles into the
position shown in the figure. The weighing can then be made as the
absorber is swinging freely in the air. After the weighing has been
made, the elevator is again lifted, the hooks are released, and by
turning the valve the elevator and load are safely lowered.

The size of the openings of the pipes into the cylinder is so adjusted
that the movement of the elevator is regular and moderate whether it is
being raised or lowered, thus avoiding any sudden jars that might cause
an accident to the absorbers. With this system it is possible to weigh
these absorbers to within 0.1 gram and, were it necessary, probably the
error could be diminished so that the weight could be taken to 0.05
gram. On a balance of this type described elsewhere,[20] weighings could
be obtained to within 0.02 gram. For all practical purposes, however, we
do not use the balance for weighing the absorbers closer than to within
0.10 gram. In attempting to secure accuracy no greater than this, it is
unnecessary to lower the glass door to the balance case or, indeed, to
close the two doors to the compartment in which the elevator is closed,
as the slight air-currents do not affect the accuracy of the weighing
when only 0.1 gram sensitiveness is required.

PURIFICATION OF THE AIR-CURRENT WITH SODIUM BICARBONATE.

As is to be expected, the passage of so large a volume of air through
the sulphuric acid in such a relatively small space results in a slight
acid odor in the air-current leaving this absorber. The amount of
material thus leaving the absorber is not weighable, as has been shown
by repeated tests, but nevertheless there is a sufficiently irritating
acid odor to make the air very uncomfortable for subsequent respiration.
It has been found that this odor can be wholly eliminated by passing the
air through a can containing cotton wool and dry sodium bicarbonate.
This can is not weighed, and indeed, after days of use, there is no
appreciable change in its weight.

VALVES.

In order to subdivide experiments into periods as short as 1 or 2 hours,
it is necessary to deflect the air-current at the end of each period
from one set of purifiers to the other, in order to weigh the set used
and to measure the quantity of carbon dioxide and water-vapor absorbed.
The conditions under which these changes from one system to another are
made, and which call for an absolutely gas-tight closure, have been
discussed in detail e