side of the absorber system (see page 66) at the
beginning of every experiment. This insures the same power of absorption
on each side of the absorption system so that the residual amount of
carbon dioxide in the chamber from period to period does not undergo
very marked changes. This has been found the best method, because if one
can is left on a day longer than the other there is apt to be
alternately a rise and fall in the amount of residual carbon dioxide in
the apparatus, owing to the unequal efficiency of the absorbers.

These cans are each day taken to the basement, where the first
section[19] only is taken out and replaced with new potash-lime. Thus,
three-quarters of the contents of the can is used over and over, while
the first quarter is freshly renewed every day. Potash-lime has not been
found practicable for the U-tubes because one can not, as in the case of
soda-lime, see the whitening of the reagent where the carbon dioxide is
absorbed.

The importance of having the soda-lime or potash-lime somewhat moist, to
secure the highest efficiency for the absorption of the carbon dioxide,
makes it necessary to absorb the moisture taken up by the dry air in
passing through the potash-lime can. Consequently a second vessel
containing sulphuric acid is placed in the system to receive the air
immediately after it leaves the potash-lime can. Obviously the amount of
water absorbed here is very much less than in the first acid absorber
and hence the same absorber can be used for a greater number of
experiments.

BALANCE FOR WEIGHING ABSORBERS.

The complete removal of water-vapor and carbon dioxide from a current of
air moving at the rate of 75 liters per minute calls for large and
somewhat unwieldy vessels in which is placed the absorbing material.
This is particularly the case with the vessels containing the rather
large amounts of sulphuric acid required to dry the air. In the course
of an hour there is ordinarily removed from the chamber not far from 25
grams of water-vapor and 20 to 30 grams of carbon dioxide. This
necessitates weighing the absorbers to within 0.25 gram if an accuracy
of 1 per cent is desired. The sulphuric-acid absorbers weigh about 18
kilograms when filled with acid. In order to weigh this receptacle so as
to measure accurately the increase in weight due to the absorption of
water to within less than 1 per cent, we use the balance shown in fig.
29. This balance has been employed in a number