The problem of absorbing the water-vapor from so rapid a current of air
is second only to that of absorbing the carbon dioxide from such a
current. All experiments with potassium hydroxide in the form of sticks
or in solution failed to give the desired results and the use of
soda-lime has supplemented all other forms of carbon dioxide absorption.
More recently we have been using potash-lime, substituting caustic
potash for caustic soda in the formula, and the results thus obtained
are, if anything, more satisfactory than with the soda-lime.
The potash-lime is made as follows: 1 kilogram of commercial potassium
hydroxide, pulverized, is dissolved in 550 to 650 cubic centimeters of
water and 1 kilogram of pulverized quicklime added slowly. The amount of
water to be used varies with the moisture content of the potash. There
is a variation in the moisture content of different kegs of potash, so
when a keg is opened we determine experimentally the amount of water to
be used. After a batch is made up in this way it should be allowed to
cool before testing whether it has the right amount of water, and this
is determined by feeling of it and noting how it pulverizes in the hand.
It is not advisable to make a great quantity at once, because we have
found that if a large quantity is made and broken into small particles
and stored in a container it has a tendency to cake and thus interfere
with its ready subsequent use.
A record was kept of the gains in weight of a can filled with
potash-lime during a series of experiments where there were three
silver-plated cans used. This can was put at the head of the system and
when it began to lose weight it was removed. The records of gains of
weight when added together amount to 400 grams. From experience with
other cans where the loss of moisture was determined, it is highly
probable that at least 200 grams of water were vaporized from the
reagent and thus the total amount of carbon dioxide absorbed must have
been not far from 600 grams. At present our method is not to allow the
cans to gain a certain weight, but during 4-hour or 5-hour experiments,
in which each can may be used 2 or 3 hours, it is the practice to put a
new can on each 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.