ining small amounts of carbon dioxide with great
accuracy, the method for determining water-vapor to be accurate calls
for the use of rather large quantities of air. From preliminary
experiments with a sling psychrometer it was found that its use was
precluded by the space required to successfully use this instrument, the
addition of an unknown amount of water to the chamber from the wet bulb,
and the difficulties of reading the instrument from without the chamber.
Recourse was had to the determination of moisture by the absolute
method, in that a definite amount of air is caused to pass over
pumice-stone saturated with sulphuric acid. It is of interest here to
record that at the moment of writing a series of experiments are in
progress in which an attempt is being made to use a hair hygrometer for
this purpose.

The method of determining the water-vapor and carbon dioxide in the
residual air is extremely simple, in that a definite volume of air is
caused to pass over sulphuric acid and soda-lime contained in U-tubes.
In other words, a small amount of air is caused to pass through a small
absorbing-system constructed of U-tubes rather than of porcelain vessels
and silver-plated cans. Formerly a very elaborate apparatus was employed
for aspirating the air from the chamber through U-tubes and then
returning the aspirated air to the chamber. This involved the use of a
suction-pump and called for a special installation for maintaining the
pressure of water constant. More recently a much simpler device has been
employed, in that we have taken advantage of the pressure in the
ventilating air-system developed by the passage of air through the
blower. After forcing a definite quantity of air through the reagents in
the U-tubes, it is then conducted back to the system after having been
measured in a gas-meter.

This procedure is best noted from fig. 30. The connected series of three
U-tubes on the rack on the table is joined on one end by well-fitting
rubber connections to the tube leading from the mercurial manometer and
on the other end to the rubber tube A leading to the gas-meter. On
lowering the mercury reservoir E, the mercury is drained out of the tube
D and air passes through both arms of the tube and then through the
three U-tubes. In the first of these it is deprived of moisture, and in
the last two of carbon dioxide. The air then enters the meter, where it
is measured and leaves the meter through the tube B, saturate