quare
centimeter. The average reading of the barometer at the sea level is 760
mm., which corresponds to a pressure of 1033.3 g. per square centimeter.
The following problem will serve as an illustration of the application
of Boyle's law.

A gas occupies a volume of 500 cc. in a laboratory where the barometric
reading is 740 mm. What volume would it occupy if the atmospheric
pressure changed so that the reading became 750 mm.?

Substituting the values in the equation VP = vp, we have 500 x 740 =
v x 750, or v = 493.3 cc.

~Variations in the volume of a gas due to changes both in temperature and
pressure.~ Inasmuch as corrections must be made as a rule for both
temperature and pressure, it is convenient to combine the equations
given above for the corrections for each, so that the two corrections
may be made in one operation. The following equation is thus obtained:

(5) V_{s} = vp/(760(1 + 0.00366t)),

in which V_{s} represents the volume of a gas under standard
conditions and v, p, and t the volume, pressure, and temperature
respectively at which the gas was actually measured.

The following problem will serve to illustrate the application of this
equation.

A gas having a temperature of 20 deg. occupies a volume of 500 cc. when
subjected to a pressure indicated by a barometric reading of 740 mm.
What volume would this gas occupy under standard conditions?

In this problem v = 500, p = 740, and t = 20. Substituting these
values in the above equation, we get

V_{s} = (500 x 740)/(760 (1 + 0.00366 x 20)) = 453.6 cc.

[Illustration: Fig. 8]

~Variations in the volume of a gas due to the pressure of aqueous vapor.~
In many cases gases are collected over water, as explained under the
preparation of oxygen. In such cases there is present in the gas a
certain amount of water vapor. This vapor exerts a definite pressure,
which acts in opposition to the atmospheric pressure and which therefore
must be subtracted from the latter in determining the effective pressure
upon the gas. Thus, suppose we wish to determine the pressure to which
the gas in tube A (Fig. 8) is subjected. The tube is raised or lowered
until the level of the water inside and outside the tube is the same.
The atmosphere presses down upon the surface of the water (as indicated
by the arrows), thus forcing the water upward within the tube with a
pressure equal to the atmospheric pressure. The full force of this
upward pressure, however, is