The volume of a gas is, in practice, usually reduced to that which it
would be at a temperature of 0∞ C., when the column of mercury in the
barometer is 760 mm. high. But, although convenient, this practice is
not always necessary. The only thing required is some way of checking
the variations in volume, and of calculating what the corrected volume
would be under certain fixed conditions.

Suppose that at the time a series of standardisings is being made, 100
c.c. of air were confined in a graduated tube over moist mercury. These
100 c.c. would vary in volume from day to day, but it would always be
true of them that they would measure 100 c.c. under the same conditions
as those under which the standardisings were made. If, then, in making
an actual assay, 35.4 c.c. of gas were obtained, and the air in the tube
measured 105 c.c., we should be justified in saying, that if the
conditions had been those of the standardising, the 105 c.c. would have
measured 100 c.c., and the 35.4 c.c. would have been 33.7; for 105:
100:: 35.4: 33.7. The rule for using such a piece of apparatus for
correcting volumes is:--_Multiply the c.c. of gas obtained by 100, and
divide by the number of c.c. of air in the apparatus._

If it is desired to calculate the volumes under standard conditions
(that is, the gas dry, at 0∞ C. and 760 mm. barometric pressure) the
calculations are easily performed, but the temperature and pressure must
be known.

_Correction for Moisture._--The "vapour tension" of water has been
accurately determined for various temperatures, and it may be looked
upon as counteracting the barometric pressure. For example, at 15∞ C.
the vapour tension equals 12.7 millimetres of mercury; if the barometer
stood at 750 mm., the correction for moisture would be made by