The methods are without doubt
capable of a more extended application. In measuring liquids, ordinary
variations of temperature have but little effect, and variations of
atmospheric pressure have none at all, whereas with gases it is
different. Thus, 100 c.c. of an ordinary aqueous solution would, if
heated from 10° C. to 20° C., expand to about 100.15 c.c. 100 c.c. of a
gas similarly warmed would expand to about 103.5 c.c., and a fall of one
inch in the barometer would have a very similar effect. And in
measuring gases we have not only to take into account variations in
volume due to changes in temperature and atmospheric pressure, but also
that which is observed when a gas is measured wet and dry. Water gives
off vapour at all temperatures, but the amount of vapour is larger as
the temperature increases.
By ignoring these considerations, errors of 3 or 4 per cent. are easily
made; but, fortunately, the corrections are simple, and it is easy to
construct a piece of apparatus by means of which they may be reduced to
a simple calculation by the rule of three.
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.
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