ccupies that number of divisions or hundredths
of a cubical inch. Or if a quantity competent to produce a certain effect
is to be measured, the effect is to be obtained, and then the indication
read off. In exact experiments it is necessary to correct the volume of gas
for changes in temperature and pressure, and especially for moisture[A].
For the latter object the volta-electrometer (fig. 66.) is most accurate,
as its gas can be measured over water, whilst the others retain it over
acid or saline solutions.

[A] For a simple table of correction for moisture, I may take the
liberty of referring to my Chemical Manipulation, edition of 1830,
p. 376.

738. I have not hesitated to apply the term _degree_ (736.), in analogy
with the use made of it with respect to another most important imponderable
agent, namely, heat; and as the definite expansion of air, water, mercury,
&c., is there made use of to measure heat, so the equally definite
evolution of gases is here turned to a similar use for electricity.

739. The instrument offers the only _actual measurer_ of voltaic
electricity which we at present possess. For without being at all affected
by variations in time or intensity, or alterations in the current itself,
of any kind, or from any cause, or even of intermissions of action, it
takes note with accuracy of the quantity of electricity which has passed
through it, and reveals that quantity by inspection; I have therefore named
it a VOLTA-ELECTROMETER.

740. Another mode of measuring volta-electricity may be adopted with
advantage in many cases, dependent on the quantities of metals or other
substances evolved either as primary or as secondary results; but I refrain
from enlarging on this use of the products, until the principles on which
their constancy depends have been fully established (791. 848.);

741. By the aid of this instrument I have been able to establish the
definite character of electro-chemical action in its most general sense;
and I am persuaded it will become of the utmost use in the extensions of
the science which these views afford. I do not pretend to have made its
detail perfect, but to have demonstrated the truth of the principle, and
the utility of the application[A].

[A] As early as the year 1811, Messrs. Gay-Lussac and Thenard employed
chemical decomposition as a measure of the electricity of the voltaic
pile. See _Recherches Physico-chymiques_, p. 12. The principles and
prec