electrodes (773. &c.), shall diminish their
quantity, or add other gases to them.

734. In many cases when the instrument is used as a _comparative standard_,
or even as _a measurer_, it may be desirable to collect the hydrogen only,
as being less liable to absorption or disappearance in other ways than the
oxygen; whilst at the same time its volume is so large, as to render it a
good and sensible indicator. In such cases the first and second form of
apparatus have been used, figg. 62, 63. (707. 708.). The indications
obtained were very constant, the variations being much smaller than in
those forms of apparatus collecting both gases; and they can also be
procured when solutions are used in comparative experiments, which,
yielding no oxygen or only secondary results of its action, can give no
indications if the educts at both electrodes be collected. Such is the case
when solutions of ammonia, muriatic acid, chlorides, iodides, acetates or
other vegetable salts, &c., are employed.

735. In a few cases, as where solutions of metallic salts liable to
reduction at the negative electrode are acted upon, the oxygen may be
advantageously used as the measuring substance. This is the case, for
instance, with sulphate of copper.

736. There are therefore two general forms of the instrument which I submit
as a measurer of electricity; one, in which both the gases of the water
decomposed are collected (709. 710. 711.); and the other, in which a single
gas, as the hydrogen only, is used (707. 708.). When referred to as a
_comparative instrument_, (a use I shall now make of it very extensively,)
it will not often require particular precaution in the observation; but
when used as an _absolute measurer_, it will be needful that the barometric
pressure and the temperature be taken into account, and that the graduation
of the instruments should be to one scale; the hundredths and smaller
divisions of a cubical inch are quite fit for this purpose, and the
hundredth may be very conveniently taken as indicating a DEGREE of
electricity.

737. It can scarcely be needful to point out further than has been done how
this instrument is to be used. It is to be introduced into the course of
the electric current, the action of which is to be exerted anywhere else,
and if 60 deg. or 70 deg. of electricity are to be measured out, either in one or
several portions, the current, whether strong or weak, is to be continued
until the gas in the tube o