132 Faraday's Researches j

educts at both electrodes be collected. Such is the case when i
solutions of ammonia, muriatic acid, chlorides, iodides, acetates j
or other vegetable salts, etc., are employed. j

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

471. There are therefore two general forms of the instrument j
which I submit as a measurer of electricity; one in which both \
the gases of the water decomposed are collected (444, 445, 446), i
and the other in which a single gas, as the hydrogen only, is * i
used (442, 443). When referred to as a comparative instrument \
(a use I shall now make of it very,extensively), it will not often i
require particular precaution in the observation; but when I
used as an absolute measurer, it will be needful that the baro- !
metric pressure and the temperature be taken into account, and i
that the graduation of the instruments should be to one scale; j
the hundredths and smaller divisions of a cubical inch are quite I
fit for this purpose, and the hundredth may be very conveniently j
taken as indicating a DEGREE of electricity. ]

472. It can scarcely be needful to point out further than has j
been done how this instrument is to be used. It is to be intro- j
duced into the course of the electric current, the action of which 1
is to be exerted anywhere else, and if 60° or 70° 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 occupies that number of divisions or hundredths of i
a cubical inch. Or if a quantity competent to produce a certain j
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 i
pressure, and especially for moisture.1 For the latter object the !
volta-electrometer (fig. 26) is most accurate, as its gas can be ;
measured over water, whilst the others retain it over acid or j
saline solutions. j

473. I have not hesitated to apply the term^degree (471), 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, etc., is there made use of to
measure heat, so the equally definite evolution of gases is here
turned to a similar use for electricity.

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