Effect of Motion in the Fluids 273

to the other; and has given his views of the cause.1 The pre-
caution against this effect was not to put the metals into the
acid until the proper temperature had been given to both parts
of it, and then to observe the first effect produced, accounting
that as the true indication, but repeating the experiment until
the result was certain.

907. Effect of motion.—This investing fluid (906) made it
necessary to guard against the effect of successive rest and
motion of the metal in the fluid. As an illustration, if two tin
wires (869) be put into dilute nitric acid, there will probably
be a little motion at the galvanometer., and then the needle will
settle at o°. If either wire be then moved, the other remaining
quiet, that in motion will become positive. Again, tin and
cadmium in dilute sulphuric acid gave a strong current, the
cadmium being positive, and the needle was deflected 80°.
When left, the force of the current fell to 35°. If the cadmium
were then moved it produced very little alteration; but if the
tin were moved it produced a great change, not showing, as
before, an increase of its force, but the reverse, for it became
more negative, and the current force rose up again to 80°.2
The precaution adopted to avoid the interference of these actions,
was not only to observe the first effect of the introduced wires,
but to keep them moving from the moment of the introduction!

908. The above effect was another reason for heating the
acids, etc. (906) before the wires were immersed; for in the
experiment just described, if the cadmium side were heated to
boiling, the moment the fluid was agitated on the tin side by
the boiling on the cadmium side, there was mor effect by far
produced by the motion than the heat: for the heat at the cad-
mium alone did little or nothing, but the jumping of the acid
over the tin made a difference in the current of 20° or 30°.

909. Effect of air.—Two platinum wires were put into cold

1 Annales de Chimie, 1830, xlv. p. 40.

2 Tin has some remarkable actions in this respect. If two tins be
immersed in succession into dilute nitric acid, the one last in is positive
to the other at the moment: if, both being in, one be moved, that is for
the time positive to the other. But if dilute sulphuric acid be employed,
the last tin is always negative: if one be taken out, cleaned, and re-
immersed, it is negative: if, both being in and neutral, one be moved, it
becomes negative to the other. The effects with muriatic acid are the
same in kind as those with sulphuric acid, but not so strong. This effect
perhaps depends upon the compound of tin first produced in the sulphuric
and muriatic acids tending to acquire some other and more advanced
state, either in relation to the oxygen, chlorine or acid concerned, and so
adding a force to that which at the first moment, when only metallic tin
and acid are present, tends to determine a current.

SI and 8. Philosophical Magazine, 1833,