118 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
2034. If lead and tin be associated in muriatic acid, the lead is positive at the first 
moment to the tin. The tin then becomes positive, and continues so. This change 
I attribute to the circumstance, that the chloride of lead formed, partly invests that 
metal, and prevents the continuance of the action there ; but the chloride of tin, 
being far more soluble than that of lead, passes more readily into the solution ; so 
that action goes on there, and the metal exhibits a permanent positive state. 
2035. The effect of the investing fluid already referred to in the cases of tin (1919.) 
and cadmium (1918.), some of the results with two metals in hot and cold acid (1966.), 
and those cases where metal in a heated acid became negative to the same metal in 
cold acid (1953, &c.), are of the same kind. The latter can be beautifully illustrated 
by two pieces of lead in dilute nitric acid : if left a short time, the needle stands nearly 
at 0°, but on heating either side, the metal there becomes negative 20° or more, and 
continues so as long as the heat is continued. On cooling that side and heating the 
other, that piece of lead which before was positive now becomes negative in turn, and 
so on for any number of times. 
2036. When the chemical action changes the current changes also. — This is shown 
by the cases of two pieces of the same active metal in the same fluid. Thus if two 
pieces of silver be associated in strong muriatic acid, first the one will be positive 
and then the other ; and the changes in the direction of the current will not be slow 
as if by a gradual action, but exceedingly sharp and sudden. So if silver and copper 
be associated in a dilute solution of sulphuret of potassium, the copper will be che- 
mically active and positive, and the silver will remain clean ; until of a sudden the 
copper will cease to act, the silver will become instantly covered with sulphuret, 
showing by that the commencement of chemical action there, and the needle of the 
galvanometer will jump through 180°. Two pieces of silver or of copper in solution 
of sulphuret of potassium produce the same effect. 
203/- If metals be used which are inactive in the fluids employed, and the latter 
undergo no change during the time, from other circumstances, as heat, &c. (1838. 
1937-), then no currents, and of course no such alterations in direction, are produced. 
2038. Where no chemical action occurs no current is produced . — This in regard to 
ordinary solid conductors, is well known to be the case, as with metals and other 
bodies (1867-). It has also been shown to be true when fluid conductors (electrolytes) 
are used, in every case where they exert no chemical action, though such different 
substances as acid, alkalies and sulpburets have been employed (1843. 1853. 1825. 
1829.). These are very striking facts. 
2039. But a current will occur the moment chemical action commences. — -This pro- 
position may be well illustrated by the following experiment. Make an arrangement 
like that in fig. 14.; the two tubes being charged with the same pure, pale, strong nitric 
acid, the two platinum wires p p being connected by a galvanometer, and the wire i, of 
iron. The apparatus is only another form of the simple arrangement fig. 15., where, 
in imitation of a former experiment (889.), two plates of iron and platinum are placed 
