104 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
tin was always positive, but with hot acid the iron was sometimes positive. The 
effects were but small, and I had not time to enter further into the investigation. 
1968. I trust it is understood that, in every case, the precautions as to very careful 
cleansing of the wires, the places of the ends, simultaneous immersion, observation of 
the first effects, &c., were attended to. 
^[. v. The exciting chemical force affected by dilution. 
1969. Another mode of affecting the chemical affinity of these elements of voltaic 
circuits, the metals and acids, and also applicable to the cases of such circuits, is to 
vary the proportion of water present. Such variation is known, by the simplest 
chemical experiments, to affect very importantly the resulting action, and, upon the 
chemical theory, it was natural to expect that it would also produce some corre- 
sponding change in the voltaic pile. The effects observed by Avogadro and CErsted 
in 1823 are in accordance with such an expectation, for they found that when the 
same pair of metals was plunged in succession into a strong and a dilute acid, in 
certain cases an inversion of the current took place*. In 1828 De la Rive carried 
these and similar cases much further, especially in voltaic combinations of copper and 
iron with lead~j~. In 1827 Becquerel;}; experimented with one metal, copper, plunged 
at its two extremities into a solution of the same substance (salt) of different strengths ; 
and in 1828 De la Rive§ made many such experiments with one metal and a fluid 
in different states of dilution, which I think of very great importance. 
1970. The argument derivable from effects of this kind appeared to me so strong 
that I worked out the facts to some extent, and think the general results well worthy 
of statement. Dilution is the circumstance which most generally exalts the existing 
action, but how such a circumstance should increase the electromotive force of mere 
contact did not seem evident to me, without assuming, as before (1874.), exactly those 
influences at the points of contact in the various cases, which the prior results, 
ascertained by experiments, would require. 
1971. The form of apparatus used was the bent tube already described (1915.) 
fig. 7- The precautions before directed with the wires, tube, &c., were here likewise 
needful. But there were others also requisite, consequent upon the current produced 
by combination of water with acid, an effect which has been described long since by 
Becquerel||, but whose influence in the present researches requires explanation. 
1972. Figs. 9 and 10 represent the two arrangements of fluids used. The part below 
m in the tubes being strong acid, and that above diluted. If the fluid was nitric acid 
and the platinum wires as in the figures, drawing the end of the wire D upwards above m, 
or depressing it from above m downwards, caused great changes at the galvanometer ; 
but if they were preserved quiet at any place, then the electro-current ceased, or very 
* Annales de Chimie, 1823, xxii. p. 361. f Ibid. 1828, xxxvii. p. 234. 
X Ibid. 1827, xxxv. p. 120. § Ibid. 1828, xxxvii. p. 240, 241. 
|| Traitd de l’Electricite, ii. p. 81. 
