102 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
by the intensity of the affinities concerned ; and so cases may easily be produced, in 
which that metal exerting the least amount of action is nevertheless the positive metal 
in a voltaic circuit ; as with copper in weak nitric acid associated with other copper in 
strong acid (1975.), or iron or silver in the same weak acid against copper in the strong 
acid (1996.). Many of those instances where the hot side ultimately becomes negative, 
as of zinc in dilute solution of sulphuret of potassium (1953.), or cadmium and lead 
in dilute nitric acid (1955.), are of this nature ; and yet the conditions and result are 
in perfect agreement with the chemical theory of voltaic excitement (1918.). 
1958. The distinction between currents founded upon that difference of intensity 
which is due to the difference in force of the chemical action which is their exciting 
cause, is, I think, a necessary consequence of the chemical theory, and in 1834 I 
adopted that opinion* (891. 908. 916. 988.). De la Rive in 1836 gave a still more 
precise enunciation of such a principled-, by saying, that the intensity of currents is 
exactly proportional to the degree of affinity which reigns between the particles, the 
combination or separation of which produces the currents. 
1959. I look upon the question of the origin of the power in the voltaic battery as 
abundantly decided by the experimental results not connected with the action of 
heat (1824, &c. 1878, &c.). I further view the results with heat as adding very strong 
confirmatory evidence to the chemical theory; and the numerous questions which 
arise as to the varied results produced, only tend to show how important the voltaic 
circuit is as a means of investigation into the nature and principles of chemical 
affinity (1967-). This truth has already been most strikingly illustrated by the re- 
searches of De la Rive made by means of the galvanometer, and the investigations 
of my friend Professor Daniell into the real nature of acid and other compound 
electrolytes]:. 
Cases of two metals and one electrolyte ; one junction being heated. 
1960. Since heat produced such striking results with single metals, I thought it 
probable that it might be able to affect the mutual relation of the metals in some 
cases, and even invert their order : on making circuits with two metals and electro- 
lytes, I found the following cases. 
1961. In the solution of sulphuret of potassium, hot tin is well positive to cold silver : 
cold tin is very slightly positive to hot silver, and the silver then rapidly tarnishes. 
1962. In the solution of potassa, cold tin is fairly positive to hot lead, but hot tin is 
much more positive to cold lead. Also cold cadmium is positive to hot lead, but 
hot cadmium is far more positive to cold lead. In these cases, therefore, there are 
great differences produced by heat, but the metals still keep their order. 
1963. In dilute sulphuric acid, hot iron is well positive to cold tin, but hot tin is 
still more positive to cold iron. Plot iron is a little positive to cold lead, and hot lead 
* Philosophical Transactions, 1834, p. 428. t Annales de Chimie, 1836, Ixi. p. 44. &c. 
1 Philosophical Transactions, 1839, p. 97. 
