CHEMICAL AND CONTACT THEORIES COMPARED. 
79 
1863. The chemical philosopher is embarrassed by none of these difficulties ; for he 
first, by a simple direct experiment, ascertains whether any of the two given sub- 
stances in the circuit are active chemically on each other. If they are, he expects and 
finds the corresponding current ; if they are not, he expects and he finds no current, 
though the circuit be a good conductor and he look carefully for it (1829.). 
1864. Again; taking the case of iron, platina, and solution of sulphuret of potas- 
sium, there is no current ; but for iron substitute zinc, and there is a powerful cur- 
rent. I might for zinc substitute copper, silver, tin, cadmium, bismuth, lead, and 
other metals ; but I take zinc, because its sulphuret dissolves and is carried off by 
the solution, and so leaves the case in a very simple state : the fact, however, is as 
strong with any of the other metals. Now if the contact theory be true, and if the 
iron, platina, and solution of sulphuret of potassium give contacts which are in perfect 
equilibrium as to their electromotive force, then why does changing the iron for zinc 
destroy the equilibrium ? Changing one metal for another in a metallic circuit causes 
no alteration of this kind : nor does changing one substance for another among the 
great number of bodies which, as solid conductors, may be used to form conducting 
(but chemically inactive) circuits (1867 3 &c.). If the solution of sulphuret of potas- 
sium is to be classed with the metals as to its action in the experiments I have quoted 
(1825, &c.), then, how comes it to act quite unlike them, and with a power equal to 
the best of the other class, in the new cases of zinc, copper, silver, &c. (1882. 
1885, &c.). 
1 865. This difficulty, as I conceive, must be met, on the part of the contact theorists, 
by a new assumption, namely, that this fluid sometimes acts as the best of the me- 
tals, or first class of conductors, and sometimes as the best of the electrolytes or se- 
cond class. But surely this would be far too loose a method of philosophizing in 
an experimental science (1889.) ; and further, it is most unfortunate for such an as- 
sumption, that this second condition or relation of it never comes on by itself, so as 
to give us a pure case of a current from contact alone ; it never comes on without 
that chemical action to wffiich the chemist so simply refers all the current which is 
then produced. 
1866. It is unnecessary for me to say that the same argument applies with equal 
force to the cases where nitrous acid, nitric acid, and solution of potash are used; 
and it is supported with equal strength by the results which they have given (1843. 
1849. 1853.). 
1867. It may be thought that it was quite unnecessary, but in my desire to esta- 
blish contact electromotive force, to do which I was at one time very anxious, I made 
many circuits of three substances, including a galvanometer, all being conductors, 
with the hope of finding an arrangement which, without chemical action, should pro- 
duce a current. The number and variety of these experiments may be understood 
