Page:Experimental researches in electricity.djvu/284

258 worthy of opposition to the numerous cases produced by the chemical philosopher (880); for without them the contact theory as applied to the pile appears to me to have no support, and, as it asserts contact electromotive force even with the balanced condition, to be almost without foundation.

860. To avoid these and similar conclusions, the contact theory must bend about in the most particular and irregular way. Thus the contact of solution of sulphuret of potassium with iron must be considered as balanced by the joint force of its contact with platinum, and the contact of iron and platinum with each other; but changing the iron for lead, then the contact of the sulphuret with the latter metal is no longer balanced by the other two contacts, it has all of a sudden changed its relation: after a few seconds, when a film of sulphuret has been formed by the chemical action, then the current ceases, though the circuit be a good conductor (873); and now it must be assumed that the solution has acquired its first relation to the metals and to the sulphuret of lead, and gives an equilibrium condition of the contacts in the circle.

861. So also with this sulphuretted solution and with potassa, dilution must, by the theory, be admitted as producing no change in the character of the contact force; but with nitric acid, it, on the contrary, must be allowed to change the character of the force greatly (965). So again acids and alkalies (as potassa) in the cases where the currents are produced by them, as with zinc and platinum for instance, must be assumed as giving the preponderance of electromotive force on the same side, though these are bodies which might have been expected to give opposite currents, since they differ so much in their nature.

862. Every case of a current is obliged to be met, on the part of the contact advocates, by assuming powers at the points of contact, in the particular case, of such proportionate strengths as will consist with the results obtained, and the theory is made to bend about, having no general relation for the acids or alkalies, or other electrolytic solution used. The result therefore comes to this: The theory can predict nothing regarding the results; it is accompanied by no case of a voltaic current produced without chemical action, and in those associated with chemical action it bends about to suit the real results, these contortions being exactly parallel to the variations which the pure chemical force, by experiment, indicates.

863. In the midst of all this, how simply does the chemical