After the establishment of the Berzelius theory, no great progress was made along elec trochemical lines until about 1835, when Fara day announced his discovery of what are now known as Faraday's laws, which will be dis cussed later. Faraday received his taste for scientific work and the training that led up to it while serving as a helper in the laboratory of Sir Humphrey Davy, and Davy is said to have once replied, in answer to a question, that his most important scientific discovery was Michael Faraday. Besides the laws governing the quantitative relations of electrochemical reactions, we also owe to Faraday our system of electrochemical nomenclature. To explain the reactions taking place he assumed the pas sage of the electricity to be associated with the movement in the solution of particles of mat ter which he called ions; the poles themselves were in general termed electrodes, the positive pole being the anode and the negative pole the cathode; the ions that moved to the positive pole were anions, and those moving toward the negative pole were cathions; the solution un-. dergoing decomposition was the electrolyte, that surrounding the anode being the anolyte, and that surrounding the cathode the catholyte; the process of decomposition was called electro lysis.
When the decomposition of water was first noticed, an explanation was sought for the si multaneous appearance of hydrogen at one elec trode and of oxygen at the other. In 1805 Grotthus proposed a theory to explain the mechanism of the conduction of the electric current through the solution and opened the discussion of a problem for which we still have no entirely satisfactory solution. According to the Grotthus theory, the current charges one electrode positively and the other negatively, and these charged surfaces in turn act on the molecules of water in such a way that the hy drogen of the water becomes positively charged and the oxygen negatively charged. The at traction of the negative pole for the positively charged hydrogen and of the positive pole for the negatively charged oxygen then causes the molecules to arrange themselves as shown in A of Fig. 1. If the charge on the two electrodes is then sufficient, the atoms a and a' have their charges neutralized at the electrode and become free gas; the atoms b and b' then recombine with c and c' and so on throughout the line, forming new molecules of water, as in B, which then, under the continued influence of the cur rent, will reorient themselves as before and the whole process is repeated. This theory held its own for about 50 years, but as the science developed, imperfections were discovered that made it no longer tenable, and it was eventually replaced by the Clausius theory. Clausius as sumed that the positive and negative portions of the molecule in the electrolyte were not firmly combined with each other, but were in a state of continuous vibration, which if it became vigorous enough would cause the positive part of one molecule to come within the sphere of influence of the negative part of another mole cule, with which it would unite, the negative and positive particles thus left temporarily free in turn soon come within the sphere of other oppositely charged particles with which to unite, so that there would be going on through the solution all the time a continuous interchange between the particles. But when an electric
current is sent through the solution, a force is generated in the direction of the flow of the current and the vibration and•exchange is no longer irregular, and in all directions, but is intensified in the direction of the current flow, thus causing a movement of positive particles toward the negative pole, and vice versa.
The Grotthus idea of fixed ions was thus replaced by the vibrating ions of Clausius, and this in turn, some 30 years later, was replaced by the Arrhenius theory of free ions. This theory has probably given a greater impulse to electrochemical research and has, directly and indirectly, been an aid to more discoveries than any other conception in the field of electro chemistry. The Arrhenius theory, or as it is frequently called, the electrolytic dissociation theory (see SOLUTIONS) was based on the as sumption that when an acid, base or salt was dissolved, yielding a solution that was a con ductor of electricity, the molecules of the dis solved substance were by the act of solution de composed into part-molecules, or ions. At any finite concentration the solution will still con tain a certain amotuit of undissociated material, and only at infinite dilution is the substance completely dissociated into ions. These disso ciated ions are positively and negatively charged, and it is the ions that act as carriers of the current, the conductivity of the solution being dependent on the degree of dissociation of the dissolved substance.
The discrepancies that constantly cropped out in the development of the details of the Ar rhenius theory led up to what is known as the Hydrate theory, which assumes that part of the water present in the solution is combined with the dissolved substance, thus leaving as free solvent only a portion of the total amount pres ent, which from a concentration standpoint would bring about the same results as the as sumption according to the Arrhenius theory of an increase of the ultimate particles in the solu tion by dissociation. And this idea, in turn, becomes the Solvate theory when its principles are extended front aqueous to all solutions, both aqueous and non-aqueous. This Solvate theory, supplementing the Arrhenius theory, extends the latter from its former constricted field of dilute solutions to a theory of solutions in gen eral. There are still, however, many points that need further development, particularly with regard to the exact relation between dissocia tion and solvation.