This recalls the “capital experiment” entry which Sir Humphry Davy wrote after the account of his decomposition of caustic potash. On the 7th of April we come to a marvellous page of speculations. He has seen that liquids, both solutions and fused salts, can be decomposed by the current, and that at least one solid is capable of electrolysis. But he finds that alloys and metals are not decomposed. He finds that electrolysis is easiest for those compounds that consist of the most diverse elements, and is led on to speculate as to the possible constitution of those conductors that the current does not decompose. This may involve a recasting of accepted ideas; but from such a step he does not shrink, as the following extracts show:—
Metals may not be compounds of elements most frequently combined, but rather of such as are so similar to each other as to pass out of the limit of voltaic decomposition.
13th April (same page).
If voltaic decomposition of the kind I believe then review all substances upon the new view to see if they may not be decomposable, &c. &c. &c.
ATTRACTION BY POLES DOUBTED.
He has now found that the facts observed do not admit of being explained on the supposition that the motion of the ions is due to the attraction of the poles, and accordingly there follows the entry:—
(Ap. 13, 1833.)
A single element is never attracted by a pole, i.e. without attraction of other element at other pole. Hence doubt Mr. Brande’s Expts on attraction of gases and vapours. Doubt attraction by poles altogether.
To this subject he returned in 1834; an intervening memoir—the sixth—being taken up with the power of metals and solids to bring about the combination of gaseous bodies. In the seventh series, published in January, 1834, his first work is to explain the new terms which he has adopted, on the advice of Whewell, to express the facts. The so-called poles, being in his view merely doors or ways by which the current passes, he now terms electrodes, distinguishing the entrance and exit respectively as anode and cathode,[42] while the decomposable liquid is termed an electrolyte, and the decomposing process electrolysis. “Finally,” he says, in a passage (here italicised) worthy to be engraved in gold for the essential truth it enunciates on a question of terminology, “I require a term to express those bodies which can pass to the electrodes, or, as they are usually called, the poles. Substances are frequently spoken of as being electronegative, or electropositive, according as they go under the supposed influence of a direct attraction to the positive or negative pole. But these terms are much too significant for the use to which I should have to put them; for though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible but still very dangerous, because continual, influence, they do great injury to science, by contracting and limiting the habitual views of those engaged in pursuing it. I propose to distinguish such bodies by calling those anions which go to the anode of the decomposing body; and those passing to the cathode, cations; and when I shall have occasion to speak of these together, I shall call them ions.[43] Thus, the chloride of lead is an electrolyte, and when electrolyzed evolves the two ions, chlorine and lead, the former being an anion and the latter a cation.” In Faraday’s own bound volume of the “Experimental Researches” he has illustrated these terms by the sketch here reproduced. ([Fig. 12].)
Faraday’s letter to Whewell when he consulted him as to the new words has not been preserved. He discarded, when the paper was printed, the terms he had first used. Whewell’s replies of April 25th and May 5th, 1834, have been preserved and are printed in Todhunter’s biography of Whewell. From the later of the two the following passage is extracted:—