amiable and liberal, induced me at last to take the bold and simple step of writing to Sir H. Davy.' He was favourably received, and, in the next year, became Davy's assistant at the Institution; and afterwards his successor. The Institution which produced such researches as those of these two men, may well be considered as a great school of exact and philosophical chemistry. Mr. Faraday, from the beginning of his course of inquiry, appears to have had the consciousness that he was engaged on a great connected work. His Experimental Researches, which appeared in a series of Memoirs in the Philosophical Transactions, are divided into short paragraphs, numbered in a continued order from 1 up to 1160, at the time at which I write;10 and destined, probably, to extend much further. These paragraphs are connected by a very rigorous method of investigation and reasoning which runs through the whole body of them. Yet this unity of purpose was not at first obvious. His first two Memoirs were upon subjects which we have already treated of (B. xiii. c. 5 and c. 8), Voltaic Induction, and the evolution of Electricity from Magnetism. His 'Third Series' has also been already referred to. Its object was, as a preparatory step towards further investigation, to show the identity of voltaic and animal electricity with that of the electrical machine; and as machine electricity differs from the other kinds in being successively in a state of tension and explosion, instead of a continued current, Mr. Faraday succeeded in identifying it with them, by causing the electrical discharge to pass through a bad conductor into a discharging-train of vast extent; nothing less, indeed, than the whole fabric of the metallic gas-pipes and water-pipes of London. In this Memoir it is easy to see already traces of the general theoretical views at which he had arrived; but these are not expressly

10 December, 1835. (At pre- lished the Twenty-first Series' sent, when I am revising the of his Researches ending with second edition, September, 1846, paragraph 2453.) Dr. Faraday has recently pub

11 Phil. Trans. 1833.

stated till his Fifth Series;' his intermediate Fourth Series being occupied by another subsidiary labour on the conditions of conduction. At length, however, in the Fifth Series, which was read to the Royal Society in June, 1833, he approaches the theory of electro-chemical decomposition. Most preceding theorists, and Davy amongst the number, had referred this result to attractive powers residing in the poles of the apparatus; and had even pretended to compare the intensity of this attraction at different distances from the poles. By a number of singularly beautiful and skilful experiments, Mr. Faraday shows that the phenomena can with no propriety be ascribed to the attraction of the poles.12 'As the substances evolved in cases of electro-chemical decomposition may be made to appear against air,13 which, according to common language, is not a conductor, nor is decomposed; or against water,14 which is a conductor, and can be decomposed; as well as against the metal poles, which are excellent conductors, but undecomposable; there appears but little reason to consider this phenomenon generally as due to the attraction or attractive powers of the latter, when used in the ordinary way, since similar attractions can hardly be imagined in the former instances.'

Faraday's opinion, and, indeed, the only way of expressing the results of his experiments, was, that the chemical elements, in obedience to the direction of the voltaic currents established in the decomposing substance, were evolved, or, as he prefers to say, ejected at its extremities. 15 He afterwards states that the influence which is present in the electric current may be described 16 as an axis of power, having [at each point] contrary forces exactly equal in amount in contrary directions.

Having arrived at this point, Faraday rightly wished to reject the term poles, and other words which could hardly be used without suggesting doctrines now

12 Researches, Art. 497. 14 495.

13 Researches, Arts. 465, 469. 15 493. 16 517.

proved to be erroneous. He considered, in the case of bodies electrically decomposed, or, as he termed them, electrolytes, the elements as travelling in two opposite directions; which, with reference to the direction of terrestrial magnetism, might be considered as naturally east and west; and he conceived elements as, in this way, arriving at the doors or outlets at which they finally made their separate appearance. The doors he called electrodes, and, separately, the anode and the cathode;17 and the elements which thus travel he termed the anion and the cation (or cathion)." 18 By means of this nomenclature he was able to express his general results with much more distinctness and facility.

But this general view of the electrolytical process required to be pursued further, in order to explain the nature of the action. The identity of electrical and chemical forces, which had been hazarded as a conjecture by Davy, and adopted as the basis of chemistry by Berzelius, could only be established by exact measures and rigorous proofs. Faraday had, in his proof of the identity of voltaic and electric agency, attempted also to devise such a measure as should give him a comparison of their quantity; and in this way he proved that 19 a voltaic group of two small wires of platinum and zinc, placed near each other, and immersed in dilute acid for three seconds, yields as much electricity as the electrical battery, charged by ten turns of a large machine; and this was established both by its momentary electro-magnetic effect, and by the amount of its chemical action.20

It was in his 'Seventh Series,' that he finally established a principle of definite measurement of the amount of electrolytical action, and described an instrument which he termed 21 a volta-electrometer. In

17 Art. 663.

18 The analogy of the Greek derivation requires cation; but to make the relation to cathode obvious to the English reader, and

to avoid a violation of the habits of English pronunciation, I should prefer cathion.

19 Art. 371.

20 Researches, Art. 537. 21 739.

this instrument, the amount of action was measured by the quantity of water decomposed: and it was necessary, in order to give validity to the mensuration, to show (as Faraday did show) that neither the size of the electrodes, nor the intensity of the current, nor the strength of the acid solution which acted on the plates of the pile, disturbed the accuracy of this measure. He proved, by experiments upon a great variety of substances, of the most different kinds, that the electro-chemical action is definite in amount according to the measurement of the new instrument. 22 He had already, at an earlier period, 23 asserted, that the chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes; but the volta-electrometer enabled him to fix with more precision the meaning of this general proposition, as well as to place it beyond doubt.

The vast importance of this step in chemistry soon came into view. By the use of the volta-electrometer, Faraday obtained, for each elementary substance, a number which represented the relative amount of its decomposition, and which might properly be called. its electro-chemical equivalent. And the question naturally occurs, whether these numbers bore any relation to any previously established chemical measures. The answer is remarkable. They were no other than the atomic weights of the Daltonian theory, which formed the climax of the previous ascent of chemistry; and thus here, as everywhere in the progress of science, the generalizations of one generation are absorbed in the wider generalizations of the next.

But in order to reach securely this wider generalization, Faraday combined the two branches of the subject which we have already noticed;-the theory of electrical decomposition with the theory of the pile. For his researches on the origin of activity of the voltaic circuit (his Eighth Series), led him to see more clearly than any one before him, what, as we have said, the most sagacious of preceding philosophers had main

22 Researches, Arts. 758, 814. VOL. III.

23 377.

L

24 792.

tained, that the current in the pile was due to the mutual chemical action of its elements. He was led to consider the processes which go on in the excitingcell, and in the decomposing place, as of the same kind, but opposite in direction. The chemical composition of the fluid with the zinc, in the common apparatus, produces, when the circuit is completed, a current of electric influence in the wire; and this current, if it pass through an electrolyte, manifests itself by decomposition, overcoming the chemical affinity which there resists it. An electrolyte cannot conduct without being decomposed. The forces at the point of composition and the point of decomposition are of the same kind, and are opposed to each other by means of the conducting-wire; the wire may properly be spoken of 25 as conducting chemical affinity: it allows two forces of the same kind to oppose one another; 26 electricity is only another mode of the exertion of chemical forces;27 and we might express all the circumstances of the voltaic pile without using any other term than chemical affinity, though that of electricity may be very convenient.28 Bodies are held together by a definite power, which, when it ceases to discharge that office, may be thrown into the condition of an electric current. 29

Thus the great principle of the identity of electrical and chemical action was completely established. It was, as Faraday, with great candour says,30 a confirmation of the general views put forth by Davy, in 1806, and might be expressed in his terms, that 'chemical and electrical attractions are produced by the same cause; but it is easy to see that neither was the full import of these expressions understood, nor were the quantities to which they refer conceived as measureable quantities, nor was the assertion anything but a sagacious conjecture, till Faraday gave the interpretation, measure, and proof, of which we have spoken. The evidence of the incompleteness of the views of his