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.

L

23 377.

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 conduet 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

predecessor we have already adduced, in speaking of his vague and inconsistent theoretical account of decomposition. The confirmation of Davy's discoveries by Faraday is of the nature of Newton's confirmation of the views of Borelli and Hooke respecting gravity, or like Young's confirmation of the undulatory theory of Huyghens.

We must not omit to repeat here the moral which we wish to draw from all great discoveries, that they depend upon the combination of exact fucts with clear ideas. The former of these conditions is easily illustrated in the case of Davy and Faraday, both admirable and delicate experimenters. Davy's rapidity and resource in experimenting were extraordinary,31 and extreme elegance and ingenuity distinguish almost every process of Faraday. He had published, in 1829, a work on Chemical Manipulation, in which directions are given for performing in the neatest manner all chemical processes. Manipulation, as he there truly says, is to the chemist like the external senses to the mind;32 and without the supply of fit materials which such senses only can give, the mind can acquire no real knowledge.

But still the operations of the mind as well as the information of the senses, ideas as well as facts, are requisite for the attainment of any knowledge; and all great steps in science require a peculiar distinctness and vividness of thought in the discoverer. This it is difficult to exemplify in any better way than by the discoveries themselves. Both Davy and Faraday possessed this vividness of mind; and it was a consequence of this endowment, that Davy's lectures upon chemistry, and Faraday's upon almost any subject of physical philosophy, were of the most brilliant and captivating character. In discovering the nature of voltaic action, the essential intellectual requisite was to have a distinct conception of that which Faraday expressed by the remarkable phrase,33 an axis of power having equal and opposite forces:' and the distinctness of this idea

31 Paris, i. 145.

32 Pref. p. ii.

33 Art. 517.

in Faraday's mind shines forth in every part of his writings. Thus he says, the force which determines the decomposition of a body is in the body, not in the poles. 3+ But for the most part he can of course only convey this fundamental idea by illustrations. Thus 35 he represents the voltaic circuit by a double circle, studded with the elements of the circuit, and shows how the anions travel round it in one direction, and the cations in the opposite. He considers 36 the powers at the two places of action as balancing against each other through the medium of the conductors, in a manner analogous to that in which mechanical forces are balanced against each other by the intervention of the lever. It is impossible to him 37 to resist the idea, that the voltaic current must be preceded by a state of tension in its interrupted condition, which is relieved when the circuit is completed. He appears to possess the idea of this kind of force with the same eminent distinctuess with which Archimedes in the ancient, and Stevinus in the modern history of science, possessed the idea of pressure, and were thus able to found the science of mechanics.38 And when he cannot obtain these distinct modes of conception, he is dissatisfied, and conscious of defect. Thus in the relation between magnetism and electricity,39 there appears to be a link in the chain of effects, a wheel in the physical mechanism of the action, as yet unrecognized.' All this variety of expression shows how deeply seated is the thought. This conception of Chemical Affinity as a peculiar influence or force, which, acting in opposite directions, combines and resolves bodies;--which may be liberated and thrown into the form of a voltaic current, and thus be transferred to remote points, and applied in various ways;-is essential to the understanding, as it was to the making, of these discoveries.

By those to whom this conception has been conveyed, I venture to trust that I shall be held to have given a

34 Art. 661.

37 950.

35 96.

38

990.

36 917.

39 1114.

faithful account of this important event in the history of science. We may, before we quit the subject, notice one or two of the remarkable subordinate features of Faraday's discoveries.

Sect. 3-Consequences of Faraday's Discoveries.

FARADAY'S volta-electrometer, in conjunction with the method he had already employed, as we have seen, for the comparison of voltaic and common electricity, enabled him to measure the actual quantity of electricity which is exhibited, in given cases, in the form of chemical affinity. His results appeared in numbers of that enormous amount which so often comes before us in the expression of natural laws. One grain of water 40 will require for its decomposition as much electricity as would make a powerful flash of lightning. By further calculation, he finds this quantity to be not less than 800,000 charges of his Leyden battery ;41 and this is, by his theory of the identity of the combining with the decomposing force, the quantity of electricity which is naturally associated with the elements of the grain of water, endowing them with their mutual affinity.

Many of the subordinate facts and laws which were brought to light by these researches, clearly point to generalizations, not included in that which we have had to consider, and not yet discovered: such laws do not properly belong to our main plan, which is to make our way up to the generalizations. But there is one which so evidently promises to have an important bearing on future chemical theories, that I will briefly mention it. The class of bodies which are capable of electrical decomposition is limited by a very remarkable law: they are such binary compounds only as consist of single proportionals of their elementary principles. It does not belong to us here to speculate on the possible import of this curious law; which, if not fully established, Faraday has rendered, at least, highly probable:42 but it is impossible not to see how closely it

40 Art. 153.

41 861.

42 697.