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.34 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 cathions 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 distinctness 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. connects the Atomic with the Electro-chemical Theory; and in the connexion of these two great members of Chemistry, is involved the prospect of its reaching wider generalizations, and principles more profound than we have yet caught sight of. As another example of this connexion, I will, finally, notice that Faraday has employed his discoveries in order to decide, in some doubtful cases, what is the true chemical equivalent;43 I have such conviction,' he says, 'that the power which governs electro-decomposition and ordinary chemical attractions is the same; and such confidence in the overruling influence of those natural laws which render the former definite, as to feel no hesitation in believing that the latter must submit to them too. Such being the case, I can have no doubt that, assuming hydrogen as 1, and dismissing small fractions for the simplicity of expression, the equivalent number or atomic weight of oxygen is 8, of chlorine 36, of bromine 78'4, of lead 103.5, of tin 59, &c.; notwithstanding that a very high authority doubles several of these numbers.' Sect. 4.-Reception of the Electro-chemical Theory. THE epoch of establishment of the electro-chemical theory, like other great scientific epochs, must have its sequel, the period of its reception and confirmation, application and extension. In that period we are living, and it must be the task of future historians to trace its course. We may, however, say a word on the reception which the theory met with, in the forms which it assumed, anterior to the labours of Faraday. Even before the great discovery of Davy, Grotthuss, in 1805, had written upon the theory of electro-chemical decomposition; but he and, as we have seen, Davy, and afterwards other writers, as Riffault and Chom pré, in 43 Art. 851. 1807, referred the effects to the poles.44 But the most important attempt to appropriate and employ the generalization which these discoveries suggested, was that of Berzelius; who adopted at once the view of the identity, or at least the universal connexion, of electrical relations with chemical affinity. He considered,45 that in all chemical combinations the elements may be considered as electro-positive and electro-negative; and made this opposition the basis of his chemical doctrines; in which he was followed by a large body of the chemists of Germany. He held too that the heat and light, evolved during cases of powerful combination, are the consequence of the electric discharge which is at that moment taking place: a conjecture which Faraday at first spoke of with praise. 46 But at a later period he more sagely says,47 that the flame which is produced in such cases exhibits but a small portion of the electric power which really acts. These therefore may not, cannot, be taken as evidences of the nature of the action; but are merely incidental results, incomparably small in relation to the forces concerned, and supplying no information of the way in which the particles are active on each other, or in which their forces are finally arranged.' And comparing the evidence which he himself had given of the principle on which Berzelius's speculations rested, with the speculations themselves, Faraday justly conceived, that he had transferred the doctrine from the domain of what he calls doubtful knowledge, to that of inductive certainty. Now that we are arrived at the starting-place, from which this well-proved truth, the identity of electric and chemical forces, must make its future advances, it would be trifling to dwell longer on the details of the diffusion of that doubtful knowledge which preceded this more certain science. Our history 44 Faraday (Researches, Art. 481, 492). 47 960. |