Thus the principle that the Co-existent Polarities of magnetism and electricity are connected and fundamentally identical, is not only true, but is far from being either vague or barren. It has been a fertile source both of theories which have, at present, a very great probability, and of the discovery of new and striking facts. We proceed to consider other similar

cases.

5. Connexion of Electrical and Chemical Polarities. The doctrine that the chemical forces by which the elements of bodies are held together or separated, are identical with the polar forces of electricity, is a great discovery of modern times; so great and so recent, indeed, that probably men of science in general have hardly yet obtained a clear view and firm hold of this truth. This doctrine is now, however, entirely established in the minds of the most profound and philosophical chemists of our time. The complete development and confirmation of this as of other great truths, was preceded by more vague and confused opinions gradually tending to this point; and the progress of thought and of research was impelled and guided, in this as in similar cases, by the persuasion that these co-existent polarities could not fail to be closely connected with each other. While the ultimate and exact theory to which previous incomplete and transitory theories tended is still so new and so unfamiliar, it must needs be a matter of difficulty and responsibility for a common reader to describe the steps by which truth has advanced from point to point. I shall, therefore, in doing this, guide myself mainly by the historical sketches of the progress of this great theory, which, fortunately for us, have been given us by the two philosophers who have played by far the most important parts in the discovery, Davy and Faraday.

It will be observed that we are concerned here with the progress of theory, and not of experiment, except so far as it is confirmatory of theory. In Davy's Memoir of 1826, on the Relations of Electrical and

2

2 Phil. Trans. 1826, p. 383.

Chemical Changes, he gives the historical details to which I have alluded. Already in 1802 he had conjectured that all chemical decompositions might be polar. In 1806 he attempted to confirm this conjecture, and succeeded, to his own satisfaction, in establishing that the combinations and decompositions by electricity were referable to the law of electrical attractions and repulsions; and advanced the hypothesis (as he calls it), that chemical and electrical attractions were produced by the same cause, acting in one case on particles, in the other on masses. This hypothesis was most strikingly confirmed by the author's being able to use electrical agency as a more powerful means of chemical decomposition than any which had yet been applied. Believing,' he adds, that our philosophical systems are exceedingly imperfect, I never attached much importance to this hypothesis; but having formed it after a copious induction of facts, and having gained by the application of it a number of practical results, and considering myself as much the author of it as I was of the decomposition of the alkalies, and having developed it in an elementary work as far as the present state of chemistry seemed to allow, I have never,' he says, ' criticised or examined the manner in which different authors have adopted or explained it, contented, if in the hands of others, it assisted the arrangements of chemistry or mineralogy, or became an instrument of discovery.' When the doctrine had found an extensive acceptance among chemists, attempts were made to show that it had been asserted by earlier writers: and though Davy justly denies all value to these pretended anticipations, they serve to show, however dimly, the working of that conviction of the Connexion of Co-existent Properties which all along presided in men's minds during this course of investigation. 'Ritter and Winterl have been quoted,' Davy says, among other persons, as having imagined or anticipated the relation between electrical powers and chemical affinities before the discovery of the pile

3 Phil. Trans. 1826, p. 389.

4 Ibid. p. 384.

f Volta. But whoever will read with attention Ritter's "Evidence that Galvanic action exists in oranised nature," and Winter's Prolusiones ad Chemiam eculi decimi noni, will find nothing to justify this pinion.' He then refers to the Queries of Newton at he end of his Optics. 'These,' he says, 'contain more rand and speculative views that might be brought to ear upon this question than any found in the works modern electricians; but it is very unjust to the sperimentalists who by the laborious application of ew instruments, have discovered novel facts and alogies, to refer them to any such suppositions as at all attractions, chemical, electrical, magnetical, d gravitative, may depend upon the same cause.' It perfectly true, that such vague opinions, though ising from that tendency to generalize which is the sence of science, are of no value except so far as they e both rendered intelligible, and confirmed by experiental research.

The phenomena of chemical decomposition by means the voltaic pile, however, led other persons to views ry similar to those of Davy. Thus Grotthus in 18055 blished an hypothesis of the same kind. 'The pile Volta,' he says, 'is an electrical magnet, of which h element, that is, each pair of plates, has a positive 1 a negative pole. The consideration of this polarity gested to me the idea that a similar polarity may ne into play between the elementary particles of ter when acted upon by the same electrical agent; 1 I avow that this thought was for me a flash of at.'

i. The thought, however, though thus brought into ng, was very far from being as yet freed from vague3, superfluities, and errours. I have elsewhere iced Faraday's remark on Davy's celebrated Mer of 1806; that the mode of action by which the ets take place is stated very generally, so generally, eed, that probably a dozen precise schemes of elecchemical action might be drawn up, differing essen

Ann. Chim. lxviii. 54.

6 Hist. Ind. Sc. b. xiv. c. ix. sect. 1.

tially from each other, yet all agreeing with the statement there given.' When Davy and others proceeded to give a little more definiteness and precision to the statement of their views, they soon introduced into the theory features which it was afterwards found necessary to abandon. Thus both Davy, Grotthus, Riffault, and Chompré, ascribed electrical decomposition to the action of the poles, and some of them even pretended to assign the proportion in which the force of the pole diminishes as the distance from it increases. Faraday, as I have already stated, showed that the polarity must be considered as residing not only in what had till then been called the poles, but at every point of the circuit. He ascribed electrochemical decomposition to internal forces, residing in the particles of the matter under decomposition, not to external forces, exerted by the poles. Hence he shortly afterwards proposed to reject the word poles altogether, and to employ instead, the term electrode, meaning the doors or passages (of whatever surface formed) by which the decomposed elements pass out. What have been called the positive and negative poles he further termed the Anode and Cathode; and he introduced some other changes in nomenclature connected with these. He then, as I have related in the History, invented the Volta-electrometer, which enabled him to measure the quantity of voltaic action, and this he found to be identical with the quantity of chemical affinity; and he was thus led to the clearest view of the truth towards which he and his predecessors had so long been travelling, that electrical and chemical forces are identical11.

9

7. It will, perhaps, be said that this beautiful train of discovery was entirely due to experiment, and not to any a priori conviction that co-existent polarities

7 See Faraday's Historical Sketch, Researches, 481-492.

8 Art. 524.

9 In 1834. Eleventh Series of Researches. Art. 662.

10 Hist. Ind. Sc. b. xiv. c. ix. sect. 2.

11 Arts. 915, 916, 917.

must be connected. I trust I have sufficiently stated that such an a priori principle could not be proved, nor even understood, without a most laborious and enlightened use of experiment; but yet I think that the doctrine, when once fully unfolded, exhibited clearly, and established as true, takes possession of the mind with a more entire conviction of its certainty and universality, in virtue of the principle we are now considering. When the theory has assumed so simple a form, it appears to derive immense probability (to say the least) from its simplicity. Like the laws

of motion, when stated in its most general form, it appears to carry with it its own evidence. And thus this great theory borrows something of its character from the Ideas which it involves, as well as from the Experiments by which it was established.

8. We may find in many of Mr. Faraday's subsequent reasonings, clear evidence that this idea of the Connexion of Polarities, as now developed, is not limited in its application to facts already known experimentally, but, like other ideas, determines the philosopher's researches into the unknown, and gives us the form of knowledge even before we possess the matter. Thus, he says, in his Thirteenth Series 12, 'I have long sought, and still seek, for an effect or condition which shall be to statical electricity what magnetic force is to current electricity; for as the lines of discharge are associated with a certain transverse effect, so it appeared to me impossible but that the lines of tension or of inductive action, which of necessity precede the discharge, should also have their correspondent transverse condition or effect.' Other similar passages might be found.

I will now consider another case to which we may apply the Principle of Connected Polarities.

9. Connexion of Chemical and Crystalline Polarities. -The close connexion between the Chemical Affinity and the Crystalline Attraction of elements cannot be overlooked. Bodies never crystallize but when their elements combine chemically; and solid bodies which

12 Art. 1658.