Dr. Faraday's brilliant series of discoveries respecting the forces of nature, and their relations to each other and to matter,—of which Volta'a discovery of the pile, Davy 'e application of it to electrolysis, Oersted'a discovery of electro-magnetism, Seebeek'a of thermoelec tricity, Tyndall's of the relation to structural density of pare- and diamagnetism, and Mr. Grove's own discovery of the gas-battery, may be cited as parallels of single terms in that series,—have probably contributed more than those of any other experimental philosopher to prepare the scientific public mind for the Idea of correlation ; and the7 have had this effect, not only on account of their aggregate magni tude, as constituting so great a part of the advance in physics made during the last sixty years, but because, from their bearing so exten sively on what Dr. Faraday calls " the dual forms of power," electricity and magnetism, they have exhibited both the actuality of physical correlation, and the convertibility of force, in a manner ao palpably intelligible. In the relations subsisting between positive and negative electricity, and north and south magnetism, respectively, we have the mental idea of correlation realised to sensible perception. The relation of those forces themselves Is probably a not less perfect cor relation, though perhaps of a different order; while their mutual convertibility, in whioh electromagnetism and magneto-electricity originate, is so readily demonstrated by experiment..
In an addition (June, 1858) to his discourse on tile " Conservation of Force," (a designation introduced we believe by Helmholz, in advocating a principle long tacitly admitted in particular cases, but which had not before been explicitly recognised in its true generality,) delivered in the preceding year (Exp. Res. In Chem. and Phys. p. 461.462), Dr. Faraday says—" Those who admit the possibility of the common origin of all physical force, and also acknowledge the principle of conservation. apply that principle to the sum total of the force. Though the amount of mechanical force (using habitual language for convenience sake ) may remain unchanged and definite iu its character for a long time, yet when, as in the collision of two equal iuclaetic bodies, it appears to be lost, they find it in the form of beat, and whether they admit that heat to be a continued mechanical action ( as is most pro bable), or assume some other ides, as that of electricity, or action of a heat-fluid, EMU they hold to the principle of conservation by admitting that the sum of force, that is, of the cause of action,' Is the same, whatever character the effects assume. With them the convertibility of heat, electricity, magnetism, chemical action, and motion, is a familiar thought." This is forcibly put, and is perfectly true ; but the mutual convertibility of the forces enumerated has only become "a familiar thought" since Mr. Grove's announcement of the
principle of their correlation. No one, however, could have more perfectly realised the conception of that principle, or appreciated its importance, than Dr. Faraday, as indeed the preceding extract may itself show. The latest course of lectures delivered by him (and published in the Chemical News ') consisted of Illustrations of the Various Forces of 3latter,—i. e., of such as are called the Physical or Inorganic Forces, including an Account of their Relations to each other.' The subject of the concluding lecture, given on the 7th of January last (1860), was, co nomine,' The Correlation of the Physical Forces,' of which many experimental demonstrations were shown, relating chiefly to the change of chemical force into electricity, and of electricity into magnetism ; and this eminent explorator of physical truth, observing to his audience that be might show them many other experiments by which he could obtain electricity and chemical action, heat and light from a magnet, terminated his course by the emphatic question, " But what more need I show you to prove the universal correlation of the physical forces of matter, and their mutual conversion one into another I" A certain amount of anticipation of Mr. Grove's views has been recently claimed by Professor Tyndall (` Glaciers of the Alps,' p. 300) for the late M. Rendu, bishop of Annecy, the author of a remarkable essay on glaciers, published in 1841. In considering what he calls the "law of circulation," and after alluding to the circulation of water through terrestrial nature, and that of the elements of organic sub stances from the solid to the fluid, and thence again to the state of organisation, N. Rendu adds, " That universal agent which we designate by the names fire, light, electricity and magnetism, has pro bably also a circulation as wide as the universe." But this is exactly one of those comparatively vague notions, however true in their degree, referring the forces of nature to a common priuciple, which have been alluded to in the introductory part of this article, and to which every period in the history of philosophy, for at least a century before, supplies a parallel. When Professor Tyndall in sequence claims also for Rendu, on the same account, a degree of anticipation of Helmholz'a more recent doctrine of the Conservation of Force,' quot ing the latter as saying in reference to the "circuit" formed by " heat, light, electricity, magnetism, and chemical affinity," " starting from each of these different manifestations of natural forces, we can set every other in action," he ascribes to Helmholz what had before been explicitly enunciated by Grove, almost in the same words, as may be seen in the preceding statement of his doctrine.