We have endeavoured in this article to do justice to Mr. Grove, and Ito the subject to which he has devoted so much attention and thought. ,The progress of science, in its approach to the abstract truth, consists partly in the ascent to higher and higher generalisations successively, and partly in the limitation of generalisations previously attained, or, in other words. the verification of hypotheses. Fully believing the doctrine of the correlation of physical forces, we stated several year* since our opinion that that doctrine is the expression of the deeper or higher I truth, that they are all, not excluding mechanical force, effects, or affections, of the ether,—the ether of Hooke, Young, and Freanel, and generally of the mathematicians and physicists who hate advocated the undulatory theory of light and of heat. In the ' Philosophical Maga zine' for January, 1859, Professor Challis of Cambridge, thus intro duces his theory of the principle :—" It appears to be established by modem experimental researches, that the different physical forces are mutually related by some common condition, or bond of connexion ; but what the precise nature of the connexion is, perhaps experiment alone is incapable of determining. This generalisation will become matter of exact knowledge only when it is brought within the domain of mathematics. The great desideratum of the existing state of natural philosophy Is a mathematical theory of physical forces. After the explanations that have been given of a great variety of the phenomena of light, which is one of those forces, by the hypothesis of a highly eLaatio medium pervading apace, it is not a little surprising that an explanation of the ' there/wipe' of the several forces should not have been sought for in the existence of this medium, which would seem to be a vast reservoir of force sufficient to account for all observed dynamical effects." Professor Challis has pursued this subject In a series of elaborate analytical investigations, also published in the ' Philosophical Magazine' for 1859 and 1860. " The principal hypo thesis of the theory Is, that the physical forces are all consequences of the motions and pressures of a uniform and highly elastic medium pervading space. The variations of the pressures of the medium are supposed to be proportional to variations of its density ; and this sup iiition forms the basis of a mathematical Investigation of the relations between the motions and the pressures. Further, it is assumed that
the medium acts immediately by pressure on the ultimate atoms of bodies, which are all supposed to be spheres of invariable magnitudes and of the same intrinsic inertia. According to these hypotheses, the different phenomena and properties of bodies depend only on the mag nitudes of their atoms, the proportions in which they are composed of atoms of different magnitudes, and on the curanerments of the atoms." As the explanation of the sensible properties of matter must 1.X2 pre ceded by a mathematical investigation of the laws of the dynamic action of the assumed etherial medium, Professor Challis has already entered on such an investigation relatively to light, heat, the force of gravity, the forces of molecular aggregation, and the• force of electricity; and is still proceeding with the explanation of his theory.
We will conclude this article by the suggestion that progress in natural philosophy, in both the directions indicated above, would probably bo effected by the experimental and mathematical investiga tion of the tenoning theorem : Taking apace to be the extension of material substance, the resultant of Its dimensions, and mere consequence of its existence ; and 'notion to be the substitution of one portion of matter for another, identical with the succession of phenomena, our perception of which is time: heat and light are correlates of each other, being also, as we receive them from the sun, and as existing in their terrestrial form, affee. tions of an ether pervading space and ordinary ponderable matter ; but acting as the initiating forces in respect of electricity, mag netism, and chemical affinity, which are correlates of each other of a different order, and derivative affections of the ether (perhaps, also, derivatively from the ether, of the matter exhibiting their phenomena), the apparent production of heat and light by them being in reality the etalzaion only of those initiating forces : all these forces have the power of substituting one portion of matter for another, or of causing motion; the apparent agency of time and motion being merely the continu ance and consequent accumulation of action.