Electricity is characterised by the author of the theory of the cor relation of physical forces, "as that affection of matter or mode of force which most distinctly and beautifully relates other modes of force, and exhibits, to a great extent in a quantitative form, its own relation with them, and their reciprocal relations with it and with each other." Commencing with it as an initiating force, we get motion directly produced by it, in various forms ; for instance, in the attraction and repulsion of bodies and in the deflection of the galvanometer needle. Electricity directly produces heat, as shown in the ignited wire, the electric spark, and the voltaic are. In these phenomena, also, it directly produces light, of the greatest known intensity. It directly produces magnetism, as originally shown by Oersted in his great discovery [ErAcYno-Maeerrism). Lastly, electricity produces chemical affinity; and by its agency we are enabled to obtain effects of analysis or synthesis with which ordinary chemistry does not fur nish us.
Light is, perhaps, that mode of force the reciprocal relations of which with the others have been the least traced out Until the discoveries of Niepce, Daguerre, and Talbot very little could be predicated of the action of light in pr ucing other modes of force. 'Viewing the phenomena of photography as resulting from a function of light, we get the Latter as an initiating force, capable of producing, mediately or immediately, the other modes of force. Thus, it immediately produces chemical action; and having this, we at once acquire a moans of producing the others. In Mr. Grove's lectures at the London Institution in 1843, already mentioned, he showed the following experiment by which the production of all the other modes of force by light is exhibited. "A prepared daguerreotype plate is enclosed in a box filled with water, having a glass front, with a shutter over it Between this glass and the plate is a gridiron of silver wire; the plate is connected with one extremity of a galvanometer coil, and the gridiron of wire with one extremity of a Breguet's helix : the other extremities of the galvanometer and helix are connected by a wire, and the needles brought to zero. As soon as a beam of either daylight or the oxyhydrogcn light is, by raising the shutter, permitted to impinge upon the plate, the needles are deflected. Thus, light being the initiating force, we get chemical action on the plate, electricity circu latiug through the wires, magnetism in the coil, heat in the helix, and /110604 in the needles." Light would seem directly to produce beat in the phenomena of what is termed absorption of light. Whenever light is absorbed heat takes its place, affording us apparently an instance of the conversion of light into beat; and of the fact, that the force of light is not, in reality absorbed or annihilated, but merely changed in character, becoming in this instance converted into heat by impinging on solid matter, u heat is converted into light when solid incombustible matter becomes intensely luminous by being Introduced into highly-heated but only slightly luminous gm. The difference between the correlation of
light and heat and that of the other forces with each other, and with them, has already been noticed, and will also be returned to In the aysetimn, as was proved by the Important discovery of Faraday [1Iscxzeo-tnrceniciyv], will produce electricity, but with this pecu liarity, that In itself it is atatio; and, therefore, to produce a dynamic force, motion must be auperadded to it; it is in fact directive, not motive, altering the direction of other forces, but not in strictness initiating them. Magnets being moved in the direction of lines joining their poles, produce electrical currents in such neighbouring bodies as are conductors of electricity, in directions transverse to the line of motion. So if the magnet be atationary, conducting bodies moved across any of the lines of magnetic force have curreuts of electricity developed in them. Magnetism can, then, through the medium of electricity, produce heat, light, chemical a _fatty, and motion. It can itself be produced by those forces, but cannot produce them except when In motion, which, therefore, is to be regarded in this case as the initiative force. Magnetism will, however, directly affect the other forces, light, heat, and chemical and change their direction or mode of action, or, at all events, will so affect matter subjected to these forces that their direction is changed. When this force, however, hi in what may bo called its dynamic condition, that is, its state of change at the commencement and the termination, or during the increment or decrement of its development, it will produce directly some of the other forces, electricity and heat for example. " But it may be said," observes Mr. Grove on this point,—and the observation is important with reference to the philosophy of the entire subject of the correla tion of physical forces,—" while magnetism Is thus progressive seine other force is acting on it, and therefore it does not initiate. This ho admits to be true, but urges that " the same may be said of all the other forces ; they havo no commencement that we can trace. We must over refer them back to some antecedent force equal in amount to that produced, and therefore tho word initiation cannot in strictness apply, but must only be taken as signifying the force selected as the first." The peculiarity, however, of the mutual relations of heat and light, already noticed, as recognised by Mr. Grove himself, points, as remarked above, to a limitation of the truth of his views in this reaped.