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Of the physical forces that are directly transformed into social ones,the like is to be said. Currents of air and water, which before the use ofsteam were the only agents brought in aid of muscular effort for performingindustrial processes, are, as we have seen, generated by solar heat. Andthe inanimate power that now, to so vast an extent, supplements human labour,is similarly derived. Sir John Herschel was the first to recognize the truththat the force impelling a locomotive, originally emanated from the Sun.

Step by step we go back -- from the motion of the piston to the evaporationof the water; thence to the heat evolved during the burning of coal; thenceto the assimilation of carbon by the plants of whose imbedded products coalconsists; thence to the carbon di-oxide from which their carbon was obtained;and thence to the rays of light which effected the de-oxidation. Solar forcesmillions of years ago expended on the Earth's vegetation, and since lockedup in deep-seated strata, now smelt the metals required for our machines,turn the lathes by which the machines are shaped, work them when put together,and distribute the fabrics they produce. And since economy of labour makespossible a larger population, gives a surplus of human power that would elsebe absorbed in manual occupations, and thus facilitates the development ofhigher kinds of activity; these social forces which are directly correlatedwith Physical forces anciently derived from the Sun, are only less importantthan those of which the correlates are the vital forces recently derivedfrom it. §73. Many who admit that among physical phenomena at large, transformationof forces is now established, will probably say that inquiry has not yetgone far enough to enable us to assert equivalence. And in respect of theforces classed as vital, mental, and social, the evidence assigned they willconsider by no means conclusive even of transformation, much less of equivalence.

But the universal truth above followed out under its various aspects,is a corollary from the persistence of force. From the proposition that forcecan neither come into existence nor cease to exist, the several foregoingconclusions inevitably follow. Each manifestation of force can be interpretedonly as the effect of some antecedent force; no matter whether it be an inorganicaction, an animal movement, a thought, or a feeling. Either bodily and mentalenergies, as well as inorganic ones, are quantitatively correlated to certainenergies expended in their production, and to certain other energies whichthey initiate; or else nothing must become something and something must becomenothing. The alternatives are, to deny the persistence of force, or to admitthat from given amounts of antecedent energies neither more nor less thancertain physical and psychical changes can result. This corollary cannotindeed be made more certain by accumulating illustrations. Whatever proofof correlation and equivalence is reached by experimental inquiry, is basedon measurement of the forces expended and the forces produced. But, as wasshown in the last chapter, any such process implies the use of some unitof force which is assumed to remain constant; and its constancy can be assumedonly as being a corollary from the persistence of force. How then can anyreasoning based on this corollary, prove the equally direct corollary thatwhen a given quantity of force ceases to exist under one form, an equal quantitymust come into existence under some other form or forms?

"What, then," it may be asked, "is the use of investigationsby which transformations and equivalence of forces is sought to be inductivelyestablished? If the correlation cannot be made more certain by them thanit is already does not their uselessness necessarily follow?" No. Theyare of value as disclosing the many particular implications which the generaltruth does not specify. They are of value as teaching us how much of onemode of force is the equivalent of so much of another mode. They are of valueas detecting under what conditions each metamorphosis occurs. And they areof value as leading us to inquire in what shape the remnant of force hasescaped, when the apparent results are not equivalent to the cause.