The Transformation and Equivalence of Forces §66. When, to the unaided senses, Science began to add supplementarysenses in the shape of measuring instruments, men began to perceive variousphenomena which eyes and fingers could not distinguish. Of known forms offorce, minuter manifestations became appreciable; and forms of force beforeunknown were rendered cognizable and measurable. Where forces had apparentlyended in nothing, and had been carelessly supposed to have actually doneso, instrumental observation proved that effects had in every instance beenproduced: the forces having reappeared in new shapes. Here has at lengtharisen the inquiry whether the force displayed in each surrounding change,does not in the act of expenditure undergo metamorphosis into an equivalentamount of some other force or forces. And to this inquiry experiment is givingan affirmative answer, which becomes daily more decisive. Séguin,Mayer, Joule, Grove, and Helmholtz, are more than others to be credited withthe enunciation of this doctrine. Let us glance at the evidence on whichit rests.
Motion, wherever we can directly trace its genesis, we find had pre-existedas some other mode of force. Our own voluntary acts have always certain sensationsof muscular tension as their antecedents. When, as in letting fall a relaxedlimb we are conscious of a bodily movement requiring no effort, the explanationis that the effort was exerted in raising the limb to the position whenceit fell. In this case, as in the case of an inanimate body descending tothe Earth, the force accumulated by the downward motion is equal to the forcepreviously expended in the act of elevation. Conversely, Motion that is arrestedproduces, under different circumstances, heat, electricity, magnetism, light.
From the warming of the hands by rubbing them together, up to the ignitionof a railway-brake by intense friction -- from the lighting of detonatingpowder by percussion, up to the setting on fire a block of wood by a fewblows from a steam-hammer; we have abundant instances in which heat arisesas Motion ceases. It is uniformly found that the heat generated is greatin proportion as the Motion lost is great; and that to diminish the arrestof motion by diminishing the friction, is to diminish the quantity of heatevolved. The production of electricity by Motion is illustrated equally inthe boy's experiment with rubbed sealing-wax, in the common electrical machine,and in the apparatus for exciting electricity by the escape of steam. Whereverthere is friction between heterogeneous bodies electrical disturbance isone of the consequences. Magnetism may result from Motion either immediately,as through percussion on steel, or mediately as through electric currentspreviously generated by Motion. And similarly, Motion may create light; eitherdirectly, as in the minute incandescent fragments struck off by violent collisions,or indirectly, as through the electric spark. "Lastly, Motion may beagain reproduced by the forces which have emanated from Motion; thus, thedivergence of the electrometer, the revolution of the electrical wheel, thedeflection of the magnetic needle, are, when resulting from frictional electricitypalpable movements reproduced by the intermediate modes of force, which havethemselves been originated by motion."
That mode of force which we distinguished as Heat, is now regarded asmolecular motion -- not motion as displayed in the changed relations of sensiblemasses to one another, but as assessed by the units of which such sensiblemasses consist. Omitting cases in which there is structural rearrangementof the molecules, heated bodies expand; and expansion is interpreted as dueto movements of the molecules in relation to one another: wider oscillations.
That radiation through which anything of higher temperature than things aroundit, communicates Heat to them, is clearly a species of motion. Moreover,the evidence afforded by the thermometer that Heat thus diffuses itself,is simply a movement caused in the mercurial column. And that the molecularmotion which we call Heat, may be transformed into visible motion, familiarproof is given by the steam-engine; in which "the piston and all itsconcomitant masses of matter are moved by the molecular dilatation of thevapour of water." Where Heat is absorbed without apparent result, moderninquiries have detected unobtrusive modifications: as in glass, the molecularstate of which is so far changed, that a Alarized ray of light passing throughit becomes visible, which it does not when the glass is cold; or as on polishedmetallic surfaces, which are altered in molecular structure by radiationsfrom objects very close to them. The transformation of Heat into electricityoccurs when dissimilar metals touching each other are heated at the pointof contact: electric currents being so produced. Solid, incombustible matterput into heated gas, as lime into the Oxyhydrogen flame, becomes incandescent;and so exhibits the conversion of Heat into light. The production of magnetismby Heat, if it cannot be proved to take place directly, may be proved totake place indirectly through the agency of electricity. And through thesame agency may be established the correlation of Heat and chemical affinity-- a correlation which is directly shown by the marked influence Heat exerciseson chemical composition and decomposition.
