206* TRANSFORMATION AND EQUIVALENCE OF FORCES. 



raising the limb to the position whence it fell. In this case, 

 - as in the case of an inanimate body descending to the Earth, 

 the force accumulated by the downward motion is just equal 

 to the force previously expended in the act of eleva- 

 tion. Conversely, Motion that is arrested produces, 

 under different circumstances, heat, electricity, magnetism, 

 light. From the warming of the hands by rubbing them 

 together, up to the ignition of a railway-brake by intense 

 friction — from the lighting of detonating powder by percus- 

 sion, up to the setting on fire a block of wood by a few blows 

 from a steam-hammer; we have abundant instances in which 

 heat arises as Motion ceases. It is uniformly found, that the 

 heat generated is great in proportion as the Motion lost is 

 great; and that to diminish the arrest of motion, by dimin- 

 ishing the friction, is to diminish the quantity of heat 

 evolved. The production of electricity by Motion is illus- 

 trated equally in the boy's experiment with rubbed sealing- 

 wax, in the common electrical machine, and in the apparatus 

 for exciting electricity by the escape of steam. Wherever 

 there is friction between heterogeneous bodies, electrical dis- 

 turbance is one of the consequences. Magnetism may result 

 from Motion either immediately, as through percussion on 

 iron, or mediately as through electric currents previously 

 generated by Motion. And similarly, Motion may create 

 light; either directly, as in the minute incandescent frag- 

 ments struck off by violent collisions, or indirectly, as 

 through the electric spark. " Lastly, Motion may be again 

 reproduced by the forces which have emanated from Mo- 

 tion; thus, the divergence of the electrometer, the revolu- 

 tion of the electrical wheel, the deflection of the magnetic 

 needle, are, when resulting from frictional electricity, pal- 

 pable movements reproduced by the intermediate modes of 

 force, which have themselves been originated by motion." 



That mode of force which we distinguish as Heat, is 

 now generally regarded by physicists as molecular motion — 

 not motion as displayed in the changed relations of sensible 



