502 PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 



longer exists as such, but is transformed into mechanical or visible mo- 

 tion, chemical and magnetic power, as also into heat and light. The 

 electric current of a galvanic element originates, as will be shown here- 

 after, in the slow oxidation of the dissolving zinc; that is, in the corre- 

 sponding heat of combustion, which, by means of the transmutation into 

 the form of the electric current, produces all the effects above referred 

 to. These reciprocal changes and transformations of natural forces are 

 treated in Grove's " Correlation of the Natural Forces." 



As shown by the preceding examples, which might be multiplied at 

 pleasure, it may be affirmed: All natural forces are closely connected ; 

 any one of these forces may be transformed into another, but none of 

 them can be destroyed. This fundamental principle of modern physical 

 research has its origin as regards the experimental part of it in the heat 

 equivalent first demonstrated by J. E. Mayer and J. P. Joule; its sub- 

 stance is now generally termed the "conservation of energy." Until a 

 few years ago this proposition was generally expressed by "conserva- 

 tion of force"; since however the conceptions of force, its work and 

 energy, have become clearer, recently the former term has generally 

 been used in preference to the latter. The proposition relative to the 

 "conservation of energy" asserts that the sum total of active energies 

 in the universe is unchangeable in whatever degree the constituents of 

 this sum might change in their relation to each other. If, for instance, 

 a stone is thrown up perpendicularly it possesses a certain kinetic en- 

 ergy in consequence of the velocity imparted at the start; as soon as it 

 has reached a certain height this energy for an instant appears lost; at 

 the turning point the velocity of the stone and its kinetic energy are 

 zero. But at its arrival at the point from which it had been thrown up 

 it again possesses the previously imparted velocity and consequently its 

 original kinetic energy: a fact which can readily be mathematically 

 demonstrated. We ask, then: Was the energy at the point of return 

 really momentarily destroyed ? 



In order to answer this query let us suppose that the stone at the mo- 

 ment of turning rests on a fiat roof; its kinetic energy as to motion is 

 without a doubt zero; but it is not absolutely lost, it simply takes 

 another form of energy. This will be plain to us if we remember that 

 if afterward that stone falls to the ground, the original kinetic energy 

 is again obtained. As long then as that stone remains lying up there 

 the apparently lost energy is preserved in a different form for future use. 



Let us take another example : If the spring of a bow is stretched a 

 certain energy is imparted to it by the labor necessary lor the stretch- 

 ing, which afterward in shooting off the arrow is returned in its en- 

 tirety, — not considering the losses from the impediments to motion. Is 

 the energy employed lost as long as the bow spring remains stretched .' 

 Or is it only laid up .' It is plain that the latter is the ease. 



If, on throwing a body up perpendicularly, we call the energy with 

 which it begins and on returning completes its motion — "kinetic energy" 



