PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 503 



(also actual or dynamic energy) ; if furthermore we call the stored up or 

 preserved energy of a lifted weight or of a stretched spring, &c, "po- 

 tential energy" (also energy of position, statical or quiescent energy, or 

 elasticity) ; we observe that in throwing up a stone, as also in the stretch- 

 ing of a spring, the original kinetic energy is by degrees transformed 

 into potential energy, until finally the kinetic energy is equal to zero, 

 while the potential energy has become a maximum quantity. In the 

 ict urn of the stone, or the resilience of the spring, the relation is the 

 reverse; i.e. the potential energy is by degrees turned into kinetic 

 energy, untjl finally the latter has attained its maximum, while the for- 

 mer on the other hand has become zero. 



A swinging pendulum, at the moment when it reaches the middle point 

 with the greatest velocity, possesses only kinetic energy ; at the highest 

 points of its excursion it possesses only potential energy. On all inter- 

 mediate points the sum of its kinetic and potential energy is a constant 

 quantity. In a similar way the kinetic and potential energy form in 

 their sum a constant quantity in a stone that is thrown and falls back 

 vertically. 



This is the real import of the proposition of the " conservation of 

 energy." Applied to the energy of the universe — as a whole, the propo- 

 sition is: The sum total of the kinetic and potential energy in tue uni- 

 verse remains ever constant, however much the values of the several 

 items may change. It is consequently impossible to destroy either 

 energy or work, or the forces producing them, but it is equally impossi- 

 ble to originate them. The mutual play of the natural forces consists 

 then only in their transformation, in their absorption and reappearance, 

 without the least loss of the effects, i. c. without any loss of energy or 

 work, or without their re-creation. 



Having grasped the meaning of the highly important theory of the 

 "conservation of energy," we will test its sufficiency by a few r examples. 

 If a stone has had kinetic energy given it in being thrown upward, and 

 at the turning point has passed through a state of potential energy, 

 and on returning to the starting point possesses again its full, original 

 kinetic, energy, does it not lose it all the same when it is suddenly 

 brought to rest on reaching the ground '.' Apparently it does! In reality 

 it does not, for the kinetic energy regained through the fall of the stone 

 has, by its striking the ground, been turned into oscillation energy, in 

 internal and external work of the stone molecules; it has been trans- 

 formed into heat and expansion by heat, as was demonstrated by Joule's 

 experiment. 



The descending water of the Rhine-fall, of mountain torrents, iK:c. in 

 dashing against the foot of rooks, in reality does not lose any of its 

 motive energy, but for the seeming loss in external motion the internal 

 motion of the molecules becomes more intense than previously; i. c. it 

 possesses at the foot of the height as much more of heat as it has lost in 



