223 



• the vapour raised from the earth^s surface by the heat of the 

 sun acquires in the clouds potential energy ; in again descend- 

 ing to the sea-level, it acquires actual energy, and may do 

 useful work in the shape of mountain torrents, the usual motive 

 power in mountainous districts, or mischief to the garden and 

 greenhouse, in the solid form of hail. In a mixture of oxygen 

 and hydrogen gases in combining proportions,, the energy of 

 chemical affinity remains potential, until by the action of 

 heat, such as that of an electric spark, some of the gaseous 

 atoms are brought within the sphere of their mutual attraction, 

 when the whole unite violently with the evolution of light and 

 heat, and form water: and the theory of conservation requires 

 that exactly the same amount of energy that was developed in 

 the forms of heat and light at the time of combination would 

 be required to tear the atoms asunder again, and to place them 

 beyond the reach of each other^s attraction. Again, the 

 energy of a pendulum is wholly potential at each extremity of 

 its oscillation, and wholly actual at the middle or lowest point. 

 By some writers of eminence the potential energy here 

 described has been termed energy of position.''^ Practically^ 

 the term "actuaP^ is not used, and potential is frequently 

 used elliptically for ^' potential energy ■'^ ; thus, we speak of 

 the potential of an electric charge, or of a voltaic current. But 

 it must be observed that the term potential, used substantively, 

 has a definite meaning as employed by Laplace and Green in 

 the analytical investigation of theories of attraction : this 

 subject, for the purposes of the present paper, it is not neces- 

 sary to consider further. 



14. The theory of the conservation of energy implies that no 

 kind of energy can be produced by human agency except at the 

 expense of an equal amount of the same kind, or an equivalent 

 amount of some other kind, of energy. From this it follows as 

 a corollary, that so far as physical law is concerned, the total 

 .amount of energy in the universe must remain unchanged ; 

 but to assert that it is, under all circumstances, unchangeable is 

 a very different matter. The creation of matter must neces- 

 sarily imply the creation of energy ; and those who deny the 

 possibility of the one, must deny that of the other also ; they 

 must, in fact, deny the existence of Omnipotence. It may 

 further be remarked, that the principle of the conservation of 

 energy is identical with that treated in all theoretical works on 

 dynamics as the " conservation of vis viva.'' 



15. It is much to be regretted that a far greater degree of 

 logical accuracy in the use of terms than is usually met with does 

 not exist amongst even the ablest writers on physics, for many of 

 the arguments adduced against physical principles lie not against 



