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the vapour raised from the eartlFs surface hv 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 lorrents, 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. 
I3v some writers of eminence the potential energy here 
described has been termed “ energy of position.” Practically, 
the term “ actual ” 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 
