101 
reference to both forms of energy.— Visible kinetic energy as 
that of a cannon-ball shot upwards) is changed, as it rises* into 
visible potential energy; and as the ball descends, its 
energy is again changed into the kinetic. The ball Conserved 'as 
strikes the earth, and again the visible kinetic energy Matter 
is changed into a kinetic energy of invisible motion, Sudden]y 
called “ Heat” Whenever visible kinetic energy is im P«iea 
suddenly impeded, it changes into “heat” (p. 80). changes into 
Energy of every kind is found to have great powers SbTe^Mo! 
of transmutation ; and Sir W. R. Groves's instructive tion - 
“Correlation of Forces" brings together many varieties of 
cases. (And see p. 106.) Life, so far as it is physical, depends 
on transformation of energy (p. 81). In any system of bodies 
there are various kinetic and potential energies, the sum 
of which remains for ever unaltered. Hence “Energy," 
even when invisible, has as much claim to be regarded as 
objective reality, as “ Matter" itself. The difference between 
them is that energy is a very Proteus in change, while matter is 
always the same. The only real things in the physi- 
cal, or, so-called visible Universe, (for it is not all ^„ al1 
really visible), being “ matter " and “ energv," equally trans- 
matter being passive, — all physical changes are foimable? 
merely transformations of energy, “ each change representing a 
kind of creation and annihilation" (p. 81). 
It is of the utmost importance, however, here to know — 
whether all forms of “ energy are equally susceptible of trans- 
formation ? " If any one form be less transformable than others 
(p. 82), though the whole quantity of energy may remain, it will 
become less and less available (p. 82). Now this is the case 
with heat. 
20. The investigation of the transformation of this form of 
energy— Heat — into work, has taught us the dvna- Heat js 
mical theory of heat; and also the principle of the Energy which 
“ Dissipation of Energy " ; and it has been shown ™ormabie 
that only a portion of the heat can, (even under the kineti^ 
most favourable conditions), be transformed into energies, 
useful work (p. 83). Some invisible finite agencies, (playfully 
called “demons" by Sir W. Thomson), may here have some- 
thing to do (pp. 127 and 148); for while it is possible to change 
mechanical energy into heat (p. 90) — only a portion can be 
retransformed ; and that too would be more and more “ dissi- 
pated " on repeating the process. Heat not, in fact, being wholly 
“ conserved," — or not in an available form, — will bring the 
system of the Universe ultimately to an end. This point is 
carefully elaborated by our authors. “ Conservation of 
