184 
NATURE 
[Fuly 7, 1870 
remote starry systems, in some of which there is a total 
extinction of luminosity for a while, to be succeeded by a 
most brilliant luminous outburst, presenting all the ap- 
pearance of a world on fire. 
We shall not enter here into great detail regarding the 
various changes of energy from one form into another ; 
suffice it to say, that amid all these changes of form, and 
sometimes of quality, the element of gwantity remains 
the same. Those of our readers who are mathematicians 
know what is meant by variable quantities, for instance, 
in the equation xt+y+2=A, if +, y, & 2 are variable andA 
constant, you may change x into y and into z, and y into 
x and into z, and in fact perform any changes you choose 
upon the left hand side of your equation, provided that 
you keep their sum always constant and equal toA. Itis 
precisely thus in the world of energy ; and the invariability 
of the sum of all the energies of the universe forms the 
doctrine known as the “ conservation of energy.” This 
doctrine is nothing else than an intelligent and scientific 
denial of the chimera of perpetual motion. 4 
Recognising the great importance of work, it was 
natural enough at an early stage of our knowledge that 
enthusiasts should endeavour to create energy or the power 
of doing work, that is to say, endeavour to construct a 
machine that should go on working for ever without 
needing to be supplied with fuel in any way, and accord- 
ingly inventors became possessed with the idea that some 
elaborate system of machinery would, no doubt, give us 
this grand desideratum, and men of science have been 
continually annoyed with these projects, until in amoment 
of inspiration they conceived the doctrine of the conser- 
vation of energy ! 
It flows from this doctrine that a machine is merely an 
instrument which is supplied with energy in one form, 
and which converts it into another and more convenient 
form according to the law of the machine. 
We shall now proceed to trace the progress of energy 
through some of its most important transformations. To 
begin with that one to which we have already alluded, 
what becomes of the energy of a falling body after it 
strikes the earth? This question may be varied in a great 
number of ways. We may ask, for instance, what be- 
comes of the energy of a railway train when it is stopped ? 
what becomes of the energy of a hammer after it has 
struck the anvil? of a cannon ball after it has struck 
the target ? and so on. 
In all these varieties we see that either percussion or 
friction is at work ; thus it is friction that stops a railway- 
train, and it is percussion that stops the motion of a 
falling stone or of a falling hammer, so that our ques- 
tion is in reality, what becomes of the energy of visible 
motion when it has been stopped by percussion or 
friction ? 
Rumford and Davy were the pioneers in replying to 
this important question. Rumford found that in the pro- 
cess of boring cannon the heat generated was sometimes 
so great as to boil water, and he supposed that work was 
changed into heat in the process of boring. Davy again 
melted two pieces of ice by causing them to rub against 
each other, and he likewise concluded that the work 
spent on this process had been converted into heat. 
We see now why by hammering a coin on an anyil we 
can heat it very greatly, or why on a dark night the 
sparks are seen to fly out from the break-wheel which 
stops the motion of the railway train, or why by rubbing 
a metal button violently backwards and forwards against 
a piece of wood we can render it so hot as to scorch 
our hand, for in all these cases it is the energy of 
visible motion which is being converted into heat. 
But although this was known nearly a century ago, it 
was reserved for Joule, an English philosopher of the 
present day, to point out the numerical relation subsist- 
ing between that species of energy which we call visible 
motion and that which we call heat. 
The result of his numerous and laborious experiments 
was, that if a pound of water be dropped from a height 
of 772 feet under the influence of gravity, and if the 
velocity which it attains be suddenly stopped and con- 
verted into heat, this heat will be sufficient to raise the 
whole mass 1° Fahr. in temperature. 
From this he concluded that when a pound of water 
is heated 1° Fahr. in temperature, an amount of mole- 
cular energy enters into the water which is equiva- 
lent to 772 foot-pounds, that is to say, to one pound 
raised 772 feet high against the influence of gravity, 
or allowed to fall 772 feet under the same influence. 
He found again that if a.pound of water were to fall 
twice this distance, or 1,544 feet under gravity, the ve- 
locity if stopped would raise its temperature 2° Fahr., 
and in fact that the rise of temperature under such cir- 
cumstances is proportional to the height from which the 
pound of water is supposed to fall. By this means 
an exact relation is established between heat and 
work, Grove was the first to point out the probability of 
a connection between the various species of molecular 
energy ; and the researches of Joule, Thomson, and others, 
have established these relations with numerical accuracy, 
No better example of the correlation of the various kinds 
of energy can be given than what takes place in a galvanic 
battery. Let us suppose that zinc is the metal used. Here 
the source of energy is the burning or chemical combina- 
tion of the zinc with oxygen, &c., in order to form a salt of 
zinc. The source of energy is in fact much the same as when 
coal is burned; it is the energy produced by chemical com- 
bination. Now, as we have said, the zinc combines with 
the oxygen, and sulphate of zinc is produced, but the 
result of this combination does not at first exhibit itself in 
the form of leat, but rather in that of an electric current. 
No doubt a great portion of the energy of this electric 
current is ultimately spent in heat, but we may, if we 
choose, spend part in promoting chemical decomposition ; 
for instance, we may decompose water. In this case part 
of the energy of the battery, derived as has been stated 
from the burning of the zinc, is spent in heat and part in 
decomposing the water, and hence we shall have less heat 
than if there were no water to decompose. But if when 
we have decomposed the water, we mix together the two 
gases hydrogen and oxygen which are the results of this 
decomposition, and explode them, we shall recover the 
precise deficiency of heat. Without the decomposition, 
let us say that the burning in the battery of a certain 
weight of zinc gives us heat equal to Ioo, but with the 
decomposition only 89, twenty units of energy have there- 
fore become spent in the decomposition, but if we explode 
the mixture of gases procured from the decomposition we 
shall get back heat equal to 20, and thus make the whole 
