224 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. [September 17, 1870. 
Bread such as is commonly eaten, may he in a 
certain sense compared to the hay with which a 
horse is fed; hut it is known that when a horse is 
fed with hay alone, all its capabilities are not fully 
developed. 
It is only necessary to compare the performances 
of German workmen, who consume bread and po¬ 
tatoes chiefly, with those of English and American 
workmen who eat meat, in order to acquire a clear 
perception of the degree in which the magnitude, 
energy and duration of the work done by the latter 
are augmented by the kind of food they live upon. 
Again, compare the English statesman who, in 
expounding his views or maintaining a debate in 
Parliament delivers a speech lasting five hours or 
more, who at sixty years of age retains the capability 
of taking part in field sports, with the German phi¬ 
losopher who at the same age keeps up with difficulty 
the remains of his powers in order to be capable of 
work, while he becomes fatigued by a walk of a few 
hours. 
For considerable and long-continued intellectual 
or bodily exertion it is necessary to have not only 
good organs of digestion, it is equally necessary that 
there should be a proper selection of food, which 
should be of such a character that the smallest pos¬ 
sible portion of its available force may be required 
for the full production of its action in the body, so 
that there may be the more of that force remaining 
disposable at the will of the individual. 
A knowledge of the conditions most proper for nu¬ 
trition with a view to the performance of work is 
therefore most of all requisite in reference to man. 
We must endeavour to obtain very different means 
of judging as to these conditions now that we know 
urea is not either a measure of the work done, or 
exclusively a measure of the increase or decrease of 
the body during rest or work, as Dr. Parke’s obser¬ 
vations show. 
WHAT IS ENERGY? 
BY BALFOUR STEWART. 
III.-THE CONSERVATION OF ENERGY. 
It is well known that certain organisms of the 
animal world do not confine themselves to one state 
of being. or to one order of existence, and the most 
familiar instance of this roving habit of life is the 
caterpillar, which passes first into the chrysalis 
state, and after that into the butterfly. This habit 
is not, however, peculiar to the organic w r orld, for 
energy delights in similar transmutations, and we 
have just seen how the eminently silent and invisible 
electrical current may occasionally be transmuted 
into the vivid, instantaneous, awe-inspiring flash of 
lightning. Nor is this element of change confined 
to our peculiar corner of the universe, but it extends 
itself to 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 out¬ 
burst, presenting all the appearance of a w r orld on 
fire. 
. We shall not enter here into great detail regard¬ 
ing the various changes of energy from one form into 
another; suffice it to say, that amid a 11 these 
changes of form, and sometimes of quality, the ele¬ 
ment of quantity remains the same. Those of our 
readers who are mathematicians know what is meant 
by variable quantities; for instance, in the equation 
x -f- y + £ = A, if x, y, and z are variable and A con¬ 
stant, 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 to A. It is 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 no¬ 
thing else than an intelligent and scientific denial of 
the chimera of perpetual motion. 
Recognizing 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 accordingly inventors became pos¬ 
sessed with the idea that some elaborate system of 
machinery would, no doubt, give us this grand desi¬ 
deratum, and men of science have been continually 
annoyed with these projects until, in a moment of 
inspiration, they conceived the doctrine^of the con¬ 
servation 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 j)rogress of 
energy through some of its most important transfor¬ 
mations. 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 ques¬ 
tion may be varied in a great number of ways. We 
may ask, for instance, what becomes 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 tar¬ 
get ? 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 faffing hammer, so 
that our question 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 hi 
the process of boring cannon the heat generated was 
sometimes so great as to boil water, and he sup¬ 
posed that work was changed into heat in the pro¬ 
cess 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 
anvil 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 con¬ 
verted into heat. 
But although this was known nearly a century 
ago, it was reserved for Joule, an English philo¬ 
sopher of the present day, to point out the nume¬ 
rical relation subsisting between that species of 
