We have now gone as far as we conveniently can in considering the
intertwining strands of the atom and of electricity. It is time to turn
to the third strand—energy.
To physicists the concept of “work” is that of exerting a force on a
body and making it move through some distance. To lift a weight against
the pull of gravity is work. To drive a nail into wood against the
friction of its fibers is work.
Anything capable of performing work is said to possess “energy” from
Greek words meaning “work within”. There are various forms of energy.
Any moving mass possesses energy by virtue of its motion. That is, a
moving hammer will drive a nail into wood, while the same hammer held
motionlessly against the nailhead will not do so. Heat is a form of
energy, since it will expand steam that will force wheels into motion
that can then do work. Electricity, magnetism, sound, and light can be
made to perform work and are forms of energy.
The forms of energy are so many and so various that scientists were
eager to find some rule that covered them all and would therefore serve
as a unifying bond. It did not seem impossible that such a rule might
exist, since one had been found in connection with matter that appeared
in even greater variety than energy did.
All matter, whatever its form and shape, possessed mass, and in the
1770s, the French chemist Antoine Laurent Lavoisier (1743-1794)
discovered that the quantity of mass was constant. If a system of matter
were isolated and made to undergo complicated chemical reactions,
everything about it might change, but not its mass. A solid might turn
into a gas; a single substance might change into two or three different
substances, but whatever happened, the total mass at the end was exactly
the same (as nearly as chemists could tell) as at the beginning. None
was either created or destroyed, however, the nature of the matter might
change. This was called the “law of conservation of mass”.
[Illustration: _Lavoisier in his laboratory during his studies on
respiration. From a sketch made by Madame Lavoisier._]
[Illustration: _Antoine Lavoisier and his wife._]
Naturally, it would occur to scientists to wonder if a similar law might
hold for energy. The answer wasn’t easy to get. It wasn’t as simple to
measure the quantity of energy as it was to measure the quantity of
mass. Nor was it as simple to pen up a quantity of energy and keep it
from escaping or from gaining additional quantity from outside, as it
was in the case of mass.
Beginning in 1840, however, the English physicist James Prescott Joule
(1818-1889) began a series of experiments in which he made use of every
form of energy he could think of. In each case he turned it into heat
and allowed the heat to raise the temperature of a given quantity of
water. He used the rise in temperature as a measure of the energy. By
1847 he was convinced that any form of energy could be turned into fixed
and predictable amounts of heat; that a certain amount of work was
equivalent to a certain amount of heat.
In that same year, the German physicist Hermann Ludwig Ferdinand von
Helmholtz (1821-1894) advanced the general notion that a fixed amount of
energy in one form was equal to the same amount of energy in any other
form. Energy might change its form over and over, but not change its
amount. None could either be destroyed or created. This is the “law of
conservation of energy”.