The principle just stated embodies one of the fundamental laws of
science, called the law of the _conservation of matter_.

A similar law holds for energy as well. We can transform electric
energy into the motion of trolley cars, or we can make use of the
energy of streams to turn the wheels of our mills, but in all these
cases we are transforming, not creating, energy.

When a ball is fired from a rifle, most of the energy of the gunpowder
is utilized in motion, but some is dissipated in producing a flash and
a report, and in heat. The energy of the gunpowder has been scattered,
but the sum of the various forms of energy is equal to the energy
originally stored away in the powder. The better the gun is, the less
will be the energy dissipated in smoke and heat and noise.




CHAPTER V

FOOD


55. The Body as a Machine. Wholesome food and fresh air are
necessary for a healthy body. Many housewives, through ignorance,
supply to their hard-working husbands and their growing sons and
daughters food which satisfies the appetite, but which does not give
to the body the elements needed for daily work and growth. Some foods,
such as lettuce, cucumbers, and watermelons, make proper and
satisfactory changes in diet, but are not strength giving. Other