Page:Popular Science Monthly Volume 76.djvu/232

228 study the physical properties of the substance before and after the change has taken place.

The Atomic Theory of Heat.—The theory of heat properly includes the whole of chemistry, and every student of elementary chemistry is familiar with the use of the atomic theory in enabling one to form clear ideas of chemical processes. For example, the burning of hydrogen is thought of as the joining together of atoms of hydrogen and oxygen to form molecules of water vapor. The atomic theory is also of use in giving one clear ideas of the physical properties of substances. Thus, a gas is supposed to consist of a great number of particles in violent to-and-fro motion, and the gas exerts a pressure against the walls of the containing vessel because of the bombardment of the walls by the rapidly moving molecules of the gas. In addition to these two highly developed branches of the atomic theory (chemistry and the theory of gases), the atomic theory has been applied in a more or less vague but very useful way in the study of a great variety of heat phenomena as exemplified in the following quotation from Tyndall's "Heat a Mode of Motion."

Thermodynamics.—To understand the essential features of the science of thermodynamics it is necessary to revert to the discussion of work and energy. Whenever a substance gives up energy which it has in store the substance always undergoes change. Thus, the fuel which supplies the energy to a steam engine and the food which supplies the energy to a horse undergo a chemical change; the steam which carries the energy of the fuel from a boiler to a steam engine cools off or undergoes a thermal change when it gives up its energy to the engine; a clock spring changes its shape as it gives up its energy in driving a clock; an elevated store of water changes its position as it gives its energy to a water wheel; the heavy flywheel of a steam engine does the work of the engine for a few moments after the steam is shut off and the fly-wheel changes its velocity as it gives up its energy.

In mechanics the theory of energy is discussed in connection with mechanical changes only, thermal and chemical changes being carefully ignored, and in taking up the study of thermal and chemical