228 THE POPULAR SCIENCE MONTHLY 



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 hydro- 

 gen 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." 



When a hammer strikes a piece of lead the motion of the hammer appears 

 to be entirely lost. Indeed in the early days it was supposed that what we now 

 call the energy of the hammer was destroyed. But there is no loss. The motion 

 of the massive hammer is transformed into molecular motion in the lead, and 

 here our imagination must help us. In a solid body, although the force of 

 cohesion holds the atoms together, the atoms are supposed, nevertheless, to 

 vibrate within certain limits, and the greater the amount of mechanical action 

 invested in the body by percussion, compression, or friction, the greater will be 

 the rapidity and the wider the amplitude of the atomic oscillations. 2 



Thermodynamics. — To understand the essential features of the sci- 

 ence 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 under- 

 goes 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 care- 

 fully ignored, and in taking up the study of thermal and chemical 



2 Tyndall goes on to explain in a general way the difference in the constitu- 

 tion of solids, liquids and gases. See "Heat: A Mode of Motion," pp. 115-119. 



