THERMODYNAMICS. 261 



atmosphere, so that it is impossible to represent it to scale on an ordinary 

 diagram. 



AdiabatiCS or IsentrOpiCS. If a body is not allowed to gain or lose 

 heat by conduction, and if the volume and pressure alter, but in such a 

 way that no kinetic energy is acquired by the body or its parts, the 

 change is said to be adiabatic. When, for example, a mass of gas is 

 enclosed in a non-conducting cylinder under a loaded piston, a very 

 gradual change in the load will produce an adiabatic change in the gas. 

 But a sudden change, such as a sudden finite decrease of load, will result 

 in rapid motion of the piston and rapid motion of the gas. Some of the 

 energy of this internal motion of the gas will be converted into heat by 

 viscosity, and the body will thus receive heat, though from itself. In the 

 celebrated experiment of Gay-Lussac and Joule on the expansion of a gas 

 when no external work is done (p. 120), no heat is given to or taken from 

 the outside, but the gas acquires considerable kinetic energy which is 

 ultimately converted into heat, each element receiving heat from the 

 surrounding elements through the viscosity. Hence the change is not 

 adiabatic. As we shall show later, the temperature would have fallen 

 about 70 had the change been truly adiabatic, the gas losing energy 

 through external work. This cooling with adiabatic expansion shows 

 that the adiabatics of a gas on the indicator diagram are steeper than the 

 isothermals, for the adiabatic through a given point must move down to 

 a lower isothermal. We shall show later that the adiabatics are steeper 

 than the isothermals in all cases. For a reason to be given later the 

 adiabatics are also termed isentropics. 



Isopiestics and Isometrics. Lines parallel to the horizontal axis 

 indicate a change of volume at constant pressure and are termed Iso- 

 piestics. The lines of equal volume parallel to the vertical axis are 

 termed Isometrics, 



Heat Engines. Any arrangement for the transformation of heat 

 into mechanical energy is termed a heat engine. 



By considering the most familiar instance, the steam engine, we may 

 see what are the essentials of a heat engine. 



There is a working substance, the water and steam. This is made to 

 expand on conversion into steam by the communication of heat from the 

 sides of the boiler the source of heat. But only a part of the heat 

 communicated by the source is turned into work. In order to make the 

 transformation continuous, the steam is ejected from the cylinder and is 

 either allowed to expand into the cooler air, carrying off with it much of 

 the heat originally given, or it is passed into a cold condenser, where it 

 yields up heat to the condenser and is turned into cold water. The 

 general nature of the process then consists in the communication to the 

 working substance of heat from a hotter body the source the trans- 

 formation of some of this heat to work by the expansion of the working 

 substance and the communication of heat not transformed to a colder 

 body, either the outer air or a condenser. In thermodynamics the colder 

 body is termed the Refrigerator. 



It is of the utmost practical importance to find what is the maximum 

 fraction of the heat leaving the source which can be transformed to work 

 when the temperatures of the source and refrigerator are known. For 

 a comparison of the actual fraction transformed in n given engine with 



