ATOM. 451 



Outline of Modern Molecular Science, and in particular of the Molecular Theory 



of Gases. 



We begin by assuming that bodies are made up of parts, each of which 

 is capable of motion, and that these parts act on each other in a manner 

 consistent with the principle of the conservation of energy. In making these 

 assumptions, we are justified by the facts that bodies may be divided into 

 smaller parts, and that all bodies with which we are acquainted are conservative 

 systems, which would not be the case unless their parts were also conservative 

 systems. 



We may also assume that these small parts are in motion. This is the 

 most general assumption we can make, for it includes, as a particular case, the 

 theory that the small parts are at rest. The phenomena of the diffusion of 

 gases and liquids through each other shew that there may be a motion of the 

 small parts of a body which is not perceptible to us. 



We make no assumption with respect to the nature of the small parts 

 whether they are all of one magnitude. We do not even assume them to have 

 extension and figure. Each of them must be measured by its mass, and any 

 two of them must, like visible bodies, have the power of acting on one another 

 when they come near enough to do so. The properties of the body, or medium, 

 are determined by the configuration and motion of its small parts. 



The first step in the investigation is to determine the amount of motion 

 which exists among the small parts, independent of the visible motion of the 

 medium as a whole. For this purpose it is convenient to make use of a 

 general theorem in dynamics due to Clausius. 



When the motion of a material system is such that the time average of 

 the quantity 2 (waf) remains constant, the state of the system is said to be 

 that of stationary motion. When the motion of a material system is such that 

 the sum of the moments of inertia of the system, about three axes at right 

 angles through its centre of mass, never varies by more than small quantities 

 from a constant value, the system is said to be in a state of stationary motion. 



The kinetic energy of a particle is half the product of its mass into the 

 square of its velocity, and the kinetic energy of a system is the sum of the 

 kinetic energy of all its parts. 



572 



