CHAPTER IX. 

 THE KINETIC THEORY OF MATTER. 



Atomic Hypotheses Solids Liquids Gases Kinetic Theory of Gases Mean 

 Value of the Square of the Velocity of Translation V 3 Mixture of Gases 

 Relation between V and Temperature Energy of Translation and Internal 

 Energy Joule's Approximate Method of Calculating the Velocity of Mean 

 Square Effusion or Transpiration through a small Orifice into a Vacuum 

 Thermal Transpiration The Mean Free Path The M.F.P. calculated from the 

 Coefficient of Viscosity Conduction of Heat in Gases The Diameter of the 

 Molecules and the number of Molecules per Cubic Centimetre Forces acting on 

 unequally heated Surfaces in High Vacua The Gas Equation of Van derWaals 



THE belief in the identity of energy throughout its apparent transforma- 

 tions naturally leads us to attempt to explain these various manifestations 

 as really identical in form, though affecting our senses differently. As 

 examples of hypotheses of this kind, we may instance the electro- 

 magnetic theory of light, in which light (or rather radiant energy) is 

 supposed to consist of a mixture of electric and magnetic energies ; and 

 the electrical theory of chemical energy, which identifies chemical action 

 with electric energy. But we obtain our primary idea of energy from 

 the power of doing work possessed by a moving body, and we are able to 

 study most thoroughly the energy-transactions in a system which we need 

 only consider mechanically i.e. with regard to its kinetic and potential 

 energies. Our aim, then, must be to frame hypotheses which shall 

 reduce other energies to these forms, for such hypotheses can be most 

 completely worked out, and their consequences most thoroughly compared 

 with the facts of observation. 



Atomic Hypotheses. The basis of these attempts must almost 

 necessarily be the hypothesis of an atomic or molecular constitution of 

 matter. This hypothesis we owe to the Greek philosophers, who 

 possibly arrived at it from considerations of the compressibility, the 

 disintegration, and the diffusion of matter. If matter is composed of 

 small particles separated by interspaces, it is easy to explain compression 

 as an approach of these particles, solution as an entry of the particles 

 of one substance into the spaces separating the particles of the other, 

 cleavage as the forcing apart of the particles by the insertion of a wedge 

 of some other substance, diffusion as the scattering of the particles, and 

 so on. If we suppose that matter is continuous, then we must accept 

 such facts as compression and solution as simple facts i.e. facts incapable 

 as yet of explanation. 



Considered only as explaining the general and most obvious properties 

 of matter, the atomic hypothesis is merely qualitative. It became 

 quantitative when used by Dalton and other chemists to explain the 

 laws of chemical combination, and the simplicity of this explanation has 



