6 The Kinetic Theory conception of Matter [CH. i 



There is one further point to be mentioned. However elastic the billiard- 

 balls and table may be, the motion cannot continue indefinitely. In time 

 the energy of this motion will be frittered away, partly by the vibrations 

 set up at collisions and partly, perhaps, by frictional forces. What, then,/ 

 does this represent in the gas, and how is it that a gas, if constituted as we/ 

 have supposed, does not in a short time lose the translational motion of its 

 molecules, and replace it by a vibrational motion internal to the molecules ? 

 An answer, as will appear later, is supplied by the extreme "elasticity" of the 

 molecules, this elasticity being put in evidence by the high frequency of the 

 light-vibrations emitted by the molecules. This frittering away of the trans- 

 lational motion is in fact continually taking place in a gas, but the process 

 is so slow that, as we shall see, gases may exist for millions of years without 

 their stock of energy being replenished, before the energy of the translationa' 

 motion is appreciably diminished. 



Numerical Values. 



8. The foregoing rough sketch will, it is hoped, have given some idea of 

 the nature of the problems to be attacked. As a conclusion to this pre- 

 liminary chapter, it may be useful to give some approximate numerical values, 

 as an indication of the magnitude of the quantities with which we shall be 

 dealing. 



Number of molecules per cubic centimetre. The number of molecules in 

 a cubic centimetre of gas at normal temperature and pressure is, in 

 accordance with Avogadro's law ( 113 infra}, independent of the chemical 

 composition of the gas. Probably the most accurate values obtained experi- 

 mentally for this number are those obtained by electrical methods which are 

 quite independent of the Kinetic Theory*. Professor J. J. Thomson f finds 

 for this number, referred to normal temperature and pressure, the value 

 3'6 x 10 19 , Mr H. A. WilsonJ finds the value 4'0 x 10 19 , whilst from some 

 earlier experiments of Professor Townsend it is possible to deduce the value 

 4*1 x 10 19 . We may, therefore, conclude that the value of this number, 

 certainly as regards order of magnitude, and possibly to within an error of a 

 few per cent., is equal to about 4 x 10 I9 ||. Since the mass of the molecule 

 must be supposed to remain constant throughout all changes of density,") \ 

 temperature and volume of the gas, the number of molecules per cubic f 

 centimetre will obviously be directly proportional to the density of the gas. J 



* A review of these methods is given by Prof. J. S. Townsend, Phil. Mag. vn. p. 278. j 



t Phil. Mag. v. p. 335. J Phil. Mag. v. p. 441. ' 



Proc. Comb. Phil. Soc. ix. Part 5. 



II A discussion of earlier attempts to estimate this number is given by Lord Kelvin, Baltimore 

 Lectures, Lecture xvn., or Phil. Mag. iv. pp. 177 and 281. 





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