. But on the average the chance of an impact in going a centimeter is the number of impacts actually made in traversing this distance. The mean free path
is the distance traversed divided by the number of impacts made in going that distance. Hence
This would be the correct expression for the mean free path of a molecule which is moving through a group of molecules at rest. If, however, the molecules are all in motion they will sometimes move into a collision which would otherwise be avoided, so that the collisions will be more numerous when the molecules are in motion than when at rest—how much more numerous will depend upon the law of distribution of the speeds of the molecules. It is through a consideration of the Maxwell distribution law that the factor
is introduced into the denominator (see Jeans, Dynamical Theory of Gases) so that equation (54) becomes
APPENDIX F
NUMBER OF FREE POSITIVE ELECTRONS IN THE NUCLEUS OF AN ATOM BY RUTHERFORD’S METHOD
If