28 THE THEORY OF IONIZATION OF GASES 



exceeds a certain critical value. On this account the 

 determinations of a/p for values of X/p larger than those 

 which are given in the above tables have not been 

 obtained with a high degree of accuracy. 



The tables show that in all cases the theory is in 

 accordance with the experimental results over a wide 

 range when the forces are large. For the smaller forces 

 the values of a are larger than those given by the 



NV 



expression Ne~ x. There are many causes which may 

 contribute to this difference, but it is impossible to give 

 any reliable explanation of it as so little is known as to 

 what exactly takes place when an ion collides with a 

 molecule. 



It has been assumed that one effect of a collision is to 

 reduce the velocity of an ion to a relatively small value, 

 an assumption which seems reasonable when the velocities 

 are so large that the ions tend to produce changes in 

 the molecules by impact. In th'ese cases a large 

 proportion of the kinetic energy of the ion would be 

 absorbed by the molecule, so that after a collision the 

 ion would begin to move under the electric force with 

 a small velocity. But when the velocities on impact 

 are much below that which is required to ionize a 

 molecule, there is no reason for supposing that a large 

 proportion of the kinetic energy of the ion is lost on 

 collision ; it may thus acquire the critical velocity more 

 frequently than is shown by the calculations for the 

 smaller forces. It is also probable that there are other 

 circumstances besides the velocity of the negative ion 

 which determine what takes place on collision, and if so, 

 that ions may be produced on some few occasions when 

 the negative ion collides with a velocity less than that 



