136 HUXLEY MEMORIAL 'LECTURES 



the ionisation so produced. But in point of fact 

 the ionisation due to an alpha ray is sui generis. 

 A glance at one of Wilson's photographs (Fig. 2) 

 illustrates this. The white streak of water par- 

 ticles marks the path of the ray. The ions pro- 

 duced are evidently closely crowded along the 

 track of the ray. They have been called into 

 existence in a very minute instant of time. Now 

 we know that ions of opposite sign if left to them- 

 selves recombine. The rate of recombination 

 depends upon the product of the number of each 

 sign present in unit volume. Here the numbers 

 are very great and the volume very small. The 

 ionic density is therefore high, and recombination 

 very rapidly removes the ions after they are 

 formed. We see here a peculiarity of the ionisa- 

 tion effected by alpha rays. It is linear in dis- 

 tribution and very local. Much of the ionisation 

 in gases is again undone by recombination before 

 diffusion leads to the separation of the ions. This 

 " initial recombination " is greatest towards the 

 end of the path of the ray where the ionisation is 

 a maximum. Here it may be so effective that 

 the form of the curve is completely lost unless a 

 very large electromotive force is used to separate 

 the ions when the ionisation is being investigated. 

 We have now reviewed recent work at suffi- 

 cient length to understand something of the 

 nature of the most important advance ever made 

 in our knowledge of the atom. Let us glance 

 briefly at what we have learned. The radioactive 



