6 



complex gases yield the greater number of ions. But the yield 

 does not depend only on the number of atoms in the molecule. 

 Acetylene (C.H^) yields 25% more than air ; yet CO.,, with only 

 one atom less, yields but 5% more ; and ethylene (C2H4) 3^ields 

 the same as acetylene, though it has two atoms more. Of 

 course, in the last case, the atoms added are very light ; and 

 H2 itself has, according to my experiments, a slightly lower 

 value (for RI) than air. Rutherford also found this to be 

 the case. 



On the other hand, the influence of complexity can be 

 illustrated by the cases of acetylene and ethylene, as com- 

 pared to benzene and pentane. 



In order to bring out the significance of these compari- 

 sons, it shouia be pointed out that the a particle spends 

 exactly the same amount of energy in every gas (Bragg, "Phil. 

 Mag.," November, 1905). Thus, in different gases different 

 numbers of ions are produced for the same expenditure of 

 energy. It is quite clear, however, that this does not imply 

 that the a particle finds it easier to produce ions in some 

 gases than others. For if so there would be some influence 

 on the stopping power of atoms dependent on the number of 

 ions produced. But the stopping power is connected to the 

 atomic weight by a simple law, and the number of ions pro- 

 duced is not. Plainly, the energy spent by an a particle in 

 an atom, and the resulting ionisation are not directly con- 

 nected ; there is an intervening link. 



Either the ions made by the a particle produce others 

 in some cases, or some of the ions made never emerge from 

 the atoms. There is something which prevents the simplicity 

 of the law governing the expenditure of energy by the a 

 particle from repeating itself in the amount of ionisation pro- 

 duced. I think it is increasingly clear from our experiments 

 that there is a secondary ionisation within the molecule itself. 

 The ions first made, or possibly X-ray pulses accompanying 

 ionisation, have in some cases enough energy to make fresh 

 ions before leaving the molecule. Thus, for example, one 

 molecule of C^H,., is found to rob the a particle of just as much 

 energy as three molecules of C.^Ho. But more ions are made out 

 of the one C^Hg than out of the group of three acetylene 

 molecules. This may be explained on the grounds that the 12 

 atoms are crowded together, so that an ion projected under 

 ionisation from one of the atoms strikes one of the others with 

 an energy undiminished by motion through the field of the posi- 

 tive from which it was originally separated, and therefore 

 sufficient to make a new ion. In further consequence the ions 

 emerging from a C,,!!,; molecule move more slowly than those 



