338 Prof. Brao-o- and Mr. Kleeman 



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on 



Now the energy which is spent by the a particle is certainly 

 spent, in part, in producing ions. If the rate of the expen- 

 diture of the energy as a whole is related to the atomic weight 

 by so simple a rule as that of the square root, it is probable 

 that the part which is spent on ionization obeys the same 

 rule. For if not, and if the expenditure on ionization follows 

 some other law, then the remaining expenditure must follow 

 such a law that the two together compound into the simple 

 rule of the square root. It is proper to put aside this more 

 complicated hypothesis until we have considered the simpler 

 one. 



We may therefore advance the law one stage and state it 

 thus : — The energy spent on ionization by an a particle in its 

 passage through an atom is proportional to the square root 

 of the atomic weight. The question now arises : Is the 

 variation from atom to atom due to a difference in the number 

 of ions produced, or in the energy required to produce an 

 ion, or in both these quantities? Because an oxygen atom 

 absorbs four times as much energy as a hydrogen atom, are 

 there four times as many ions produced from an oxygen 

 atom, but the energy per ion the same ; or is the act of 

 ionization four times as difficult, but the number of ions the 

 same ; or are neither of these suppositions true, and is 

 the difference between oxygen and hydrogen in this respect 

 due to a variation in each of the two quantities ? 



Now the total conductivity imparted to a gas by the passage 

 of a particle through it is very nearly the same for the simpler 

 gases. This is true even though some of them differ widely 

 from each other in their atomic weights, e. g. oxygen and 

 hydrogen. From this it appears probable that the energy 

 required to produce an ion is always the same. It is true 

 that in some of the complex gases the total conductivity is 

 less than in the simpler gases, but this can be explained on 

 the hypothesis that the same number of ions is made in these 

 heavy molecules, but that they find a difficulty in escaping 

 from the molecule and are recombined, so that they add 

 nothing to the conductivity of the gas. If this hypothesis is 

 not entertained, we are driven to suppose that the energy 

 required to make an ion is not the same in some of the 

 complex molecules, and that the number made varies by just 

 so much as to make the total loss of energy of the particle in 

 each molecule follow the same simple law as holds for the 

 simpler molecules. For the present at least this complicated 

 supposition must also be set aside, and again we may make 

 an advance in the statement of our law. It now stands : — 

 The number of ions made by an a particle in passing 



