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LXVI. On the a Particles of Radium. 



To tlie Editors of the Philosophical Magazine. 



The University of Adelaide, 

 GENTLEMEN", — 20th September, 1905. 



Professor Rutherford has recently announced his dis- 

 covery that the a particle projected from RaC ceases 

 ionizing when its velocity falls below 60 per cent, of its 

 velocity of projection ; when therefore it still possesses a 

 velocity of 1*6 x 10 10 cms. per second. 



In a recent paper by Mr. Kleeman and myself (Phil. Mag. 

 Sept. 1905) we described a number of experimental results 

 bearing on the relation between the velocity of the particle and 

 its ionizing power and on the loss of range in passing through 

 various atoms and molecules. In our discussion of the results 

 we assumed that the velocity of the a particle fell to a very 

 small value when ionization ceased. Professor Rutherford's 

 important discovery therefore necessitates some modification 

 of our arguments ; and I ask your permission to point out 

 briefly what changes must be made. 



.In the first place, we showed that the loss of range of the 

 ol particle in passing through an atom was in all the substances 

 examined proportional to the square root of the weight of the 

 atom. This conclusion requires no modification. But in 

 discussing the theoretical significance of this fact, we had to 

 make some assumption in regard to the amount of energy 

 spent in different gases by the a. particle ; and we supposed 

 that this amount was independent of the nature of the gas 

 traversed. We argued that, if the velocity of the a particle 

 was very small when it ceased ionizing, the actual value of 

 the critical velocity could be of no importance; and the same 

 amount of its energy, viz. the whole of it, was spent by an 

 a particle before it ceased ionizing, no matter what the gas 

 traversed might be. It now appears that only a portion of 

 the energy is so spent. But the point which we wished to 

 make is clearer than before. It is plain that the a particle 

 stops ionizing at exactly the same speed in all gases (so far 

 as examined), and that our supposition was correct, though 

 our argument was not. For we have traced the ionization 

 curve in the case of a mixture of gases, and of gases of complex 

 molecular structure. Now if the « particle were able to 

 ionize in, say, hydrogen at a lower speed than in oxygen or 

 carbon, then in a mixture we should get a complex curve. 



