Various Gases by the a Particles of Radium. 355 



shown that the stopping-power of a metal depends on the 

 velocity of the a. particle, and certainly some effects observed 

 by them and Kleeman and myself would be explained i£ this 

 were true. On the other hand, this hypothesis is contradicted 

 by Rutherford's direct measurements of the velocity after 

 passing through various sheets of foil. It might be argued 

 that Kucera and Masek measure the relative stopping-powers 

 of metal and air, and that their results really show that the 

 stopping-power of air decreases with increasing velocity of 

 the ot. particle, not that that of the metal increases. But this 

 is inconsistent with their own measurements in the earlier 

 part of the paper, where they show that two equal Al foils 

 cause twice as much drop as one ; and also with the similar 

 and fuller measurements of this effect made by Levin. I 

 may add that in comparing their own results with those of 

 Kleeman and myself, they make a natural mistake in sup- 

 posing that we placed our foils in contact with the Ra. But 

 as a matter of fact they were about 1*5 cm. away, so that 

 the average speed of the particle in going through a metal 

 equal to 3 cm. of air was about the same as the average speed 

 in a gas.] 



One other point invites some consideration. Whilst the 

 saturated ionization curve seems to be the same for all gases, 

 yet the effects of initial recombination vary from gas to gas 

 and from point to point on the curve. This fact can be 

 explained by the consideration that the amount of the ioniza- 

 tion produced is an intramolecular effect, and is therefore 

 independent of the physical conditions of the molecule and 

 of the relations of one molecule to another, whilst the amount, 

 of initial recombination depends on extramolecular relations, 

 on pressure, perhaps on temperature, and so on. The in- 

 crease of initial recombination towards the end of the path 

 ■of the particle may be due in part to the existence of a 

 greater number of molecules that have lost more than one 

 ion, since in such cases recombination would be harder to 

 prevent. This raises the question as to how the ionization 

 is distributed between the molecules which the a particle 

 traverses. There does not appear to be any evidence, as yet, 

 that the chance of an ion being formed from a molecule is 

 dependent on whether the molecule has already lost one or 

 more ions : rather the contrary. If this is the case, occa- 

 sional molecules must lose several ions. ISTor is it yet clear 

 in what mode ionization occurs. Does the a particle simply 

 cause the removal from the molecule of one or more electrons ? 

 May there not possibly be a more complete disruption of the 

 molecule, or even the atom ? There is one curious parallelism 



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