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PHYSICS: K.-L. YEN 
Here too an abundance of free negative electrons was found, although the 
amount was not so great as that found in hydrogen under the same pressure. 
Conclusion and Discussion. — As the results of the present experiment ex- 
hibit no deviation from the law Up = constant it follows that both the posi- 
tive and the negative ions did not disintegrate at the potentials employed. 
It may be seen from the tables that the values of X/p were very close to the 
values at which sparking would occur in the respective gases. And since the 
cluster hypothesis demands the disintegration of the ions when the values of 
X/p are much lower than those employed, 15 it therefore follows that these 
results are directly contradictory to this hypothesis. 
On the other hand, the results are in perfect accord with the small-ion 
hypothesis. Taking this in conjunction with the results of other experi- 
ments, especially those of Wellisch and Loeb, there does not seem any question 
at all regarding the validity of this hypothesis. 
There remains, however, an experimental fact which the cluster hypothesis 
seems to be able to explain better than the small-ion hypothesis, and that is 
the difference between the mobilities of the positive and the negative ions. 
For, if both the positive and the negative ions are single molecules carrying 
elementary charges different only in signs, why should they have different 
mobilities? Whereas if the ions are clusters the difference in their mobilities 
may be ascribed to the difference between the numbers of molecules constitut- 
ing the two kinds of ions. 
This difficulty of the small-ion theory, however, is more apparent than real 
in view of the recent theories as to the electronic constitution of matter. If 
an atom consists of a positive nucleus surrounded by a satellite of negative 
electrons held together by the attractive force from the nucleus, the phe- 
nomenon of ordinary molecular collision must be attributed to the repulsion 
between the two systems of negative electrons in the colliding molecules 16,17 . 
Since, according to the small-ion theory, the only difference between the un- 
charged molecules and the ions lies in the number of negative electrons in the 
satellites — the negative ion having one more electron, and the positive ion one 
less than the uncharged molecules — it follows that the only difference be- 
tween the ordinary molecular collisions and the collisions between ions and 
uncharged molecules is that between the numbers of electrons in the colliding 
systems. It is only reasonable, therefore, to extend the conception . of the 
ordinary molecular collision to cover the case of collision between ions and 
uncharged molecules. Now since the negative ion has more of these peripheral 
negative electrons than the positive ion it follows that the repulsion between 
the negative ion and the uncharged molecule is greater than that between the 
positive ion and the uncharged molecule; and the attractive force between 
the negative ion and the uncharged molecule is smaller than that between 
the positive ion and the uncharged molecule. This results in a difference in 
the effective mean free paths of the two kinds jf ions. The positive ion, by 
virtue of the greater attractive force existing between it and the uncharged 
