PHYSICS: K.-L. YEN 107 
was offered on the basis of the Rutherford nucleus-atom theory. It was 
shown there that the difference between the two mobilities is due to the 
difference between the numbers of negative electrons in the two kinds of ions. 
There exists a greater attractive force between the positive ions and the un- 
charged molecules than that between the negative ions and the uncharged 
molecules on account of the fact that negative ions have more electrons than 
the positive. This results in a smaller mean free path, and hence a smaller 
mobility, for the positive ions than for the negative. 
From this point of view it is evident that an excess of positive over nega- 
tive mobility is scarcely to be expected in either gases or vapors. Since 
there has been very little work done on vapors and since some careful measure- 
ments of mobilities in them are necessary for the verification of this expla- 
nation in particular and of the small-ion theory in general, it was considered 
desirable to make some careful determinations in vapors, and hence the 
following work was undertaken. 
Method and Procedure. — The method and procedure here employed were 
the same as those employed in the previous experiment. The only difference 
between this experiment and the former is that in the present one only low 
frequency alternating field was employed. This was because the vapors 
worked with required that pressures be sufficiently low for them to remain 
in the vaporized state, and at such pressures the high frequency high poten- 
tial oscillating field proved inapplicable on account of the sparking across the 
gauze and the collecting plate. However, this difference does not at all 
effect the results, as the main purpose of employing the high potential field 
was to find out whether the mobilities would increase abnormally, and — since 
it had already been proved that they did not — the employment of the high 
potential field in the present experiment was entirely unnecessary. 
Another difference between the present and the previous experiment is 
that in this one a different ionization chamber was used. This chamber was 
constructed on precisely the same plan as the former, but covered with a bell 
jar of about one-sixth the size of that covering the former apparatus. This 
last arrangement is more convenient in that it allowed the contents of the 
chamber to be evacuated and refilled with ease, and that the vapors were 
rendered as free of impurities as possible. 
The vapors were produced by the same method as that employed by 
Wellisch. 1 
The measurements were made in the same manner as before, and the results 
calculated from the same formula. 
Results. — The following tables show the results obtained for the vari- 
ous vapors used. It may be seen from these tables that the results for 
all these vapors at the various pressures are perfectly consistent with the 
law that the pres'sure times the mobility is constant, and that the mean 
values for the positive mobilities are — excepting in the case of C2H5I — ■ 
smaller than those for the negative mobilities. In the case of C 2 H 5 I the mean 
