416 
DE. HAROLD A. WILSON ON THE 
results oljtained are in good agreement with those described in my paper, and show 
with greater detail the changes in the potential fall due to changes in the tem¬ 
perature of the electrodes. Cooling the positive electrode diminishes the fall of 
potential close to the negative electrode. This effect is easily explained, as Dr. Maex 
points out, on the assumption that the ionization occurs mainly on the surface of the 
liot platinum, and that cooling the platinum diminishes the amount of this ionization. 
The negative drop is due to the presence of positive ions in the gas near the negative 
electrode, and cooling the positive electrode diminishes the supply of positive ions, 
and consecpiently also the negative drop. In my paper, referred to above, I described 
this effect and explained it in this way. Dr. Marx has also used his results on the 
fall of potential to make an estimate of the velocities of the ions in the flame. The 
calculation is based on the investigation given by Professor J. J. Thomson* of the 
distrilmtion of potential between the electrodes when all the ionization is uniformly 
distributed throughout the volume of the gas, as in the case of the conductivity of 
air due to Ptontgen rays when aluminium electrodes are used. 
In the tlieory given l)y Professor Thomson the space between the electrodes, when 
the current is far from its saturation value, is considered to be divided into three 
parts by planes parallel to the electrodes. In the middle region the intensity (Xq) is 
constant, so that the numbers of positive and negative ions present are equal, and the 
ionization is equal to the recombination. The value of the current density is given 
Ijy L = (Tj -h To), where n is the number of positive or negative ions present in 
unit volume, e the charge carried by an ion, and To the velocities of the positive 
and negative ions respectively due to unit intensity. 
In the regions near the electrodes, wliose thicknesses will he denoted by and Xo 
no recombination is supposed to take place, so that if the middle region is taken away 
the distance between the electrodes becomes X^ -j- X,, and then the current density 
has the saturation value for this distance, viz., qc (X^ Xo), where q is the number 
of ions produced in unit time in unit volume of the gas. 
Dr. Marx’s determination of tlie velocities of the ions in the flame depends 
therefore on the assumption that the ionization occurs uniformly throughout the 
space l)etween the electrodes which, according to my experiments [loc. cit.), is not 
even approximately the case in flames containing salt vapours. In his first paper 
Dr. Marx agreed vuth the conclusion that nearly all the ionization occurs on the 
electrodes, and brought forward fresh evidence in favour of its truth, and at the same 
time applied Professor Thomson’s theory to calculate the velocities of the ions. 
In his paper in WfEDEMANNS ‘ Annalen,’ Dr. Marx again describes the same 
experiments, Imt comes to the conclusion that at any rate when the E.M.F. is small, 
the surfiice ionization is not large compared witli tlie volume ionization, so that when 
“Oil tlie Theory of the Conduction of Electricity throug'h Gases by Charged Ions,” ‘Phil Mag., 
March, 1899. 
