(54 
mr. j. c. Mclennan on electrical conductivity in gases 
measured, was about 2 centims. From Lenard’s conclusions, it is obvious that in this 
distance the absorption of the rays by carbon dioxide would be greater than by air, 
and very much greater than by hydrogen. The effective intensities of the rays in the 
three gases at the same pressure would then be very different, and numbers such as 
those given in Column I. follow naturally under these circumstances, without assuming 
any difference in the character of the two ionizations. 
9. Ionization in Air at Different Pressures. 
In order to study more closely the influence of absorption, a number of experiments 
were carried out similar to that just described. The same apparatus was used, and the 
same method followed, but the ionizations, instead of being measured in different 
gases at the same pressure, were determined for the same gas at different pressures. 
Table II. 
Pressure. 
Ionization measured. 
millims. 
767 
1-00 
530 
1-44 
340 
1-92 
205 
‘2-32 
104 
2-68 
53 
2-74 
Between 40 and 45 millims. a sudden large 
increase was obtained in the ionization. 
This was found to be due to the action 
of the field itself in 
dissociating the gas. 
The results obtained with air are shown in Table II. The pressures are expressed 
in heights of columns of mercury at the same temperature. The ionizations given 
are relative, that corresponding to atmospheric pressure being taken as unity, and 
each value is the average of a large number of readings. 
The results are also shown graphically in fig. 6, where the abscissae represent 
pressures, and the ordinates corresponding relative ionizations. 
i a 
The numbers show that as the pressure decreased the ionization obtained witl 
saturating electromotive force steadily increased, until a pressure of about 75 millims. 
of mercury was reached. This result, though surprising, can be readily explained 
by the great absorption of the rays at atmospheric pressure. 
The rays had to travel at least 1'5 centims. from the window before they reached 
that part of the chamber from which the saturation current was obtained. For this 
