650 Mr. G. W. Walker on 
shown that radium E deposited on a platinum plate is not 
appreciably volatile until a temperature of nearly 1000° C. 
It is intended to examine this point still further in order to 
see if this difference of behaviour is only apparent or real. 
The experiments as a whole are, I think, best explained on 
the view that polonium and radio-tellurium both contain the 
same radioactive constituent, which is the fifth disintegration 
product of radium. The most definite method of settling the 
matter is to compare the rates of decay of the activity of 
the three active substances, and experiments of this character 
are already in progress. 
It would be of scientific value to isolate the product 
radium D from pitchblende, for in many respects it would 
be as useful scientifically as radium itself. Its activity in 
the pure state, measured by the @ rays, would be about 
25 times that of radium, and the rate of change of its activity 
is sufficiently slow to be neglegible in most experiments. 
Experiments are in progress to see whether a simple method 
can be found for separation of radium D from pitchblende*. 
McGill University, Montreal, 
Aug. 25, 1904. 
LXIV. On Saturation Currents in Ionization. By GnHorcE 
W. Warxer, V.A., A.R.C.Se., Fellow of Trinty College, 
Cambridge, Lecturer on Physics in the University of 
Glasgowt. 
FP HE theory and measurement of saturation currents forms 
the basis of much important work on the action of 
ionizing agents. 
The fact that, when a current passes through an ionized 
ges between two plates, a stage is reached when a considerable 
increase of the potential-difference between the plates gives 
practically no increase of current, is explained on the simple 
* Mr. B. Boltwood, of New Haven, Conn., very kindly forwarded me, 
a few days ago, a specimen of radioactive lead, which he had separated 
from pitchblende four months before. This lead gave out an unusually 
large proportion of 8 compared with « rays, and the total amount of 
8 rays from it was about the same as from the uranium present in the 
pitchblende from which it was separated. On dissolving the lead, some 
of the # ray activity was removed on a bismuth plate. I think that it 
is probable that the lead contains the product radium D. These results 
suggest that Hoffmann, whose earlier work on radio-lead was subjected 
to much criticism, was probably right in believing that he had separated 
a new radioactive constituent with the lead, the activity of which did 
not die away with the time. 
+ Communicated by the Author. 
