IOWA ACADEMY OP SCIENCE 
159 
source accounts for the marked differences in the velocities of his clouds for 
successive observations. In a series of eleven sets of observations we find that 
the timing of his clouds varies from 12 seconds for a distance of 5 millimeters 
to 33 seconds for a distance of 5 millimeters. In my own work with radium as 
a source of ionization, the timing of the clouds, as will be observed from data 
later, was practically constant for any given difference of pressure. 
Again, as has been stated above, Wilson worked at a dp. of 17 cm., which, 
according to Earns, is right within the region of persistent nuclei. Earns 
observes that these persistent nuclei induced by X-rays are ionized, carrying a 
variable number of ions; one, two or three, perhaps. He notes also that their 
number in the fog chamber varies with the time of exposure and also with 
distance of the X-ray bulb from the fog chamber. Their masses, accordingly, 
vary considerably, which no doubt accounts for the different layers witnessed 
by Wilson in his clouds. A serious objection to Wilson’s work is seen in the 
fact that the number of persistent nuclei vary with the time of exposure. If the 
time of exposure was not the same in all determinations his velocities would 
necessarily vary considerably, as his data shows. X-rays were first used as the 
ionizing source for the determination of e at the Ryerson laboratory, but the 
results were so unsatisfactory that radium was substituted. 
The data for the determination of e given in this paper were obtained under 
conditions which preclude in a measure the criticisms on Wilson and Thomson. 
A one per cent compound of radium was used as the ionizing source. Earus 
shows conclusively in his experiments that the gamma rays of radium are the 
source of nucleation in the fog chamber. These rays are productive of fieeting 
nuclei or ions and not of the persistent type arising from X-rays described 
above. Of course, the persistent type of a dust-like nature would still occur, 
even with radium, where the air is not filtered, as was the case in these ex- 
periments. As was stated above, the X-ray persistent type are cumulative with 
the time of exposure. This cumulative effect, accordingly, does not occur with 
radium and any slight variation in the time of exposure between observations 
vrould not be a source of error. 
All observations were taken at dp. varying from 22 cm. to 24 cm., which 
it will be observed, is well within *the region of maximum nucleation as given 
by Earus for an expansion apparatus that is subject to some resistance, as is 
the case of the Wilson type. During successive observations the radium was 
continuously kept in position very close to the fog chamber. The nucleation 
within the fog chamber was very uniform. This was evident from the fact 
that the coronnas due to the beam of light penetrating the fog chamber were 
of uniform width throughout their extent. The surfaces of the clouds were 
usually sharply defined, permitting good time observations. 
Although the apparatus used was entirely different from that of Earus, 
yet in the course of the work all of the phenomena of nucleation described by 
Earus were experienced and renderd familiar. The velocity of the dust-like 
persistent nuclei were frequently timed at dps. varying from 16 cm. to 18 cm. 
These velocities were usually found to be about five to six seconds for a distance 
of five millimeters, showing that their masses were relatively large. The 
radium, of course, was not in place in timing the persistent nuclei. With 
radium in' place denser clouds made up of smaller particles were obtained at a 
dp. as low as 16 cm. Furthermore, the particles in these clouds were ionized, 
negatively charged, since their falling velocities were markedly increased when 
