Vol. 6, 1920 
PHYSICS: b. BLACKWOOD 
255 
The experimental curves which have been determined by the author 
for ionization from spray, and also for that from hot wires, are all of the 
type shown in fig. III. The fact that no curve has a plurality of peaks 
proves that — contrary to the findings of other workers^ — a series of widely 
separated, well-defined mobility groups does not exist. 
Decrease of Mobility with Time. — It has been found that if the ions are 
tested soon after their production the mean mobility is relatively high. 
If, however, metal tubes are interposed between the sprayer and the test- 
ing tube, so as to increase the period between the production of the ions 
and their testing, the mean mobility decreases and higher voltages are 
necessary to produce maximum ion currents. (Fig. Ill, {A) and {B).) 
The author, using a red hot platinum wire as a source of ionization, finds 
that the rate of decrease of mobility can be retarded by drying the air in 
which the ions are produced. (Compare curves {B) and (C), fig. III.) 
This indicates that the decrease of mobility is due, in part at least, to the 
growth of the ions by condensation of water vapor. 
Nature of the Large Ion. — It has now been shown that — contrary to the 
findings of other observers^ — a series of widely separated, well-defined 
mobility groups does not exist in the ionization from spray. Their hypoth- 
esis that the large ion is a complex cluster of stable water globules may 
consequently be abandoned in favor of the simpler conception of J. J. 
Thomson and others who have pictured the large ion as merely a charged 
