0. Bancs — Ions and Nuclei in Dust-free Air. 449 



is usually much larger than n. In the cases of persistent 

 nucleation due to the X-rays or other causes, _ZV" is obviously to 

 be taken ; but here from the low values of Bp which suffice for 

 condensation, the difference is not so important. 



2. To vary the intensity of radiation, the anti-cathode of the 

 X-ray tube or the radium tube (of thin aluminum, hermetically 

 sealed, holding 10 mg. of weak radium — 10,000 X — within), 

 is placed at a distance, D, from the outside of the fog-chamber. 

 This was a horizontal cylinder of glass, 50 cm long and 15 cna in 

 diameter, with the end toward the bulb l cm thick and the side 

 wall 3 C[n thick. When D is measured from the end, persistent 

 nucleation is not usually producible* because of the thickness 

 of the glass to be penetrated. When D is measured from the 

 sides, however, persistent nucleation just begins at D — 50 cm 

 and increases at a rapidly accelerated rate for smaller distances. 

 Hence the ionization corresponding to D — 50 cm is a transi- 

 tional value at which fleeting nuclei or ions merge into persis- 

 tent nuclei. 



3. If the data are constructed graphically, it appears that all 

 the curves are eventually intersected by the curve for dust- 

 free non-energized air. In the latter we may recognize a 

 region of ions (say from Sp = 21 cm to 27 cm ) and a region of 

 colloidal nuclei for larger values of Sp-, but the whole phe- 

 nomenon is continuous. The effect of radiation as seen in the 

 other curves is therefore to decrease the efficient nucleation of 



Table II. — Persistent (large) nuclei (ZV, number per cm 3 ) produced by intense 

 X-radiation, in dust-free air. dp = 18 cm , being, decidedly below the fog- 

 limit. D measured from side of glass fog chamber (wall '3 cin thick) to 

 anticathode. Aluminum screen inserted. 



D = 



12 



20 



30 



40 



50 cm 



: io~ 3 = 



140 



56 



10 



1 



1 



dust-free air more noticeably when the radiation is weaker and 

 the supersaturation higher. These results may be tried directly 

 for instance, with the radium tube at different distances, D; for 

 a fixed pressure difference, 8p. Thus at Sp — 41 cm the nuclea- 

 tion passes through a minimum at D = 25 cm when D 

 increases from to 50 cm . The effect of radiation is then virtu- 

 ally an aggregation of the colloidal nuclei of dust-free air. If 

 the effect of ionization were merely to mask the presence 

 of the smaller colloidal nuclei, the same effect should occur at 

 intense ionization. Here, however, nuclei larger as well as 

 indefinitely smaller than the mean ionic gradation are produced 

 like the latter in continually greater numbers as the radia- 

 tion increases. The case is rather one in which relatively 



* I have since succeeded in producing persistent nucleation through thin 

 tin plate. 



