48 VAPOR NUCLEI AND IONS. 



further nuclei are destroyed or rendered relatively inefficient by the 

 low pressure until there is a sufficient excess of supersaturation to actually 

 capture the colloidal nuclei of dust-free air more and more fully in the 

 presence of the ions (few in number and small in size) remaining. Pos- 

 sibly the relative number of ions and colloidal nuclei is itself a suffi- 

 cient reason; but it since seems clear that an apparatus which ceases 

 to produce condensation on colloidal nuclei much below 0^ = 35 cm. 

 can not be expected to regain efficiency between ^ = 35 and dp = 45 

 cm., unless there is destruction of whatever has been holding down the 

 effective nucleation to low numbers. As the ions decrease either in size 

 or number, there is a reappearance of the normal colloidal nucleation 

 of dust-free air. Its asymptote is less and less depressed. 



Under moderate ionization, such as is obtained from the X-ray bulb 

 at a distance of 1 to 2 meters, the fog limit is definitely reduced, showing 

 the presence of an order of larger nuclei; the asymptotes are correspond- 

 ingly higher and they are reached later {i.e., at higher exhaustions), 

 indicating the presence of smaller nuclei than in the preceding cases, 

 as well as larger nuclei, unless the increased presence of the latter retards 

 condensation on the former. The range of sizes within which the repre- 

 sentative nucleations lie is definitely extended in both directions, but 

 particularly on the sides of the smaller nuclei. The maximum and the 

 minimum are flattened, though destruction of ions still occurs at low 

 pressure. 



The new feature of these curves is their failure to rise from the mini- 

 mum toward the asymptote of dust-free air. It follows that the ions 

 are now present in too large a number, even relatively to their reduced 

 sizes at high exhaustion (if this obtains), to leave an excess of super- 

 saturation sufficient to catch the colloidal air nuclei; or, from the other 

 point of view, the kinetic ionization pressure is too strong to admit of 

 sufficient rupture of the nuclei even at the lowest pressures applied. 

 (Section 43.) 



In the comparison of both series of experiments, the occurrence of 

 the intersection of the curve is noteworthy. In other words, the nuclea- 

 tion, apparently produced at high exhaustion by an X-ray bulb at 6 

 meters from the fog chamber, may be much larger than the nucleation 

 produced from 200, 100, or even 50 cm., due (as we have supposed) to 

 the reappearing efficiency of the colloidal nuclei in the former case. 

 Similar complications surround the distance effects at other pressures; 

 but under conditions of sufficiently moderate exhaustion, i. e., below 

 the fog limit of filtered air, the distance effects between 50 and 600 cm. 

 remains disproportionately small. 



When the ionization is relatively intense, as when the anticathode is 

 from 10 to 60 cm. from the thinner side of the glass fog chamber, the 



