94 VAPOR NUCLEI AND IONS. 



vanished to a few hundred, the upper curve is only half way on its march 

 toward the asymptote. This shows the remarkable sensitiveness of the 

 method as a test for the presence of ions or of any nuclei larger than the 

 colloidal sizes. Moreover, measurement of the large coronas is relatively 

 easy. Finally, the curve dp = 31, if prolonged backwards, would seem to 

 start nearly from the origin ; in such a case one would have to picture to 

 oneself a single particle breaking to pieces, in the absence of radiation, 

 into fragments of continually decreasing size, until the debris ultimately 

 numbers 150,000 colloidal nuclei. 



The intermediate curve (dp = 28) also coalesces* approximately with 

 the other curves for lapses of time less than / = 1 3 seconds. It has its 

 own minimum, however, and from the lower pressure difference, neces- 

 sarily its own asymptote at n = 40,000, since only the coarser order 

 of air nuclei fall within the given limits of condensation in the apparatus 

 used. For the same reason the minimum is lower and later, seeing that 

 the ions are present throughout in relatively greater numbers as com- 

 pared with the efficient colloidal nuclei, than was the case at dp = 31. 



The curves as a whole have so close a resemblance to the data inves- 

 tigated in section 61 for the effect of radium at different distances 

 from the fog chamber that the same cause must underlie both series of 

 observations. In the former case (distance effects) any given intensity 

 of ionization between the maximum and the vanishing values may be 

 maintained indefinitely by properly placing the radium tube; in the 

 latter case (decay) all stages are passed through in 2 or 3 minutes. 

 Beginning with dust-free non-energized air, the number of efficient 

 nuclei decreases as the number of ions increases (for either or possibly 

 both of the reasons already given) until the condensation takes place 

 wholly on ions. For greater intensities of ionization the number of 

 ions must increase further, and hence the efficient nucleation rises again 

 while the curve passes through a minimum. 



I , The curves enable us to make certain interesting comparisons, inas- 

 much as the same nucleation results from radium decaying for a stated 

 length of time, as results from the action of radium at a certain distance 

 from the line of sight. From the importance of secondary radiation 

 in connection with these observations, such comparisons are probably 

 not simple. The essential feature is the passage of the nucleation 

 through the same stages of variation, whether of size or of number, in 

 both cases no matter how the given successive intensities of ionization 

 may be produced, or whether they come from within or without. 



* Considered relatively to the wide divergence after ^ = 13 sec. is passed. The coales- 

 cence need not be perfect . Small coronas fall out too rapidly for close measurement . 



