DISTRIBUTIONS OF NUCLEI. 47 



they are the same in any apparatus, within the limits of its efficiency. 

 In fact, even the curve which I have endeavored to reduce to the same 

 scale from Wilson's disk colors for non-energized dust-free air (apart 

 from differences in the meaning of dp to be explained in the next sec- 

 tion) lies in the same region of pressure difference, and shows a slope 

 quite in keeping with the other curves.* For an imperfect apparatus, 

 on the other hand, the slope terminates abruptly at a lower asymptote 

 and in a region of higher exhaustion. It will be convenient to refer 

 to the nuclei belonging to the slopes specified as representative nuclei. 



To obtain the curve for the colloidal nuclei of dust-free air, it is often 

 necessary to wait a week or more, until unknown internal sources of 

 nucleation have spent themselves. Moreover, the isolated observations 

 made with advantage ought to be made hours apart, for it is with the 

 exhaustion that the release of spurious internal nuclei occurs, suggesting 

 that water nuclei due to the evaporation of small fog particles are possibly 

 in question. 



The air curve passes through the first two cycles of large coronas, 

 terminating beyond the highest green-blue-purple, actually in the 

 stone-blue or higher coronas, the first of the series producible in the 

 fog chamber by any means whatever. It corresponds with the opaque 

 of the steam jet, beyond which, however, there exists another cycle 

 of yellows, to my knowledge beyond the reach of the fog chamber. It 

 is not probable that the medium is as yet quite optically inactive after 

 the bluish coronas specified, but merely appears so in small thicknesses. 



If we turn from the curve of non-energized air to the case of weak 

 ionization such as is produced by 10 mg. of impure radium (i 0,000 X) 

 or by the X-rays from a distance of 6 meters, we may note in the first 

 place that the coronal fog limit has enormously decreased as compared 

 with its position in dust-free non-energized air. The nuclei are therefore 

 throughout correspondingly larger in size. The nucleation soon rises 

 with an approach to the characteristic slope referred to, showing that 

 the representative ions lie within a relatively narrow range of sizes, the 

 curve of distribution terminating in a maximum. With increasing 

 exhaustion the nucleation first decreases to a minimum; hence to all 

 appearances the ions are now partially destroyed or else materially 

 reduced in size at low pressure, so as to fail of capture even in the higher 

 exhaustions. Beyond the minimum the final and most interesting stage 

 of variation characteristic of weak ionization may be observed. The 

 nucleation rapidly rises again, eventually to approach the asymptote of 

 dust-free air. In explanation of this phenomenon, we may agree that 



*The coincidence between my interpretation of Wilson's disk colors and my own data 

 holds only when the drop of pressure is observed at the fog chamber. .See section 48. 



