EFFICIENCY OF FOG CHAMBER. 27 



20. The same, continued. Action of radium. The action of radium 

 is characterized by a slight rise of the asymptote over the preceding 

 case and of displacement of the curve into smaller supersaturations. 

 This was to be anticipated; nuclei are caught more easily, and a some- 

 what smaller order of size and increased number is detected in conse- 

 quence. The other distinctive feature is the smaller range of pressure 

 differences within which practically all condensation occurs. In the 

 preceding experiments this range lay between dp = 18 and 24, whereas in 

 the present case the marked changes fall between dp = ig and 2 1 . There 

 results another apparent rise of the fog limit corresponding to the coronal 

 effects, though it is slight and uncertain. What is noteworthy is the 

 greater steepness of the rising branch of the curve, already indicated. 

 The question may be asked whether for a rigorously instantaneous 

 exhaustion this part of the curve would become even more nearly 

 vertical. One may note that the radium curve eventually passes 

 through a maximum. Furthermore, while in its initial stages it lies 

 very near the former X-ray curve (D = $o), the latter shows no tendency 

 to reach a near asymptote. 



21. The same. Action of X=rays. Finally, the results with the 

 X-rays are similar. High asymptotes, a tendency of the curve to lie 

 within a region of lower supersaturations than in table 7, etc., are 

 apparent. Nuclei smaller in size and larger in number have been caught 

 more easily than heretofore. If we regard the present and the earlier 

 curves for D = $o, the height of asymptote in the former case is much 

 above the one in the latter case, and moreover passes through the 

 maximum (reached at dp = 26) as dp increases indefinitely. This cor- 

 responds to the result for radium. 



It is therefore interesting that if the radiation is further intensified, 

 as in the last case, where D = 1 2 cm. , the curve continues to rise apparently 

 throughout higher ranges of pressure. The limits are not reached (dp 

 continually increasing) until nuclei of a size equal in smallness to the 

 order holding for the colloidal nuclei of air have become available for 

 condensation. A final point deserving comment is the break of the 

 X-ray curve near dp = 22. This is not only marked in both curves of 

 table 10, but similarly apparent in table 7. Unfortunately, the cycle of 

 coronas changes at this point (green-blue-purple corona) ; but apart 

 from this it seems probable that with intense X-ray radiation, two suc- 

 cessive groups of nuclei are in question ; the first group prominent below 

 dp = 22 and the second above dp = 22. 



22. Remarks on the w=curves. Non=energized air. The w-curves, 

 which assume that the nuclei are replaced sooner than they can be 



