SOURCES OF NUCLEI. 9 



as the distance, b c, varying with the number of axial nuclei and the 

 distribution constant, a, increases, all the figures, 2, 3, 4, 5, 6, may be 

 logically evolved. 



On the left-end face, moreover, there would be special interference 

 with the distribution of nuclei giving rise to the corresponding dis- 

 tortion seen in the coronas. Further distortion due to the decrease 

 from left to right of the intensity of the radiation must also be 

 apparent, and the gradient of distribution will be slightly altered by 

 diffusion. One may note that if anything issues from the walls of 

 the vessel, it comes as abundantly out of the water below as out of 

 the wet cloth above. 



10. Possible origin of nuclei at walls of receiver. As has already 

 been suggested, the observed gradation of fog particles may result 

 from the (real or virtual) evolution of effective nuclei at the top and 

 the bottom of the apparatus, in consequence of the impact of X-rays 

 on those parts. There is much electric evidence against such an 

 explanation ; nevertheless it is worth a brief examination, particularly 

 as it includes the effect of secondary radiation to be discussed below 

 (Chapter III). 



The enormous coronas which have been obtained with the above 

 (shallow) apparatus, as compared with the small coronas seen in the 

 cases of more bulky apparatus, is in keeping with this view. Again, 

 the rapid decrease of the nucleating power of the X-rays might to 

 some extent be associated with the increasing obliquity of the rays, 

 but no evidence of this was found. 



The observed distortion of coronas is clearly due to a gradation of 

 nuclei, either as to size or number, or both. If efficient nuclei issue 

 from the top and bottom, they must be present in greatest number near 

 those parts of the apparatus, and consequently the largest diameter of 

 coronas should apparently be found there. But if the largest number 

 of effective nuclei is present near the top and bottom, the tendency 

 to growth by cohesion will also be most marked in those regions. 

 Hence, with this admission, the largest nuclei must be looked for 

 nearest the top and bottom, while the gradation in size decreases regu- 

 larly toward the axis. The large nuclei, therefore, may be sufficiently 

 numerous near the walls to capture all the available moisture on con- 

 densation , leaving the small nuclei without a load of water and unable 

 to appreciably descend. Hence the marked rain effect, the rapidity 

 with which the first coronas usually drop out, the turbulent motion 

 which succeeds condensation, the occurrence of large persistent coronas 



