1 6 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



15. Conclusion. Generally, when the bulb is close to the end 

 of the chamber, the coronas obtained after a short exposure are all 

 roundish, but taper from a large size near the bulb to the vanishing 

 diameter or apex, figure i, near the middle of the fog chamber, with 

 all intermediate gradations of aperture in corresponding intermediate 

 positions. The pressure difference, 8p, applied is thus more and more 

 in excess of the fog limit as the line of sight is nearer the bulb. 

 Beyond the apex the pressure difference used is below the fog limit. 

 Smaller nuclei occur throughout the chamber, but they are probably 

 more and more fleeting in character. The colloidal nuclei of dust-free 

 air are always present. The number of nuclei within the given range 

 of condensation, i. e. , above a certain lower limit of diameter, increases 

 with the intensity of the ionization, axially as well as transversely. 

 If the exposure is prolonged and the radiation sufficiently intense, 

 the nuclei are everywhere within the given pressure difference, but 

 the axial excess of efficient nuclei is retained. Beyond this, the 

 endeavor to come to a decision as to the origin of the persistent nuclei 

 on the basis of the above experiments seems as yet premature ; for in 

 addition to the hypothesis which refers them to the impact of X-rays 

 on the walls of the vessel, the diffusion hypothesis, the effect of sec- 

 ondary radiation within the vessel (such radiation outside of the vessel 

 produces fleeting nuclei only as will be detailed in Chapter III), etc., 

 there is something to be said in favor of the spontaneous production 

 of water nuclei in the presence of the intense X-radiation arriving at 

 any point from both primary and secondary sources. 



