C. Barus — Distribution of Nuclei. 183 



quence of the impact of X-rays on those parts, associated with 

 secondary radiation. There is much electric evidence against 

 such an explanation ; nevertheless it is worth a brief examina- 

 tion. 



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 nucleat- 

 ing power of the X-rays is to some extent referable to the 

 increasing obliquity of the rays. 



The observed distortion of coronas is clearly due to a grada- 

 tion of nuclei, either as to size, or number, or both. If effi- 

 cient 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 appar- 

 ently 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 the largest nuclei must be looked for nearest the top 

 and bottom, while the gradation in size decreases regularly 

 towards the axis. The large nuclei, therefore, may be suffi- 

 ciently numerous near the walls to capture all the available 

 moisture on condensation, leaving the small nuclei without a 

 load of water and unable to 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 on second exhaustion 

 even after the first coronas have quite dropped out, etc. 



Finally one may note that secondary radiation issuing from 

 the top and the bottom of the condensation chamber would 

 accentuate the present effect. 



Thus it seems not unreasonable to infer that nuclei are pro- 

 duced by the impinging X-rays in much the same way in 

 which they are produced by high temperature (ignition), or by 

 high potential ; and the question arises whether the nuclei 

 thus easily set free may not be associated with the electrons to 

 which the cohesions between the molecules may be ascribed. 



11. Absorption of the ions at the walls of the receiver. — If 

 the nuclei due to the ionization of air by the X-rays are 

 absorbed at the walls of the receiver* a diffusion gradient will 

 be established, resulting in a decreasing number of nuclei 

 from the axle outward, a distribution the reverse of the pre- 

 ceding. The observed distortion will, therefore, here be due 

 to a gradation in the number of nuclei. 



* A number of similar cases have been worked out in Smithsonian Contri- 

 butions, Xo. 1809, 1901, " Experiments with ionized air ;" and ibid., No. 

 1373, chapter v, 1903. 



