PERIODICITY. 



141 



98. Cause Of periodicity. In cases of large and small coronas, 

 whether the persistence be attributed to electrical potential or to solu- 

 tion, a water nucleus is always in question. Small fog particles are 

 caught on small nuclei near the limit of exhaustion and these evapo- 

 rate and become the water nuclei available for the next exhaustion. 

 In general, there are three groups of nuclei (x,y, <?) concerned in any 

 exhaustion : A group, ,r, of water nuclei form the preceding exhaustion; 

 a group, 2, which will evaporate to make the water nuclei of the suc- 

 ceeding exhaustion ; finally, the group y, comprising nuclei adapted 

 to become the efficient nuclei of the exhaustion in question. 



In case of periodicity the successive exhaustions follow the scheme 



Exhaustion i 

 2 

 3 

 4 



yi 



-T3 = ^2 = o j2+3'3 



.v = 23 



xi, = zi = o J4+:V5 



z\ 

 22 

 23 



Superior corona on y\ 

 Inferior corona on z\ 

 Superior corona on j' 2 -f y 3 

 Inferior corona on 2-3 

 Superior corona on 



All the details observed with alternations are thus explained. In 

 view of the rapidity of decay, the corona will be formed on the satura- 

 tion value 



99. Persistence in general. This may reasonably be ascribed to the 

 formation of water nuclei, a point of view carried out in my memoir 

 on the structure of the nucleus (Smithsonian Contributions, No. 1373, 

 vol. 29, 1903), but much enhanced by the data of the present inves- 

 tigation. The heavy rains accompanying condensation in case of the 

 persistent X-ray nuclei are attributable to spontaneous condensation 

 without supersaturation, the nucleus acting under intense X-radiation 

 like a hygroscopic solute. The same result may follow the action of 

 ultraviolet light, as it certainly must result from the presence of phos- 

 phorus and of sulphuric-acid nuclei. 



100. Secondary generation. This is a curious phenomenon, show- 

 ing that the decaying persistent nucleus produced by the X-ra)^s is 

 apparently radio-active, or that the walls of the fog chamber are so, 

 or else that the large nuclei, if left without interference, break into a 

 number (on the average about three) of smaller nuclei, whereby the 

 nucleation is actually increased in the lapse of time after exposure. 

 In other words, if the uucleation is observed without cutting off the 

 radiation in one case, and if in the second case the nucleation iden- 

 tically produced is observed at a stated time after the radiation has 

 ceased, the number in the latter case (anomalously enough) is in 

 excess. (Cf. fig. 70-76, Chapter III.) The following examples make 

 this clear, the X-ray bulb being 5 cm. from the fog chamber, and the 



X 



"C> 



*,* *-v ''V^ 



