148 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



of the vessel, recalling the way in which similar nuclei are produced by 

 ignition and by high electrical potential, etc. Or one may state that 

 the secondary X-radiation, which plays near the walls of the vessel, is 

 particularly intense near those walls, so that the growth of nuclei in 

 the field of ionized air adjoining is most rapid near those parts. In 

 any case the number of efficient nuclei near the horizontal plane of 

 symmetry is apparently large, because these nuclei are nearly of a size 

 and all are therefore available for condensation. The number of 

 efficient nuclei near the walls is smaller because large and small nuclei 

 are here intermixed, and the former capture nearly all the moisture in 

 those parts. The actual nucleation here must, however, be exceed- 

 ingly large, and it is because of the relatively great density of the 

 nucleation in question that rapid and pronounced growth of nuclei 

 become possible. 



Thus there must be many nuclei which fail of capture in the first 

 exhaustion, and for this reason, finally, the coronas on second (other- 

 wise identical) exhaustion, without fresh nucleation or exposure to 

 the X-rays, are invariably phenomenally large and may correspond to 

 one-third or one-half as many nuclei per cubic centimeter as the first 

 coronas. 



108. Order of size of persistent X-ray nuclei. This may be expressed 

 in terms of the pressure difference needed to produce condensation. 

 Unfortunately the coronas on first exhaustion are apt to be distorted 

 or dense fogs, while as the pressure difference, 8p, decreases they 

 become more and more diffuse and equally unsuitable for measure- 

 ment. I have, therefore, computed the .number of particles, N z , 

 present on second exhaustion for a given nucleation . These coronas 

 are smaller, but sharp, and the fog particles are condensed on the water 

 nuclei resulting from the first exhaustion. One may estimate roughly 

 that about 10 per cent of the original number of nuclei are condensed 

 in this way. The following is an example of results for 3-minute 

 intervals of exposure to the X-rays (curves 38, 39, Chapter II) : 



S P = 33 25 17 9 4 17 



" 15 46 47 28 14 46 



The passage through a maximum at 8p = 16-20 is capable of a variety 

 of explanations, and therefore of little interest. The important point 

 at issue is the fact that these nuclei require almost no supersaturation 

 for condensation. Filmy coronas are produced by vanishing pressure 

 differences. It follows, then, that these nuclei are about of the size 

 of ordinary dust-like nuclei. 



