April 14, 1905.] 



SCIENCE. 



563 



(the radiation having been cut off imme- 

 diately after the first exhaustion) for one 

 or more minutes or longer, a second ex- 

 haustion to the stated limits will show a 

 large (secondary) corona relatively to the 

 primary corona. In other words, relatively 

 many of the fleeting nuclei or ions caught 

 in the first fog have persisted, whereas 

 without condensation, they would have van- 

 ished at once after the radiation was cut 

 off. The following is an example of data 

 bearing on this point, t denoting the time 

 elapsed from the evaporation of the first 

 corona to the precipitation of the second, 

 iVj the number of nuclei in the first and 

 No the number in the second corona. 



t= RO 120 ;M0 seconds. 



A\ X 10-= = 53 27 5:i 



iVj X 10-= = 16 7 15 



The experiments are complicated by the 

 variable X-ray bulb ; but it is obvious that 

 while all the nuclei would have vanished 

 in a few seconds without condensation, 

 about one fourth (in other experiments 

 more) persist indefinitely if reevaporated 

 after condensation from fog-particles. 



This result has an important bearing on 

 the whole phenomenon of condensation and 

 nuclei. Clearly the latter, after the evap- 

 oration specified, become solutional or water 

 nuclei, in which the original fleeting nu- 

 cleus or ion behaves as a solute. The de- 

 creased vapor pressure due to solution 

 eventually compensates the increased vapor 

 pressure due to curvature, after which at a 

 definite radius, evaporation ceases and a 

 water nucleus results. Such a nucleus, 

 however small, must be large in comparison 

 with the dissolved ion. Hence on conden- 

 sation the water nuclei will capture the 

 moisture soonest and grow largest. Now 

 in any exhaustion about one eighth of the 

 fog particles, i. e., those which are smallest 

 and whose nuclei have been caught at the 

 end of the exhaustion, regularly evaporate 

 into the larger particles to a residue of 



water nuclei. These are then the first to 

 be caught in a succeeding exhaustion. This 

 is the explanation of the rain which not 

 only accompanies all coronas in dust-free 

 air, but is often dense. It is also an ex- 

 planation of those indefinite alternations 

 of large and small coronas (periodicity) 

 which I described in detail elsewhere. 



4. Secondary Generation.— This is a 

 curious phenomenon, showing that the de- 

 caying nucleus is apparently radioactive, 

 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 aboiit three) of smaller nuclei, 

 whereby the nucleation is actually in- 

 creased in the lapse of time after exposure. 

 In other words, if the nucleation is ob- 

 served without cutting off the radiation 

 in one ease, and if in the second case the 

 nucleation identically produced is observed 

 at a stated time after the radiation has 

 ceased, the number in the latter case 

 (anomalously enough) is in excess. The 

 following examples make this clear, the X- 

 ray bulb being 5 cm. from the fog-chamber, 

 and the exhaustion carried to Bp = 20 cm. 



Rays on 2 2 2 2 2 2 2 2 2 minutes 



Rays off. 4 4 2 20 " 



NX 10-' 20 52 20 32 25 30 13 34 30 



These data are computed from the second 

 exhaustion, as the first show the densely 

 stratified fogs unavailable for measure- 

 ment. With the bulb at different distances 

 from the fog-chamber, the following data 

 admit of the same interpretation. 



Distance, Z)= 5 10 15 5 10 15 cm. 



Ravs on 2 2 2 2 2 2 minutes. 



Rays off " 



NX 10-= 22 3 1 58 9 1 



The phenomenon vanishes when the radia- 

 tion is too weak to produce persistent nu- 

 clei, therefore either when the bulb loses 

 efficiency or when it is" too far from the fog- 

 chamber. These results recall the corre- 

 sponding behavior evoked by radium in 



