22 



NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



measurements refer to an initial large corona of say ^ = 5 cm.; but as 

 this can not be measured without destroying the nuclei, the present 

 data merely show the usual occurrence of inferior coronas in the lapse 

 of time (fig. 19). In 15 hours the aperture is reduced to one-third and 

 the nucleation possibly to one-thirtieth. Certain relatively high 

 results at the end of the table seem to be referable to the presence of 

 radium in the laboratory, but no definite statement can be made. 



TABLE 12. Evanescence of nucleation of dust-free air in lapse of time. 



* Volume ratio of fog and vacuum chambers. f Vacuum chamber disconnected. 



The marked occurrence of inferior coronas in the lapse of time 

 (under conditions, therefore, where the air must be saturated with 

 moisture) seems to be positive proof against the view that these coro- 

 nas owe their origin to undersaturation. The corona immediately 

 following (second exhaustion) is always a superior corona. One may 

 note that if extremely fine nuclei (colloidal molecules) pass the filter, 

 a time loss like the present would accompany their decay. 



20. Effect Of pressure difference on exhaustion. The reason for irreg- 

 ular results in tables 13, 14, and 15 is now apparent, for in these experi- 

 ments the tendency to periodicity was not yet understood. Nor can it 

 in any case be effectually combatted. After the fog chamber has been 

 cleared of foreign nuclei, which occurs at a pressure difference above 

 20 cm. of mercury and at about the same volume expansion in both 

 chambers, the effect of further increasing the pressure difference, 8p, is 

 an exceedingly rapid increase of the apertures of coronas. The first 

 coronas after the air is made dust-free are usually particularly large. 

 Though this looks like a foreign effect, it is probably due to periodicity. 



