DISTRIBUTIONS OF NUCLEI. 7 1 



an oil film which has here collected, after which the walls remain clear for 

 some time on wetting. Prior to the systematic investigation of table 

 2 3, however, the apparatus was thoroughly overhauled and cleaned. The 

 dp referred to is henceforth to be p-p 3 , i.e., the difference between 

 atmospheric pressure and the pressure (p 3 ) observed when fog chamber 

 and vacuum chamber are in communication at the given temperature. 

 Some time having elapsed since the last observation, the first experi- 

 ments (parts I, II, etc.) of the tables show low nucleation, due to the 

 presence of large nuclei originating internally, as already specified. 

 Series III, with X-rays, agrees very fully with the preceding cases 

 (table 22) and there is nearly the same steepness of curve (dp=p- p s ) 

 already instanced. Conformably with this, series IV, V, etc., also show 

 increased steepness of curve, the nucleation, moreover, being higher 

 because the interior sources of relatively large nuclei have been gradually 

 removed. But there is throughout much irregularity, and periods are 

 frequent. Remembering that above the dp referred to is the drop of 

 pressure observed in the fog chamber, while at present dp = p- p 3 , a slight 

 advance of the highest nucleations over the preceding cases is discernible. 



51. Observations, continued. In table 24 observations are recorded, 

 made once or twice a day, far enough apart to allow water nuclei to 

 vanish by time loss. After each measured corona a low exhaustion was 

 (as usual) made to remove the greater number of such nuclei at once in 

 the large particles of the small corona produced. The pressure difference 

 (dp) is again p-p 3 , as explained in section 48, so that the correction 

 there adduced may be applied. The fog chamber was rigorously tight as 

 regards the influx of external air, but air flowed slowly from fog cham- 

 ber into the vacuum chamber, through the leaky stopcock. Between 

 the exhaustions the air was kept at low pressure for about ten hours, and 

 new air was admitted, just before exhaustion, through the filter into the 

 fog chamber, until the barometer pressure had been reached. As the 

 whole apparatus had been left standing a long time, dust-free air only 

 was present in the fog chamber and vacuum chamber. The walls of the 

 former were rubbed clean before beginning the work. 



The table also contains the above data for air energized by the X-rays 

 with anticathode distant 20 cm., observations being made while the 

 radiation acted and immediately after the exposure began, to avoid the 

 presence of persistent nuclei. The computed values for the drop in 

 pressure (p-p 2 , where p 2 is the computed isothermal pressure in the 

 isolated fog chamber alone), and the corresponding values of n, are also 

 given. The nucleation refers as usual to the exhausted fog chamber. The 

 results are constructed graphically in fig. 33. 



