60 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



A comparison of the i-minute exposures in parts II and III is note- 

 worthy (curve 51). The high-pressure difference in part III (8p = 25) 

 catches over four times as many nuclei as the low-pressure difference 

 (8p 2o), ccet. par., but the excess vanishes at once to the value of 

 those within the reach of Sp = 20 cm. Hence evanescent and persist- 

 ent nuclei are always present together. 



49. Persistence of fleeting nuclei after solution. A result occurring 

 throughout the observations is the following : Whether the nuclei are 

 fleeting in character or not, there is invariably a second strong corona. 

 This is obtainable on the succeeding exhaustion without fresh nuclea- 

 tion, even if the fog particles of the first corona are allowed to com- 

 pletely subside, before the addition of the dust-free air prior to the 

 second exhaustion. If there were no first exhaustion during the 

 exposure of the fog chamber to the radiation, no nucleation would 

 have been found after the lapse of time needed preparatory to the 

 second exhaustion . The reevaporation of fog particles from the first 

 exhaustion, in every case changes about one-eighth of the fleeting nuclei 

 into the stable nuclei observed in the second corona. This is obviously 

 an important observation, bearing on the whole phenomenon of nuclei, 

 condensation and rain. 



To investigate this case the data of table 37 were collected, in which, 

 as soon as possible after first condensation, dust-free air is admitted 

 into the fog chamber to dispel the fogs by evaporation. When this is 

 done the coronas on second exhaustion are invariably larger and denser 

 (curves 56, 57), showing that more nuclei have been preserved, i.e., 

 converted from the fleeting into the stable form. The only effect 

 producible by the premature influx is to diminish the number of fog 

 particles lost by subsidence. It follows, then, that all nuclei upon 

 which condensation has once taken place become stable nuclei. 



It is somewhat difficult to measure the first corona and at the same 

 time to provide for a quick influx of air so that but little subsidence 

 of fog particles may take place. The data of table 37, therefore, quite 

 apart from periodicity and other difficulties, can not be expected to 

 be very uniform. It was not thought necessary to prolong the interval 

 of persistence (time from influx of air to second exhaustion) beyond 

 a few minutes, for these suffice to indicate persistence. The table 

 shows that from 25 to 50 per cent of the nuclei may be preserved 

 indefinitely by reevaporating them from fog particles. The mean 

 datum of all results apart from the time interval entering the test is 

 actually 39 per cent. 



