40 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



8/> = 2i, after which the true exponential march due to the excited 

 activity begins. It would be better to state these data in terms of the 

 number of nuclei produced after indefinite exposure (nuclear satura- 

 tion), the number to be found by a given sufficiently high pressure 

 difference below the fog limit of dust-free air. But the observations 

 of table 21 are scarcely advanced enough for this purpose. In fact, the 

 irregularity of size of the coronas s l obtained at the given pressure- 

 difference, 8/) = 2i.y, is worthy of remark. The curves obtained in 

 the second exhaustion (s 2 ), whereas but a few minutes of exposure of 

 the air to the excited activity are in question, are much smoother. 

 The w-curves do not suggest any further comment. 



The above experiments on radio-activity, induced by radium con- 

 tained in a closed aluminum tube placed within the fog chamber, 

 having been made soon after the tube was filled and sealed, it was 

 thought necessary to repeat the work two months later with the same 

 tube. As a safeguard this was additionally sealed at the ground joint 

 (screw plug) with resinous cement. Data so obtained with the wood 

 fog chamber were curiously irregular, showing periodicity occurring 

 in triads. Large coronas appeared after long waiting (15 hours), and it 

 is therefore probable that there was some slight leak whereby air nuclei 

 entered the fog chamber and captured much of the precipitated water. 

 In contrast with these suspicious results, the data for decay curves were 

 consistent, showing an increase of nucleation within about 20 to 30 

 minutes subsequent to removal, after which a gradual decay occurred 

 to about one-half in 6 hours. But the data as a whole were discarded. 



The experiments were now repeated in the glass fog chamber, which 

 had been specially tested for freedom from leaks. The exhaustions 

 made below the fog limit for dust-free air are shown in table 22 and 

 figure 37. 



TABLE 22. Radium in fog chamber. Sealed aluminum tube. 8$ ==25 cm. Succes- 

 sive exhaustions, i m -2 m apart. 



* Renewed after waiting 5 minutes or more. 



f Next day. 



