38 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



given when the radium is placed at 45 cm., 100 cm., 200 cm., respec- 

 tively, from the end of the fog chamber, or, better, from the line of 

 sight. Whenever radium is not too far from the apparatus the 

 change of fog limit may be made out sharply as 



Distance, 



o 

 19 



45 



2O. 2 



TOO 

 20-8 



2OO 

 21-5 



24.5 



cm., 

 cm., 



showing that the nuclei increase in size with the intensity of the ion- 

 ization. These and the following data are mapped out in figure 34 of 

 the chart, n instead of N (reduced to normal pressure) being usually 

 inserted. In case of long distances (200 cm.) the results are apt to 

 be irregular. On removing the radium to a great distance the fog 

 limit of air is soon restored, and it will be seen that all observations 

 are introduced by test experiments with dust-free air. 



TABLE 21. Coronas for a given pressure difference and fog limits after removal of 

 radium (o.oi gram, io,oooX, in hermetically sealed aluminum tube) from inside of 

 fog chamber. 



* Radium present. 



f Radium removed. 



Successive exhaustions. 



35. Nucleation. An important interpretation of the preceding results 

 is obtained by mapping out the nucleations, n, in relation to the cor- 

 responding pressure differences, S/> (fag. 34). The slope of these 

 curves falls off rapidly for radium at o, 45, 200 cm. from the fog cham- 

 ber, while the fog limit rises. In other words, the initial slope of 

 the ^-curves is steeper as the fog limit is lower. Thus per increment 

 of S/> of i cm. of mercury, above the fog limit of the ionized medium, 

 and below the fog limit of nouenergized dust-free air, there will be 

 found in succession for radium in- 

 Sealed aluminum tube within fog chamber, S=i2,ooo 

 Sealed glass tube within fog chamber, 6,000 

 Sealed glass tube, outside, 45 cm. from fog chamber, 3,500 

 Sealed glass tube, outside, 200 cm. from fog chamber, 1,000 

 Dust-free air (above fog limit, radium at infinity), 10,000 



