136 NUCLEATION OF THE UN CONTAMINATED ATMOSPHERE. 



of the X-ray bulb. These nuclei are large, requiring very little super- 

 saturation for condensation, and are much like any ordinary nuclei. 

 They are pronouncedly of all sizes, and the initial coronas are apt to 

 be distorted and stratified beyond recognition. Whirling rains (sec- 

 tion 94) and fog accompany the first condensation. While small nuclei 

 occur throughout the chamber, the end near the bulb is at first the 

 seat of growth, which gradually extends to the other end, as I have 

 shown elsewhere.* The following two series of data, showing the 

 generation and decay of nuclei in question, may be cited as illustra- 

 tions. The pressure difference, Bp= 20 cm., is much below the fog 

 limit for dust-free air, in the given apparatus. 



Time of exposure o 5 10 20 60 120 180 sec. 



JVXio~ 3 o 2 ii TO 80 t(ioo) t(5) 



Time after exposure.. o 36 85 240 minutes. 



JVx io~ 3 (100) 36 20 Vanishing. 



Hence there is a decay of one-half in 10 minutes and of one-fifth in 

 80 minutes, or the degree of persistence is 200 to 300 times larger than 

 in the first paragraph. The data indicate, moreover, that both of 

 these extreme types of nuclei and all intermediate types now occur 

 together, as may be tested by changing the pressure difference, 8/>, on 

 exhaustion. (Cf. fig. 51, Chapter III, for 8p= 25.) Intermediate rates 

 of generation and decay may be obtained by moving the bulb nearer 

 to or farther from the end of the fog chamber. Finally, the rates at 

 which the nuclei and the ionization severally decay, between which 

 it would be difficult to distinguish in the case of the very fleeting 

 nuclei, stand in sharp contrast with the persistence of the nuclei of 

 the present paragraph. 



If TV be expressed in thousands of nuclei per cubic centimeter, and 

 time of decay, /, in seconds, the equation ilN=a-\-bt (for which 

 there is here but little justification) shows that = 0.000013, over 20 

 times smaller than in section 91. (Cf. fig. 50, Chapter III.) 



First exhaustion data of the generation of persistent nuclei are diffi- 

 cult to obtain, because the coronas soon become heavy fogs, distorted 

 beyond recognition, and the fog limits variable over wide limits, often 

 approaching vanishing smallness. The number of persistent nuclei 

 generated varies with the time of exposure at an accelerated rate, as 

 if the nuclei themselves assisted in the generation. (Cf. section 100 ; 

 also fig. 84, Chapter III.) For instance, at 8^=19.7 cm., after the 

 times of exposure, i, 2, 3 minutes, the nucleations were N-- io- 3 22, 77, 

 (120), respectively. 



* American Journ. Sci., XIX, 175. 



t Computed from second exhaustion, after subsidence of the dense fogs of first. 



