go 



VAPOR NUCLEI AND IONS. 



140 



120 



100 



80 







40 



20 



.2 



.6 



1.0 



1.2 



1.6 



Fig. 45. Depression of efficient nucleation (w) of dust-free air, ionized by gamma-rays 

 of radium penetrating through different thicknesses of lead. Table 35. 



Table 35. Absorption of 

 lead. 7-rays of radium 

 10,000 x. D = 100 cm.; 

 dp = 3 1 cm . 



high asymptote for dust-free air practically at a distance of D = 150 cm. 

 In the second series the asymptote is lower, due to details in the ad- 

 justment in the apparatus, and reached later, i. e., at a distance of D = 

 180 cm. between the radium and the fog chamber. This curve has been 



worked out completely for D = o cm., and 

 shows the very interesting feature of a well- 

 developed minimum. In other words, as the 

 radium is removed from the fog chamber, the 

 ions which at first predominate and capture 

 all of the moisture decrease more and more 

 in number, until the conditions are ripe for 

 the simultaneous condensation of moisture 

 on the colloidal nuclei of dust-free air. In 

 proportion as the radium is further removed, 

 the latter predominates, fully so when the 

 asymptote is reached. 



The corresponding results when the radium 

 tube is inclosed in a thick lead pipe (walls 

 5 mm., length 60 cm.) shows a much sharper 

 minimum, occurring at lower exhaustions. Such irregularities as are 

 apparent here may be referred to the unequal distribution of radiation 

 within the fog chamber discussed in Chapter I, sections 4 et seq. All of 

 these curves have a similar bearing on the question of the distribution 

 of the precipitate of graded nuclei. 



