PERSISTENT NUCLEI. 135 



slightly above the fog limit of the ionized medium the following data 

 are typical. JV 2l shows the number of nuclei caught when 8/> =- 21 

 cm., which lies within the pressure interval stated. 



= 4 

 Radium ,,o.ooo X) ] g ,0.4 _, ^ 



o 20. i 18 20 



f 200 20.3 10 16 



X ravs -! 5 (20>l) 22 (24) 



10 (19-7) 45 (33) 



I 5 !9-6 50 36 



The full N curves are best given graphically. Data for the radium 

 at D = 100-200 cm. are scarcely distinguishable from air(fig. 68, Chap- 

 ter III) except near the fog limit; for radium at D 10-25 cm - tne Y 

 lie close together, above the former and below the data for Z? = o. 

 With these, the X-ray effects at D = 10-50 are in sequence. Thus 

 there is general continuity between the air curves, the radium curves, 

 and the X-ray curves. As a whole, the curves for D and p are 

 doubly inflected; relatively large (efficient) nuclei are rapidly fewer 

 in number as 8/> decreases ; relatively small nuclei are also rapidly 

 fewer in number as 8p increases. Nevertheless the efficient nuclei of 

 intermediate dimensions are of all sizes, while this size tends to become 

 more uniform as the ionization is greater, i. e., the slopes of the curves 

 become steeper. Gradation is accentuated for the case of weak ioniza- 

 tion. The case of nonionized air is shown in figs. 45, 46, Chapter III. 



The efficiency here referred to depends not merely on the apparatus 

 (suddenness of exhaustion), but in particular on the degree to which 

 larger nuclei are present, remembering that all the nuclei in question 

 are produced in dust-free air. As the nuclei are essentially graded in 

 size, the larger soon capture all the available moisture to the exclusion 

 of the smaller, and the coronal diameter ceases to increase with 8p. 

 The question will be specifically treated elsewhere. 



93. Persistent nuclei. If the X-ray bulb is approached nearer the 

 fog chamber, or if a more efficient bulb is used, so that the density of 

 the ionization within the fog chamber is sufficiently increased, the 

 rate of production of nuclei will eventually exceed the rate of decay. 

 (See figs. 50, 51, 67, 84, Chapter III.) Under these conditions there is 

 not merely an increase of number in the lapse of the time of exposure 

 to the radiation, but essentially an increase of size, i. e., the nuclei 

 grow indefinitely. They are now persistent for hours after the radia- 

 tion ceases. The number, N, per cubic centimeter increases, therefore, 

 in marked degree and at an accelerated rate with the time of exposure, 

 certainly for 10 minutes or more, barring the invariable loss of efficiency 



