142 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



exhaustion carried to S/> = 20 cm. These data are computed from the 

 second exhaustions, as the first show the densely stratified fogs un- 

 available for measurement. 



Rays on .......... 2 2 2 2 2 2 2 2 2 minutes. 



Rays off .......... o 4 o 4 o 2 020 o minutes. 



" 3 ......... 20 52 20 32 25 30 13 34 30 



With the bulb at different distances from the fog chamber, the fol- 

 lowing data admit of the same interpretation (figs. 74, 76): 



Distance, D = 5 10 15 



Rays on 2 2 2 



Rays off o o o 



^VXio- 3 22 3 i 



5 10 15 cm. 

 222 minutes. 

 222 minutes. 

 58 9 i 



The phenomenon vanishes when the radiation is too weak to produce 

 persistent nuclei, therefore, either when the bulb loses efficiency or 

 when it is too far from the fog chamber. 



101. Space surrounding the X-ray tube a plenum of radiations.- 



While the phosphorescent , photographic , and electric effects of X-radia- 

 tion decrease rapidly with the distance, D, from the tube, the nucleat- 

 ing effect (A 7 ", nuclei generated per cubic centimeter, instantly) is nearly 

 constant over relatively enormous distances.* (C/. fig. 69, Chapter 

 III.) Thus to give two examples among many (S/> = 25 cm.): 



6 200 600 



88 83 83 



6 200 600 cm. 

 79 79 79 



The laws of inverse squares would predicate a reduction of 10,000 to 

 i between these limits ; and, in fact, at 6 cm. the phosphorescent 

 screen is intensely luminous, at 200 cm. very dim, at 600 cm. quite 

 dark, as in the case of any ordinary illumination. The leaves of an 

 electroscope within a glass bell jar collapse in a time which is directly 

 as the square of the distance from the energized X-ray bulb. The 

 result obtained with nuclei is astonishing ; the nuclei-producing 

 radiation would, at first sight, seem to be of an extremely penetrating 

 kind, akin to the gamma rays of radium, and distinct from the ordi- 

 nary phosphorescence-producing X-rays. This impression is accen- 

 tuated by the fact that the radiation can not be stopped by lead screens 

 many centimeters in thickness, placed between bulb and fog chamber. 

 (Cf. figs. 79, 81, 82, 83, Chapter III.) The following are typical ex- 

 amples, in which the distance between the lead plates screening the fog 

 chamber and the X-ray tube is D=6oo and 200 cm., respectively. N 



* Supposing that the fog chamber is not inclosed in impervious metal. In the latter 

 case, with the lead covering open toward the X-ray bulb only, there is constancy of N 

 within 20 per cent over 6 meters. 



