EFFECT OF SCREENS. 



II 



dental conditions. This is best worked out with water nuclei. The 

 subsequent life-history of the nucleation and the ionization is distinct, 

 nuclei, when produced by intense radiation as above, being surprisingly 

 persistent, ions by contrast characteristically fleeting. Hence it seems 

 to me to be best in keeping with all the data in hand to regard the 

 nucleation as the product which owes its growth or origin to the 

 expulsion of the corpuscles representing the concomitant ionization. 

 Ignition and high potential nuclei, X-ray and radiation nuclei in 

 general, phosphorus and water nuclei, produced throughout in strictly 

 dust-free air, all admit of this account of their occurrence and prop- 

 erties. There is no observable case of a process producing ionization 

 without nucleatiou, although there are many cases of nucleation free 

 from ionization. 



13. Tentative experiments with lead screens, inside and outside of the 

 fog Chamber. These experiments were made in large number; but 

 owing to the variability of the X-ray bulb and the action of the coil, 

 as well as the difficulty of realizing truly geometric conditions with 

 X-radiation, they are not satisfactorily conclusive. It will be seen in 

 the following chapters that results like the present can not in any case 

 be more than preliminary in character. 



The screens were lead plates with holes cut in them, or lead tubes 

 soldered to the edges of the holes normal to the plate. They were 

 placed between the X-ray bulb and the A end of the fog chamber to 

 guide the radiation. 



15* 



9 



10 



FIGS. 8-10. Forms of lead screens. 



In case of the observations i to 6, the screen was in the shape of 

 figure 8, with a horizontal slit 10 cm. long and 1.5 cm. wide, stretching 

 nearly across the end of the fog chamber. Often screens of this kind 

 were adjusted 2 to 3 cm. apart, as shown in figure 9. The screen was 

 earthed and the bulb placed as near it as practicable. 



