144 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



from the following example of results when the distance between bulb 

 and fog chamber was 2 meters : 



Thickness of lead penetrated^ o 0.14 0.28 0.42001. 

 N X io~ 3 77 10 7 5 



i. e., 14, 9, and 7 per cent of the total intensity passes one, two, and 

 three plates, respectively. (Figs. 77, 78, Chapter III.) A glass plate, 

 7 mm. thick, and an iron plate, 0.5 mm. thick, allowed about 90 per 

 cent to pass ; when the casket was left open, and the lead plate placed 

 near the bulb, 17 per cent of the total radiation w r as effective, the 

 excess being of secondary origin. The passage through a plate of 

 tinned iron (cf. figs. 85, 86, Chapter III) may be observed for a bulb 

 6 meters distant, as follows : 



Thickness of plate. ... o 0.05 o.io 0.20 cm. 

 TVXio- 3 36 28 ii 7 



It fellow's, then, that in the above examples (101) nearly one-half of 

 the total radiation was derived from secondary sources, since the pri- 

 mary radiation was certainly stopped off to within 10 per cent by the 

 lead plates. To the eye of the fog chamber, therefore, the walls of 

 the room are aglow with radiation, and no matter in what position the 

 bulb may be placed (observationally from 6 cm. to 6 m. between bulb 

 and chamber), the X-illumination, as derived from primary and sec- 

 ondary sources, is constant everywhere. It is to be understood that 

 the X-illumination here referred to may be corpuscular. In fact, so far 

 as I see, the primary and secondary radiation here in question may be 

 identical ; for the corpuscles may come from the circumambient air 

 molecules shattered by the shock of gamma rays. 



The fog chamber, if open at the end toward the bulb, shows the 

 same total intensity ; but in such a case the inner walls of the casket, 

 etc., become the source of secondary rays. 



The behavior of the wooden fog chamber in relation to rays coming 

 from different distances being such as if the circumambient medium 

 were equally energized with something recalling the character of gal- 

 vanic polarization throughout, the following mean data are designed 

 to throw further light upon this behavior (fig. 69, Chapter III). 



Fog chamber D 6 50 200 600 cm. 



Wood, lead-cased io~ 3 N-= 50 50 38 



Glass, walls 0.3 cm., bottom i cm. thick 



1 55 34 J 4 



Glass, cased in close-fitting lead tube, pro- 

 longed 50 cm. toward bulb * 52 25 12 



Media pervious with difficulty eliminate the secondary radiation enter- 

 ing the broadsides of the fog chamber, and to close the end toward 



