84 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



65. Glass fog chamber Radiation from a distance. The experiments 

 on the nucleation produced by X-rays coming from a distance were 

 now continued by aid of the cylindrical fog chamber (glass walls 0.3 

 mm. thick and i cm. thick at the bottom), the lead casket being here 

 removed (curve 69). The data are given in table 52, and may be 

 restated from the mean results, 



D + 30 cm. TV ==70,000, 



200 -f- 30 41,000, 



620 -j- 30 22,000, 



where N is measured from the line of sight, 30 cm. from the end of 

 the fog chamber nearest the bulb. Very much of the lateral radia- 

 tion is thus cut off by the thick glass walls and bottom of the fog 

 chamber; but the decrements are far from suggesting the law of 

 inverse squares even in a remote degree. As the distance from the 

 line of sight increases over 20 times, A 7 " decreases only 3 times. 



The repetition of these experiments with a less active bulb gave 

 about the same results (table 53). For distances from the line of sight, 

 D=20, 210, 610, the average nucleation was N--=55,ooo, 34,000, 

 14,000. About one-half the total radiation is absorbed by a frontal 

 lead plate, or a double glass envelope, as usual (curves 69). 



66. Radiation from a distance Glass fog chamber in lead case. The 

 endeavor was finally made to stop off all secondary radiation by pro- 

 viding a close-fitting lead tube (L, fig. 42), which not only incased 

 the fog chamber A, but extended about 50 cm. beyond the end 

 nearest the bulb. If distances are measured from the line of sight, 

 the mean results may be estimated as D = 6o, 210, 610 cm., corre- 

 sponding to A 7 " X io~ 3 = 52, 25, 12. The nucleation falls off a little 

 more rapidly than before (a part of which may be referable to imper- 

 fect alignment of the distant bulb), but after 200 cm. the decrease is 

 slow (curves 69). 



In the present case a single lead plate (thickness, 0.14 cm.) cuts off 

 nearly all the radiation, z. <?., all but 4 to 6 per cent. Hence very 

 little secondary radiation has entered, while the small penetration of 

 the lead is probably referable to the distance of the plate from the end 

 of the chamber (cf. distance effect for gamma rays, next paragraph). 

 Compared with lead, the absorption of tinned iron is small (curves 86), 

 the plates (eventually 0.25 cm. thick) allowing 26 per cent of the radi- 

 ation to pass for the same thickness of plate which was used in the 

 case of lead. This result is quite out of proportion with the relative 

 densities. 



