PENETRATION OF X-RAYS. 



8l 



Hence it follows in the above experiments with the lead envelope 

 removed, since one-half of the radiation was cut off by a single frontal 

 lead plate 0.14 cm. thick, that about half of the radiation enters the 

 wooden fog chamber, not from primary, but from secondary sources 

 (using this term in its broadest sense), through the lateral walls of the 

 apparatus. When the chamber is inclosed in the lead case open in 

 front, the inside walls of the lead become a source of radiation, so that 

 the corona need not decrease in size, as the data show. In general, 

 the behavior is such as if the whole medium between the bulb and 

 chamber were equally "polarized" (to use this word with a special 

 meaning). At the lower pressure difference (8p 2o cm.) the lead 

 plate proves to be quite impervious, but the tin plate certainly admits 

 an accumulation of 3,000 nuclei. 



64. Continued Radiation from a distance. Experiments made to 

 find the effect if the distance, D, of the bulb from the lead-incased 

 fog chamber, open toward the bulb only, are given in table 50. 



TABLE 50. Wood fog chamber in lead casket. Effect of distance, D. 8p = 25 cm. 



Exposure, 3 sec ; lapse, o sec. 



*5 2 always taken, but not recorded. Usually $2 = 1.5 cm. 



The bulb is as usual variable, but the nucleation produced is about 

 the same for all distances up to 200 cm., after which there is a possible 

 decrease of about one-fifth as far as 600 cm., the limit of observation 

 (curves 69). This relatively insignificant effect of distance is again 

 remarkable, inasmuch as all remote secondary radiation is excluded. 

 Whatever produces the nucleation, if secondary, must come from the 

 inside of the casket, or it must be primary. At all events it is again 

 manifest that the whole medium within the room is almost equally 

 energized throughout. 



Table 51 and curve 85 show the penetration of tinned iron plates, 

 each 0.5 mm. in thickness, when the X-ray bulb is 600 cm. from the 

 lead-cased fog chamber. Several millimeters of iron plate are still 



