PENETRATION OF X-RAYS. 



79 



TABLE 48. Penetration; miscellaneous experiments on case of lead plates; 0.14 cm. 



thick ; 8p = 25 cm. Wood fog chamber. 



* Another bulb. 



62. Generation through lead plate and through iron. The data of 

 table 48 show the usual accelerated increase of the nucleation, N, with 

 the time of exposure, when nuclei of the persistent type are produced 

 (curve 84). When, however, the lead screen (thickness 0.14 cm.) 

 intervenes, there is no accelerated increase and no accumulation above 

 a relatively small value, N --= 13,000. These nuclei may be a transi- 

 tional type, but it is difficult to interpret the case of very small coronas. 

 Another similar test was made with a screen of tinned sheet iron ^2 

 mm. thick. The results are in the main the same; in other words, 

 marked persistent nucleation is not produced through the iron plate 

 in spite of its slight thickness and lower density. Parallel observa- 

 tions for 3 seconds' exposure show that about as many nuclei are thus 

 generated as are obtained after a 3-minute exposure through the plate. 



On the other hand, for 3 minutes' exposure there is growth of nuclea- 

 tion, if observation is made 2 minutes after exposure ; in the presence 

 of the plate the nucleation falls off to a negligible datum in the same 

 lapse of 2 minutes after exposure. The decrement in this case is of 

 the same order as is observed for the short exposure (3 seconds) tested 

 a minute after exposure ceases. The use of another bulb with the 

 plate does not change the results. 



