80 NUCLEATION OF THE UNCONTAMINATED ATMOSPHERE. 



FOG CHAMBERS INCLOSED IN METAL CASKETS. 



63. Wood fog chamber in lead casket Penetration. In table 49 data 

 of a crucial kind are given for the purpose of separating the radiation 

 which actually passes the lead screens from that derived from second- 

 ary or lateral sources. The first part of the table is at once decisive 

 (curves 77). Less than 7 per cent of the radiation which passes the 

 wood and glass walls of the chamber will pass through the front face 

 (toward the bulb), if this face is closed by a lead plate 0.14 cm. thick. 

 When the chamber is freed from the casket and the plate placed at the 

 bulb, more than half of the radiation gets into the fog chamber sec- 

 ondarily, as shown in the second part of the table and curve 77. 



TABLE 49. Wood fog chamber in a lead casket, open in front, toward the bulb. 

 Z>='2OO cm ; 5 p = 25 cm. Thickness of lead plate, 0.14 cm ; of glass plate, 0.7 cm. 

 Exposure, 3 sec. ; lapse, o sec. 



With an improved and more fully lead-incased chamber, the data 

 given in the third part of the table were investigated, in which succes- 

 sive thicknesses of 0.14, 0.28, 0.42 cm. of lead plate allow 14, 9, and 

 7 per cent of the radiation to pass (curve 78). The differences from 

 the above datum are due to the greater intensity of the radiation here 

 applied and to other incidental conditions. Furthermore, a glass plate 

 0.7 cm. thick, and a tinned iron plate 0.05 cm. thick, each allow nearly 

 all the radiation to pass, i. e., about 90 per cent, while a lead plate 

 placed near the bulb at D = 200 cm. cuts off about 17 per cent of the 

 radiation (curve " 78) . 



