175 



turned from C, Al, and Zn. A Pb dome 4 mm. thick re- 

 duced the effect to approximately 20 per cent., indicating a 

 radiation for which the value of A. is 4, if we can suppose 

 it homogeneous. 



In the case of Zn and Al it will be seen from table i. 

 that a 4 mm. Pb dome reduces the effect of the emergence 

 radiation to about 45 per cent. There is thus a considerable 

 difference in the quality of the emergence and the incidence 

 7 radiations from these substances. 



That the incidence radiation from these substances is 

 softer than the emergence, is also supported by the experi- 

 ments of Kleeman and Eve already referred to, both these 

 observers having found the value of A. for the incidence rays 

 from these or similar substances to be of the order 4, meas- 

 ured by Pb. In the case of the incidence radia- 

 tion from Pb the measurements were not sufficiently accu- 

 rate to decide whether the incidence radiation was softer 

 than the emergence; it appeared to be of much the same 

 quality, i.e., \ = r3 approximately. 



§m. 



As the apparatus described in fig. i. was unsuitable for 

 measuring the amount of secondary emergence radiation from 

 substances such as Al and C, and as we do not know that 

 the distribution of the emergence radiation is the same in 

 all directions for all substances an experiment was set up as 

 shown in fig. vii. 



The lower portion of the diagram shows the arrange- 

 ment of the Ra, magnet, and iron case which is the same as 

 previously described. Some distance above the Ra was sup- 

 ported — upon an insulated stand — a flat cylindrical ionization 

 chamber. The floor of this chamber was of sheet lead 1*25 

 mm. thick, the sides were of Zn, and for a lid was used a 

 thin Al plate. The conical chamber, shown by dotted lines, 

 was used in experiments to be described later. The radia- 

 tors were in the form of flat plates, the first of which in 

 any experiment was placed in the position p,p. Additional 

 thicknesses were added immediately below p,p and in contact 

 with the top plate, while for very great thicknesses plugs of 

 the material could be inserted in the conical hole in the Pb 

 block, through which the y radiation streamed. 



In the first set of experiments the Pb plates L,L were 

 used in the position shown, the large central hole in them 



