752 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



between 15*6 cm. and 17 cm., but the end point at 22*1 cm. 

 is on the straight line. This effect is, no doubt, due to in- 

 effectual shutting out of a small amount of secondary pene- 

 trating radiation which manifests itself at these points. As 

 the air space between the top of the absorption plates 

 and the electroscope base was filled in (thereby reducing the 

 secondary penetrating radiation) the points came on to the 

 straight line once more. In the main curve and curve B 

 (fig. 16) greater precautions were taken to prevent any trace 

 of such radiation from entering the electroscope. The value 

 of \ obtained from the principal curve of fig. 16 over a range 

 from 2 to 22*1 cm. of lead is 0'498 (cm.) -1 , that is to say, 

 1*392 cm. of lead cuts down the y-rays of radium to half 

 value. This value agrees (1) with that previously obtained 

 (0-495, F. Soddy and A. S. Russell, he. cit. p. 644) ; (2) 

 with the value given by Tuomikoski, over a range of 22 to 

 12*0 cm. (0*51) ; (3) with the value in the present paper, 

 using, however, a brass electroscope (0*50) ; and (4) in 

 Part II. of the present paper with truncated hemispheres 

 (•050). This value for X (cm.)" 1 0*50, making X/^=0*0438, 

 may be used with confidence in calculations of the pene- 

 trating rays from the earth's crust. A new electroscope, 

 made entirely of lead, was constructed in order to save 

 trouble in blocking up the windows and the cork with lead. 

 It is shown in section in fig. 17. It consists of a cylinder 

 of lead of internal height 

 12*9 cm., internal diameter 

 9*0 cm. ; thickness of walls 

 1*30 cm. and of top 1*25 cm. 

 Two cylinders of lead, 6*5 cm. 

 long and 3*5 cm. in diameter, 

 0'4 cm. thick in the wall, were 

 soldered into the sides of the 

 electroscope to protect the 

 windows, which were circular 

 and of the same diameter as 

 the lead cylinders. The latter 

 were just large enough to allow the microscope to be inserted. 

 The sulphur of the leaf system was surrounded by an earthed 

 ring of brass. Over the cork and charging rod a third 

 cylinder of lead, 0'4 cm. thick, was placed while measure- 

 ments were being taken. To the instrument a permanent 

 base could be soldered, or the thickness and nature of the 

 base could be altered at will by clipping up different thickness 

 to two pieces of brass attached to the sides of the electroscope 

 at its base. This may be considered a standard form of 

 electroscope for work on absorption of Y-rays. 



Fier. 17. 



