Question of the Homogeneity of y-Rays. 745 



These plates were placed directly on the radium. The range 



of thickness of lead placed over the radium was from to 



18*6 cm. Up to 11 cm. the exponential law of absorption held 



true. The value of X was 0'500 (cm.) -1 . Beyond 11 cm., 



however, the curve continued no longer straight but became 



convex to the origin (fig. 12, PI. XII.). The departure of the 



curve from the straight line occurred when the intensity leak 



. ... 



(corrected for natural leak, which was 3'6 divisions per 



minute) was about one division per minute. This departure 

 must be due, either to the presence of a very penetrating 

 primary radiation from the radium, or to a constant secondary 

 radiation entering the electroscope otherwise than through 

 the base, and which, being very small, did not begin to affect 

 the straightness of the curve until very great thicknesses of 

 lead had been penetrated. 



The next experiment was carried out very similarly to the 

 first except that an electroscope of brass was substituted for 

 the lead one and no circular lead screens round the electro- 

 scope were used. The electroscope was a cylindrical one of 

 the ordinary type, 13 cm. in height and 10*8 cm. in width. 

 The thickness of the walls was 0*32 cm. and of the base 

 0*6 cm. The lead plates were placed on the radium as before 

 (which was placed in a recess in a piece of wood) and a 

 range of to 14 cm. of lead investigated. The curve obtained 

 in the same way as the last is no longer straight over any 

 part of the range but is convex to the origin, X varying from 

 0*62 (cm.) _1 to 0'08 (cm.) -1 over the range. The results are 

 shown in fig. 13, curve A (PI. XII.). The difference between 

 the character of this curve and of the last it would be natural 

 to ascribe to the nature of the material of which the electro- 

 scope is made as the other factors of the disposition have been 

 unchanged. Experiments as to the cause of this difference 

 have shown, however, that it is due as much to the nature of 

 the absorbing plates as to that of the material of which the 

 electroscope is made. These will now be described shortly. 

 A base 0'8 cm. was substituted for the 0'6 cm. base and the 

 walls were increased in thickness to 1*8 cm., but the general 

 convexity of the absorption curve was not affected. It was 

 found impossible with this disposition to obtain the straight 

 line relation by thickening the brass electroscope with brass. 

 A series of measurements was next done under exactly similar 

 conditions as last except that the two circular screens of lead 

 before used were placed around the electroscope. The effect 

 of the lead screens on the character of the curve was very 

 marked (fig. 13, curve B). The curve which had been 

 convex before became now straight over a large part of the 



