Question of the Homogeneity oj y-Rays. 



755 



the base for all bodies used as absorbing screens, except the 

 one of which the electroscope is made. In the case of brass 

 a thin base (0'58 cm.) gives higher A/<i than a thick base 

 (1*2 cm.) for bodies of Class II., and lower for Class III. 

 In the case of a lead base and Class III., X is independent of 

 the thickness of the base, but for Class II. a thin base (1 cm.) 

 gives higher A/s than a thicker one (2*85 cm.). For an elec- 

 troscope made entirely of brass the absorption of all bodies 

 except Class I. (which have already been discussed) for dis- 

 position A is absolutely exponential. Class III. has a higher 

 X/d than Class II., for which the values of X/d are practically 

 constant, as in the earlier experiments with the lead electro- 

 scope (ibid. p. 644). Using a thick lead electroscope base 

 (1 cm.), but clamping up the absorbing metal, bare radium 

 or uranium X gave exponential absorption in all cases, but 

 with different values of X than had been obtained before 

 (pp. 644 and 646) by method A (same electroscope). 



For copper and uranium X X is greater for B than A. 



For copper and radium X is smaller for B than A. 



For lead and uranium X X is smaller for B than A. 



For lead ana radium X is greater for B than A. 



These results also explain the abnormal value of the ratio 

 ^UrxARa for lead (1*465) as compared with that for copper 

 (1*186). With disposition B the value of the ratio is 1*28 

 for lead, while that for copper is 1*25. 



For radium (a thick brass electroscope and brass base were 

 used) the value of X for a body laid on the radium depends 

 to some extent on whether the rays have passed through 

 other bodies first. In Table VL, given below r , values of X 

 for paraffin wax, magnesia brick, and zinc are given when 

 the rays have first passed through lead, tin, graphite, and no 

 other body, respectively, before entering the absorber. 1 cm. 

 of lead was used and 2 cm. of tin and graphite. All the 

 curves obtained were strictly exponential. 



Table VI. 



Rays first pass through 



Values of X. 



Paraffin Wax. 



Magnesia Brick. 1 Zinc. 



Lead 



0032 

 0037 

 0-037 



0070 

 0-071 

 0-073 

 0-078 



0-212 

 0238 

 0-255 

 0-251 



Tin 



Graphite 



Nothing 



0-040 



3C 2 



