142 Messrs. A. S. Russell and F. Soddy on tlie 



from the exponential curve over the first part of the range 

 (0 to 1 cm. thickness of lead or its equivalent) now call for 

 remark. If we place a source of radium at a distance of 

 13 cm. below a lead electroscope and clamp up as the base 

 varying thicknesses of absorbing material, we invariably get 

 a similar result. Between a thickness sufficient to absorb all 

 /3-rays and a thickness equivalent to 1 cm. of lead the 

 absorption curve is convex to the origin, i. e. the value of X 

 decreases continuously. From about 1 cm. onwards X is 

 constant. Experiments under identical conditions were 

 carried out with lead, tin, zinc, and aluminium as absorbers. 

 The absorption curves for each consist of two parts, an upper 

 steep curved part and a lower straight part. The greater 

 the density of the body the steeper is this upper part and 

 the longer it takes to join the straight part of the range. If 

 now 1*3 cm. of lead be placed on the radium and the results 

 repeated, the general character of the curves is the same as 

 before. The slopes of the lower part are less, since the rays 

 are hardened by the lead, and the upper part joins with the 

 lower part at a smaller equivalent thickness (0*5 instead of 

 1*0), thus smoothing out somewhat, but not eliminating, 

 the convexity of the curve. With lead as absorber the 

 placing of 1 cm. of lead over the radium has little effect, the 

 convexity over the first centimetre being only slightly 

 diminished. This shows that the effect has nothing to do 

 with a soft type of 7-rays initially present with the hard 

 type, as has previously been supposed. If we now carry out 

 similar experiments with another disposition (permanent base 

 of lead or brass, absorbing material laid directly on the 

 radium) the results are very different. For lead the upper 

 part of the curve is again convex, whether the base be brass 

 or lead, the convexity being the greater for a brass base : 

 with zinc and aluminium, the upper part is concave, the 

 concavity being about the same with either base. Typical 

 results are given below. The ranges are expressed in actual 

 thicknesses of lead and zinc. 



1. Lead. Brass base 1'2 cm. thick. 



Range (cm.). ... 



to 25 



025 to 1-27 



1-27 to 9-9 



\ (cm.) 



0-875 



0612 



0502 



