144 Messrs. A. S. Russell and F. Soddy on the 



the centre of which communicated with a leaf system contained 

 in a lead electroscope. Fig. 1 (p. 143) shows the disposition. 

 Absorbing screens could be clamped tightly against the 

 ionization chamber in position A, A. BB was a thick lead plate. 

 Experiments were carried out with lead, tin, zinc, and alu- 

 minium, first with the short cylindrical ionization chamber and 

 secondly with one three times as long but of the same diameter. 

 This was done to find out if the shape of the ionization 

 chamber had an effect on the values of the coefficients of 

 absorption. Results are given below. 



A. Single Ionization 

 Chamber. 



B. Triple Ionization 

 Chamber. 





X. 



ioo yd. 



Range (eq.cm.! 

 of Pb). 



Pb. 



850 to 



0-728 



7-46 to 

 6-38 



0-4 to 1-7 



Sn . 



0-355 



4-90 



0-34 onward 



Zn. 



0-267 



377 



0-21 onward 



Al . 



0080 



2-83 



0'16 onward 





X. 



100 \/d. 



Range (eq.cm. 

 of Pb). 



Pb. 



0-850 to 

 0-61 



7'46 to 

 5-33 



4-88 



0-3 to 1-8 

 - 34 onward 



Sn . 

 Zn. 



0-354 



0264 



3-74 



0-21 onward 



Al . 



0-083 



293 



0"20 onward 



Thus X/d increases with d for this disposition. All the 

 curves are exponential, except that for lead w r hich is convex. 

 The length of the ionization chamber made no practical 

 difference. These dispositions are analogous to those used 

 in Table V. (II. p. 753). For these the rays were confined 

 in the same cone, which however entered directly through 

 the base of an electroscope. The distance between the base 

 of the electroscope and the mouth of the cone was 3'5 cm. 

 as compared with 2*0 for the present dispositions. The 

 results, however, were quite different from those detailed 

 above. Thus zinc had for Xjd x 100, 3*0 to 4*2 over a range 

 of 0*35 to 2' 6, the absorption curve being concave. 



These results show how meaningless a value of X for an 

 absorbing substance is unless a full account of the experi- 

 mental disposition under which the measurements are made- 

 is given. 



