12 Dr. A. S. Eve on the Coefficient of Absorption 



test-tube containing the radium was placed inside a cylinder 

 of zinc between the poles of a powerful electromagnet, and 

 the magnetic field was increased in strength until the 

 /3 radiation was eliminated. After measuring the ionization 

 due to the <y rays, the electromagnet was run back, the 

 zinc cylinder removed, and the joint /3 and y ionizations 

 determined. 



It is difficult to remove the /5 radiation by screens alone, 

 or by a powerful magnetic field alone, but a combination of 

 screens and field is very effective. 



The values of //, found for distances from 2 to 5 metres, 

 under various conditions, were as follows : — 



4'4, 4-5, 3-9, 5-0 ; cm. -1 x 10*, 



and the mean value was fi = *0045 cm. -1 



Eleven milligrams of radium bromide in solution in a 

 thick-walled glass bottle emitted some /3 radiation, which 

 could be screened off by aluminium and zinc sheets placed 

 near the bottle. The values of //, for these /3 rays were : — 



distance 1 to 2 metres fi = '00385, 

 „ 2 „ 3 „ ^='0041, 



Mean jjl = '0040 cm." 1 



It is concluded that for absorption at distances from r = 2 

 to r = 5 metres the value of /jl is approximately *0045 cm. 



Short Ranges. 



Some weaker specimens of radium were next tested at 

 shorter distances. The specimens chiefly used were, (1) a 

 capsule with a thin mica cover containing 0*56 nig. of 

 radium bromide (0'33 mg. of radium), and (2) some radium 

 bromide in a thin-walled small glass tube, containing 

 •53 mg. cf radium, the preparation being at about one- 

 quarter of full strength. The latter specimen was kindly 

 lent to me by Professor H. A. Wilson. 



The radium tube was hung by a thread at distances 

 varying from 40 to 160 cm. from the electroscope, and 

 readings were taken both when advancing and retiring 

 the radium. When measuring the joint /3 and y ionization, 

 it was necessary to withdraw the large and massive electro- 

 magnet to a distance, otherwise the effect of " reflected " 

 radiation was strongly marked. The /3 rays were then cut 

 off by screens and a strong field, and the ionization due to 



