produced by ft and 7 Rays at High Pressures, 175- 



it has been assumed that no sensible error in the relative 

 values of the ionization currents has been introduced by 

 using a voltage of 1520 when the plates were 1 cm. apart. 



Absorption by Air of the ft rays from Ur X. 



When 7 rays pass through any material some of the 

 7 rays are absorbed and ft rays are consequently produced. 

 " Emergent" ft rays continue in the direction of the original 

 7 rays, and " incident " ft rays are returned in the opposite 

 direction. In order to obtain some idea of what hnppens to 

 the emergent and incident ft rays in the ionization vessel 

 when the pressure was increased, a comparison was made 

 with the ft rays of UrX. This active material was chosen 

 because a thin film could be easily prepared of any desired 

 area, and the ft rays correspond approximately in penetrating 

 power to the ft rays from Radium C. 



A uniform layer of UrX was deposited on platinum 

 foil about 3 cm. in diameter. This was covered with alu- 

 minium 0'2 mm. thick to absorb soft ft rays. This active- 

 material was fixed to the lower plate. The upper plate was 

 aluminium foil (0*00024 cm. thick), so that reflexion in the 

 case of the upper plate was reduced to a minimum ; yet 

 there would undoubtedly be a certain amount of reflexion 

 from the interior of the vessel, and this it was impossible to 

 estimate. The distance between the plates was 1 cm. The 

 pressure in the vessel was increased, and the ionization 

 currents were measured for the different pressures. The 

 results are shown graphically in fig. 3 (PL I.) . Assuming that 

 ionization and absorption are proportional to the pressures,, 

 the saturation current at any pressure should be given by 



i = A(l — e-P xd ), 



where A = a constant ; 



p^z pressure in atmospheres ; 



\ = coefficient of absorption at unit pressure ; 



d = distance apart of the plates. 



The curve, fig. 3, can be expressed approximately by an 

 equation of this type, and the value of X was thus found to- 

 be 0*04 cm. -1 . If we calculate the absorption coefficient of 

 the ft rays in air from the known absorption coefficient in 

 aluminium, assuming the density law, \ = 0*0064 approxi- 

 mately. This discrepancy between the observed value and 

 the calculated value cannot be ascribed to lack of saturation 

 at the high pressures. 



