64 IOWA ACADEMY OF SCIENCE Vol. XXV, 191S 



coefficients for the two metals is nearly equal to the ratio of 

 this same quantity as quoted from Crowther's paper, although 

 the velocities of the B rays are a thousand times greater than 

 the maximum velocity of the photoelectrons. 



The values of p as tabulated are constant through quite a 

 range in thickness. The constancy of the coefficient of absorp- 

 tion has been found to hold up to thicknesses limited by the 

 depth of penetration of the indicent energy. This indicates 

 that the mean distribution of velocity of the electrons is the 

 same whether they come from sheets of metal less than or equal 

 to the depth of penetration of the light. (t>10-%„.) 



The coefficient of absorption of silver (d=.0135'^M'^) is less 

 than that for platinum (d:=.033lMMi) which indicates that the 

 velocity of the electrons, although excited by the same fre- 

 quency, and hence starting initially from their respective atoms 

 with the same velocity, do not in appearing at the surface of 

 the metal, although coming from the same depth, give up on 

 the average the same amount of energy in reaching that surface. 

 Since a large value of corresponds to a small velocity it fol- 

 lows that electrons originating from the same depths in the 

 two metals, appear at the surface of silver with greater velocities 

 than those originating from the correspondingly placed plati- 

 num atom. This would imply that the distribution of velocity 

 amongst the electrons would be totally different in these two 

 metals and that the "distribution of velocity curve*' for silver 

 would lie to the left of that for platinum and hence would have 

 a greater slope than the latter. 



If it is the mass of the material alone that effects absorption 

 then - should be constant for all metals, since this socalled 

 mass absorption coefficient gives a measure of the absorption 

 per unit mass of the screen per unit cross section for a normally 

 incident beam of electrons. The values of - are, however, only 

 superficially constant, so that it might be worth while giving 

 some evidence for the argument that atomic structure and not 

 density exclusively influences the absorption of the electrons 

 as they move through the material. Unfortunately we are here 

 confronted with a most difficult problem, because no allowance 

 has been made for any scattering of the electrons. We know 

 that scattering increases in amount with the thickness of the 

 metal traversed. Just what contribution scattering has made 

 towards the above data cannot be determined from them. An 



