402 - Scientific Intelligence, 



cubical electroscopes of the type usually employed iu gamma-ray 

 work. For determining tlie initial absorption, the lead front of 

 the electroscope was cut away and replaced by thin aluminum 

 foil. In cases where greater thicknesses of absorber were neces- 

 sary, lead electroscopes having sides respectively 3 mm. and 

 8 mm. thick were employed. The absorbing lead screens were of 

 much larger area than the face of the electroscope and, since they 

 were placed close to the front of this instrument, the greater 

 part of the radiation scattered in a forward direction by the 

 absorber entered the electroscope. 



The experimental data are tabulated in four columns which 

 give respectively, the maximum voltage (79,000 to 196,000 volts), 

 the range of thickness in lead (0.7 mm. to 10.0 mm.), the absorp- 

 tion coefficient /x (27 to 8.5 cm.-^), and the mass absorption coeffi- 

 cient ii/p (2.37 to 0.75). The following facts are brought out 

 by this table. In the first place, the thickness of lead through 

 which the radiation was measurable increased with the voltage 

 applied. This was due not only to the increase in the penetrating 

 power of the radiation but also to the large increase with voltage 

 of the intensity of the radiation. At 196,000 volts the radiation 

 was detected and measured after passing through 10 mm. of 

 lead. The intensity had then fallen to less than the one-millionth 

 part of its initial value. Again, for the end radiations, fx does 

 not change very much between 79,000 {fx ^= 26) and 144,000 volts 

 (/x = 22), and between 105,000 and 144,000 volts /x remains con- 

 stant. "Within the latter range of voltages the radiation is 

 absorbed nearlj^ exponentially with a value of fi equal to 22 cm.~^ 

 Above 144,000 volts the absorption is no longer exponential, but 

 the value of /x decreases progressively with increase of thickness 

 of absorbing layer. For example, at 183,000 volts /x decreases 

 from 26 to 12 as the thickness of the absorber is increased from 

 0.7 mm. to 7.0 mm. These apparently peculiar results are 

 readily explained by taking into account the characteristic 

 absorption band of lead. By assuming that the portion of the 

 graph {x = log jx/p, y ^ log A ; A = wave-length) corresponding 

 to wave-lengths less than those of the absorption band is an 

 approximately straight line parallel to the segment of the locus 

 on the longer wave-length side of the band, the author extrapo- 

 lates to the value /x = 5, for the minimum value associated with 

 196,000 volts. The experimental value was found to be 8.5. 

 Since this last number is known to be too large and as the value 

 5 was obtained from the data of other observers, the author 

 concludes that his results are not inconsistent with the quantum 

 relation e F = /t/Amin This equation has been shown by Hull 

 and Eice to hold up to 100,000 volts and probably as far as 

 150,000 volts. Rutherford's investigation therefore confirms 

 their work and extends the range of validity of the quantum 



