Retardation of a. Particles by Metals. 



189 



others." Information on this point might be obtained from 

 a consideration of the number of atoms in a particular foil 

 which come within the sphere of influence of an a particle 

 traversing the foil. However, no accurate data are available 



Fis. 2. 



561 234 56 l^345 

 Emergent fr/incE in Cms. 



as to the atomic diameter which would of course be required 

 for this calculation. In an experiment we made to throw 

 light on the question, foils of aluminium and gold equivalent 

 to about 1 cm. of air were placed at a definite distance near 

 to the zinc sulphide screen, and their air- equivalents 

 measured (1) when the a particles previous to traversing 

 the foil had passed entirely through air, and (2) when 

 they had previously traversed a gold foil equivalent to 

 2 cm. of air. The results for both metals agreed to within 

 \\ per cent., showing that longitudinal straggling is either 

 not very serious, or that it varies little from gold to air 

 under the experimental conditions. 



If we assume that straggling is not an important factor 

 in the above measurements, we can deduce general ex- 

 pressions for the air-equivalents of any foil at any part of 



