Retardation of a Particles hy Metals. 185 



The results also showed that the rates at which the air- 

 equivalents varied with the velocity of the a particles were 

 functions of the atomic weight— the rate of variation for 

 gold, for example, being greater than the variation for 

 aluminium. 



The method employed in these experiments consisted 

 in the observation of the ionization current between two 

 parallel sheets of gauze placed normal to a pencil of 

 a particles and near the end of the range. It is known 

 that the ionization changes rapidly with distance near the 

 end of the range ; and the alteration in air-equivalent of a 

 foil when it was moved between the source and the ioniza- 

 tion-chamber was deduced from the change in the ionization 

 current. In these experiments, however, the amount by 

 which the foil could be moved between the source and the 

 end of the range was limited by mechanical difficulties, and 

 the measurements were not made at the beginning or end of 

 the range. In particular, they were not extended to cases 

 where the range of the particles left after traversing the foil 

 under investigation was greater than about 9 mm. More- 

 over, it is known that the velocity of an a particle falls 

 very rapidly in the last centimetre of range, so that obser- 

 vations in this portion are important. Further, the 



ionization method of measuring ranges is somewhat affected 



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by the scattering of a particles, which becomes very pro- 

 nounced for low velocities. 



It seemed to us, therefore, that the scintillation method, 

 by which the individual a particles themselves are observed, 

 is in some respects more direct for such measurements. 

 Further, the scintillation method is often more convenient 

 for determining the air-equivalents of metal foils, and it 

 seemed of value to determine absolutely the connexion 

 between mass per unit area and air-equivalent for different 

 foils and different velocities of a particles. 



The apparatus used is shown in fig. 1. It consisted 

 essentially of a travelling microscope M, with a zinc- 

 sulphide screen Z, attached rigidly to the objective, so 

 as to be in focus. As source of a particles a conical 

 tube A containing about 1 millicurie of radium emanation 

 was used. The end of the tube was closed air-tight by 

 a thin mica window fused on to the tube by AgCl. The 

 air-equivalent of the mica was about "8 cm., so that a 

 pencil of the Ra C a particles emerging from the tube had 

 a range of about 6'3 cm. A metal foil F, whose air- 

 equivalent was under investigation, could be placed on a 



