Taylor — Retardation of Alpha Bays by Metals. 371 



In the upper portion, the curve for gold would probably lie 

 within the air curve about the same amount as the air curve 

 does withiu the hydrogen curve, figure 2 ; and in the lower 

 portion the curve for gold would probably lie without the air 

 curve by the same amount as the air curve does without the 

 hydrogen curve. At least some such differences would be in 

 accordance with the square root law, since the square root of 

 the atomic weight of air is a mean proportional between the 

 square root of the atomic weights of gold and hydrogen. The 

 curves for the other metals would occupy intermediate posi- 

 tions between the curves for gold and air. 



We have seen that for different metal sheets of about the 

 same air-equivalents the rates at which the air-equivalents 

 decrease with speed of the alpha particle, are proportional to 

 the square roots of the atomic weights of the respective metals. 

 Consequently the rates of decrease of the ratios of the quan- 

 tities of energy used up in the sheets to the energy that would 

 be consumed by a centimeter of air at the same positions in 

 the path of the particle decreases also as the square roots of 

 the atomic weights of the respective metals. On the basis of 

 our hypothesis that for a given speed of the alpha particle the 

 same amount of energy is required to produce an ion in all 

 substances, and from the results in our experiments, it appears 

 indeed very probable that for the high velocities the alpha 

 particle loses its energy, in going through a substance, more 

 rapidly the higher the atomic weight of the substance ; but as 

 the speed of the alpha particle becomes less this changes, until 

 for the low velocities the loss of the energy of the particle is 

 more rapid the lower the atomic weight of the substance. 



In conclusion I wish to express my gratitude to Professor 

 Bumstead, at whose suggestion these experiments were under- 

 taken, for his valuable suggestions and interest in the work ; 

 also to Professor Bolt wood, who kindly prepared the polonium 

 and secured the radium bromide for me, and gave me many 

 valuable suggestions. 



Summary of Results. 



1. The air-equivalents of metal foils decrease with the speed 

 of the alpha particles entering them. The decrease is very 

 small for thin foils of the lighter metals when the speed of the 

 a-particles is high ; but when the speed is low for thin sheets, 

 or when the sheets are thicker, the change becomes more marked. 

 For different sheets of the same metal the rates of change are 

 proportional to the air-equivalents of the sheets. For sheets of 

 different metals of equal air-equivalents the rates of change 

 are approximately proportional to the square roots of the 

 respective atomic weights. 



