362 Taylor — Retardation of Alpha Rays oy Metals* 



had the same value for all positions of the sheets, and hence 

 their air-equivalents remained constant. 



The behavior of the sheets of paper and celloidin, the 

 atomic weights* of which are about the same as that of air, 

 suggested the idea of undertaking to obtain sheets of some 

 substance such as hydrogen whose atomic weight is less than 

 that of air. For this purpose a ring about one centimeter 

 wide was cut from a brass tube six centimeters in diameter 

 and two small brass tubes were put in the ring diametrically 

 opposite each other. Thin films of celloidin were stretched 

 across each side of the ring and held in place by universal 

 wax. This formed a cell which could be filled with hydrogen 

 and then used in the same manner as the metal foils. To be 

 certain that the cell was always full of hydrogen a slight cur- 

 rent of the gas was kept flowing through it all the time dur- 

 ing an experiment. A current of air was kept circulating 

 through the case surrounding the apparatus in order to pre- 

 vent the hydrogen, that might possibly leak from the cell, from 

 entering the ionization chamber. The air-equivalent of the 

 hydrogen cell or sheet when 0'9 cm from the radium was deter- 

 mined by plotting the ionization curve first with hydrogen and 

 then with air in the cell. The ordinates of the latter curve 

 were all increased by the thickness of the cell of hydrogen, 

 which gave the position of the curve if the cell had been 

 evacuated. The difference between the ordinates of the two 

 curves corresponding to a given abscissa was the air-equiva- 

 lent of the hydrogen sheet. 



When the cell containing the hydrogen was moved away 

 from the radium, which was kept at a given position as in the 

 previous cases, it was found that the ionization decreased, which 

 signified that the total range in air of the alpha particle was 

 less when the hydrogen sheet was far away from the radium 

 than when it was near the radium. Thus the amount by 

 which the range of the alpha particle was cut down by its 

 passage through the cell was greater when the cell was at a 

 distance from the radium than it was when it was near the 

 radium. Consequently the air-equivalent of the hydrogen 

 cell increased as the range of the entering alpha particle 

 decreased. The particles had to pass through the celloidin 

 sheets, but this did not influence the effect because, as we have 

 seen, the amount by which the range was cut down by the 

 celloidin sheets was constant for all positions of the cell. 

 Determinations of the air-equivalents in centimeters of three 

 hydrogen cells given in Table I were made for various dis 

 tances of the cell from the radium and the results obtained 

 are recorded in Table III. 



* By atomic weight of air, paper and celloidin is meant the average 

 weight of the constituent atoms. 



