THE 



AMERICAN JOURNALOFSCIENCE 



[FOURTH SERIES.] 



Art. XXIV. — On the Ionization of Different Gases by the 

 Alpha Particles from Polonium and the Relative Amounts 

 of Energy Required to Produce an Ion ; by T. S. Taylor. 



Introduction. 



In previous papers,* the writer has shown that the air-equiv- 

 alents f of metal foils decrease with the speed of the alpha par- 

 ticles entering the foils. For sheets of different metals of 

 equal air-equivalents, the rates of decrease are approximately 

 proportional to the square roots of the respective atomic 

 weights. On the contrary, the air-equivalents of hydrogen 

 sheets increase while the hydrogen-equivalents of air sheets 

 decrease with the speed of the entering alpha particles, and at 

 such a rate as to be in agreement with the square-root law ob- 

 served for the decrease of the air-equivalents of the metal 

 sheets. 



A comparison of the Bragg ionization curves, obtained in 

 atmospheres of air and hydrogen, when the pressure of the air 

 was so reduced that the range of the alpha particles from polo- 

 nium was the same in air as it was in hydrogen at atmospheric 

 pressure, showed differences which are sufficient to account for 

 the variations in the air-equivalents of the hydrogen sheets 

 with the speed of the alpha particles. These differences be- 

 tween the Bragg ionization curves in air and hydrogen sug- 

 gested that some such differences might be found between the 

 ionization curves obtained in other gases, and it was for the 

 purpose of making a detailed comparison of the ionization 



* This Journal, vol. xxvi, pp. 169-179, Sept. 1908; ibid., vol. xxviii, pp. 

 357-372, Oct. 1909. Phil. Mag., vol. xviii, p. 604, Oct. 1909. 



f By air-equivalent is meant the amount by which the range of the alpha 

 particle is cut down by its passage through the foil. 



Am. Jour. Sci.— Fourth Series, Vol, XXXI, No. 184.— April, 1911. 

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