Olli 



Mr. T. S. Tavlor on tlie Ionization of Different 



approximately proportional to the square roots of the 

 respective atomic weights. On the contrary, the air-equiva- 

 lents 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 observed 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 

 polonium 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 between the Bragg ionization curves in air and 



hvdrooen suo-oested that some such differences might be 



" i • • • i ' • i • i 



found between the ionization curves obtained in other gases, 



and it was for the purpose of making a detailed comparison 



of the ionization curves obtained in different gases that the 



present experiments were begun. 



Continuation of Experiments. 



The apparatus used was the same as had been used in the 

 previous experiments*. The sheet-iron case, enclosing the 

 apparatus proper, was replaced by a solid iron case which 

 .could be readily exhausted. Polonium was used, as the 

 source of rays,, and was placed in a brass cylinder of .such 

 dimensions that the rays emerging from -the cylinder fell 

 well within the limits of the ionization chamber for all 

 available distances of the source of rays from the ionization 

 chamber. 



In the determination of the ionization curve in any gas, 

 the vessel enclosing the apparatus was first evacuated and 

 then the gas admitted very slowly till the pressure it exerted 

 was such that the range of the alpha particles was exactly 

 11*1 centimetres, which was the maximum range available 

 with the apparatus. The Bragg ionization curve was then 

 obtained in the usual manner by observing the deflexion of 

 the needle of the Dolezalek electrometer in scale-divisions 

 per second for various distances of the source of rays from 

 the ionization chamber. In this manner, the Bragg ioniza- 

 tion curves were obtained in the gases and vapours given in 

 Table I. The curves in figs. 1 and 2, and the dotted ones 

 in fig. 3 represent the ionization curves obtained in the 

 above manner in the gases as indicated below the figures, 



* Loc. cit. 



