﻿442 Dr. H. Gcigcr and Mr. J. M. Nuttall on the 



in the latter. When filled with hydrogen to 9 cm. pressure 

 the depth o£ the ionization vessel corresponded to 2*5 mm. of 

 air at atmospheric pressure, since the a particles had to travel 

 on an average through about &5 cm. in the electroscope 

 before they were stopped by the upper plate. The ionization 

 at different parts of the range of the a particles could be 

 measured by varying the pressure in the lower bell-jar. 

 When the latter was completely exhausted the range of the 

 a particles on entering the electroscope was only diminished 

 by about 8 mm., the distance corresponding to the stopping- 

 power of the mica. When the pressure in the lower vessel 

 exceeded 25 cm., the a particles were completely stopped 

 and the ionization then observed in the electroscope was due 

 to the natural effect only. By measuring the ionization for 

 different pressures in BB between the above limits a complete 

 ionization curve was obtained. A film of uranium which 

 contained *4 mgr. per square cm. gave an easily measurable 

 effect — about *3 div./min. corrected for natural leak. 



The films used in the experiments were prepared in the 

 following way. A round glass plate of about 15 cm. 

 diameter was carefully ground on one side by means of very 

 fine emery. .Pure uranium oxide which was well powdered 

 was then distributed uniformly over the plate by rubbing- 

 down with the finger. In this way very thin and uniform 

 films were obtained which contained approximately *4 mgr. 

 a square cm. In all cases the uranium X was first separated 

 by the " iron method " in order to avoid any effects due to soft 

 or hard J3 rays. For the purpose of making a comparison 

 under practically identical conditions of the ionization curves 

 of uranium and ionium, a trace of highly concentrated ionium 

 was mixed with about a hundred times the quantity of uranium 

 oxide. The materials were dissolved in acid in order to 

 secure their complete mixture ; the solution was then evapo- 

 rated to dryness and the residue changed into oxide. This 

 material was then distributed on a glass plate in exactly the 

 same way as described above for the pure uranium oxide. 

 On account of the presence of the ionium, this uranium film 

 was now about 30 times as active as an ordinary pure uranium 

 film. Another uranium film containing a trace of polonium 

 was also prepared in a similar way. In this case the activity 

 of the film was about 28 times greater than that of a pure 

 uranium film. This method of preparing the films ensured 

 that the absorption, if any, in the matter itself would have 

 affected the ionization curves in the three cases in exactly 

 the same way. 



