

616 Dr. H. Geiger and Mr. J. M. Nutta.ll on the Ranges 



pressure. It will be seen that the critical pressure is sharply 

 defined in each case. 



A few details regarding the radioactive substances used in 

 these experiments may be given. 



1. Uranium. — 3 mgr. of uranium oxide were spread 

 uniformly over an area of 2 5 cm. diameter. Experiments 

 made with thicker films gave practically the same result. 



2. Thorium.— It is well known that thorium cannot be 

 separated from radiothorium. Through the kindness of 

 Prof. Boltwood we have, however, obtained some thorium 

 which, on account of frequent precipitations over a period 

 of many years, was repeatedly freed from mesothorium, and 

 thus the radiothorium had practically died away. Imme- 

 diately before the measurements were taken the thorium 

 was precipitated several times in order to get rid of the 

 thorium X and the subsequent products which might have 

 been present. 



3. Thorium and Radiothorium in equilibrium. — Some 

 thorium with the radiothorium in equilibrium was separated 

 from thorite mineral, and by several precipitations freed from 

 thorium X and the following products. The curve shows 

 two distinct breaks corresponding to the a rays from thorium 

 and radiothorium. 



4. Ionium. — The source consisted of a small and hardly 

 visible spot of ionium mixed with thorium on a thin 

 aluminium-foil. The activity due to the thorium was too 

 small to be detected. 



5. Radium. — A radium solution was first freed from 

 polonium, emanation, and active deposit. A drop of the 

 solution was evaporated on a platinum plate and the measure- 

 ments taken immediately. 



6. Polonium.— The polonium films were prepared by 

 bringing the polonium solution in contact with a carefully 

 cleaned copper-foil. By electrolytic action the polonium is 

 then deposited on the copper-foil. 



7. Radiothorium. — Radiothorium was separated from a 

 mesothorium preparation and freed from thorium X and the 

 following products. A thin film was prepared and measure- 

 ments taken at once. 



The results for the different products employed in these 

 measurements are collected in the following table. In each 

 case the average value of all the measurements is given and 

 reduced to a pressure of 76 cm. and a temperature of 15° 

 and 0° centigrade respectively. But it must be remembered 

 that recent investigations have shown that actually only a 

 small fraction of the a particles traverses the whole range. 



