812 Sir E. Rutherford and Dr. J. Chad wick on the 



In genera], absorbing screens of mica were inserted in 

 front of the zinc-sulphide screen so as to make the total 

 absorption in the path of any particles liberated from the 

 bombarded material equivalent to 32 cm. of air. Control 

 experiments were made (1) by placing a sheet of paraffin 

 wax 25 /J, thick at F, (2) by passing a mixture of C0 2 and 

 H 2 through the apparatus, the source being uncovered. It 

 was found that no H particles could be detected through an 

 absorption of more than 29 cm. of air. Hence the results 

 obtained for the materials under examination were inde- 

 pendent of contamination with H or any H compound. 



The following table contains in the first column a list of 

 the elements, from lithium to sulphur inclusive, which have 

 been examined in this way ; and the second column shows 

 the material actually used in the experiments. 



The third column gives the number of scintillations per 

 minute per mgm. activity of the source observed at an 

 absorption of 32 cm. of air under certain conditions. These 

 numbers afford only a rough comparison of the effects given 

 by different elements, for the conditions of experiment, e. g. 

 the thickness and distribution of the film of material, varied 

 from element to element. The fourth column gives the 

 approximate maximum range of the particles. 



Maximum range 



No. of particles of particles, 



Element. Material. per min. per mgm. in cm. of air. 



Lithium Bi 2 — — 



Beryllium BeO — — 



Boron B '15 ca. 45 



Carbon C0 2 — — 



Nitrogen Air '7 40 



Oxygen 2 — — 



Fluorine CaF 2 -4 over 40 



Sodium Na 2 -2 ca. 42 



Magnesium MgO 



Aluminium Al, A1 2 3 M 90 



Silicon Si 



Phosphorus P (red) '7 ca. 65 



Sulphur S, S0 2 



In addition to these, the following elements of higher 

 atomic weight were examined : chlorine as Mg01 2 ; potas- 

 sium as KC1 - ; calcium as CaO ; titanium as Ti 2 3 ; man- 

 ganese as Mn0 2 ; iron, copper, tin, silver, and gold in the 

 form of metal foils. In no case were any particles observed 

 of range greater than 32 cm. of air. The question as to 

 whether any of these elements give particles of range less 

 than 32 cm. has not yet been examined. 



