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LIV. Collisio?i of a Particles with Light Atoms. IV. An 

 Anomalous Effect in Nitrogen. By Professor Sir 

 E. Rutherford, F.E.S.* 



IT has been shown in paper I. that a metal source, coated 

 with a deposit o£ radium C, always gives rise to a number 

 of scintillations on a zinc sulphide screen far beyond the range 

 of the a particles. The swift atoms causing these scintillations 

 carry a positive charge and are deflected by a magnetic field, 

 and have about the same range and energy as the swift H atoms 

 produced by the passage of a particles through hydrogen. 

 These " natural " scintillations are believed to be due mainly 

 to swift H atoms from the radioactive source, but it is difficult 

 to decide whether they are expelled from the radioactive 

 source itself or are due to the action of a particles on occluded 

 hydrogen. 



The apparatus employed to study these " natural" scintil- 

 lations is the same as that described in paper I. The intense 

 source of radium C was placed inside a metal box about 3 cm. 

 from the end, and an opening in the end of the box was 

 covered with a silver plate of stopping power equal to about 

 6 cm. of air. The zinc sulphide screen was mounted outside, 

 about 1 "mm. distant from the silver plate, to admit of the 

 introduction of absorbing foils between them. The whole 

 apparatus was placed in a strong magnetic field to deflect the 

 8 rays. The variation in the number of these " natural " 

 scintillations with absorption in terms of cms. of air is shown 

 in fig. 1, curve A. In this case, the air in the box was 

 exhausted and absorbing foils of aluminium were used. When 

 dried oxygen or carbon dioxide was admitted into the vessel, 

 the number of scintillations diminished to about the amount 

 to be expected from -the stopping power of the column of 

 gas. 



A surprising effect was noticed, however, when dried air 

 was introduced. Instead of diminishing, the number of 

 scintillations was increased, and for an absorption corre- 

 sponding to about 19 cm. of air the number was about twice 

 that observed when the air was exhausted. It was clear from 

 this experiment that the a particles in their passage through 

 air gave rise to long-range scintillations which appeared to 

 the eye to be about equal in brightness to H scintillations. 

 A systematic series of observations was undertaken to account 

 for the origin of these scintillations. In the first place we have 

 seen that the passage of a particles through nitrogen and 



* Communicated bv the Author. 



