« Particles with Light Atoms. 583 



were introduced in place of dry air, the number of scintil- 

 lations was much less than with dry air. 



It is well known that the amount of hvdrogen or gases 

 containing hydrogen is normally very small in atmospheric 

 air. No difference was observed whether the air was taken 

 directly from the room or from outside the laboratory or was 

 stored for some days over water. 



There was the possibility that the effect in air might be due 

 to liberation of H atoms from the dust nuclei in the air. No 

 appreciable difference, however, was observed when the dried 

 air was filtered through long plugs of cotton-wool, or by 

 storage over water for some days to remove dust nuclei. 



Since the anomalous effect was observed in air, but not in 

 oxygen, or carbon dioxide, it must be due either to nitrogen 

 or to one of the other gases present in atmospheric air. The 

 latter possibility was excluded by comparing the effects 

 produced in air and in chemically prepared nitrogen. The 

 nitrogen was obtained by the well-known method of adding 

 ammonium chloride to sodium nitrite, and stored over water. 

 It was carefully dried before admission to the apparatus. 

 With pure nitrogen, the number of long-range scintillations 

 under similar conditions was greater than in air. As a result 

 of careful experiments, the ratio was found to be 1*25, the 

 value to be expected if the scintillations are due to nitrogen. 



The results so far obtained show that the long-range 

 scintillations obtained from air must be ascribed to nitrogen, 

 but it is important, in addition, to show that they are dne to 

 collision of a particles with atoms of nitrogen through the 

 volume of the gas. In the first place, it was found that 

 the number of the scintillations varied with the pressure 

 of the air in the way to be expected if they resulted from 

 collision of a particles along the column of gas. In addition, 

 when an absorbing screen of gold or aluminium was placed 

 close to the source, the range of the scintillations was found 

 to be reduced by the amount to be expected if the range of 

 the expelled atom was proportional to the range of the 

 colliding a particles. These results show that the scintil- 

 lations arise from the volume of the gas and are not due to 

 some surface effect in the radioactive source. 



In fig. 1 curve A the results of a typical experiment are 

 given showing the variation in the number of natural scintil- 

 lations with the amount of absorbing matter in their path 

 measured in terms of centimetres of air for a particles. In 

 these experiments carbon dioxide was introduced at a pressure 

 calculated to give the same absorption of the a rays as ordinary 

 air. In curve B the corresponding curve is given when air 



