576* Sir E. Rutherford on Collision of 



pressure and 15° C. The scintillations, presumably due to 

 swift N and atoms, are bright and easily counted for a 

 total absorption corresponding to about 7*5 cm. of air. At 

 this stage they appear equal in brightness to those given by 

 an a particle of range about 1 cm. 



In other experiments with air, nitrogen, and oxygen, and 

 carbon dioxide, the screen and source were placed in a 

 rectangular box and a slow current of the dried gas passed 

 through during the experiment. This prevented contami- 

 nation of the screen by diffusion of active matter from the 

 source, and the range was determined by altering the distance 

 between source and screen. 



The scintillations in pure oxygen and carbon dioxide were 

 about the same brightness for corresponding ranges, and had 

 nearly the same equivalenl ranges in air as those due pre- 

 sumably to N atoms from the air. 



This was rather surprising, as we should expect the 

 atoms to have considerably less range than the lighter 

 N atom. The calculated ranges (see table above) are 7*8 

 and 9*3 cm. respectively. This suggested the possibility 

 that the scintillations might be due not to N or atoms but 

 to actual « particles of range 9 cm. which were expelled 

 from the radioactive source. If this were the case, the total 

 range of the a particles should not be altered by placing an 

 absorbing screen of aluminium or gold of known stopping- 

 power close to the source in the path of the « rays. On the 

 other hand, if the scintillations were due to swift N or 

 atoms from the air, the range should be diminished. For 

 example, if a screen of stopping power equal to 3*5 cm. of 

 air were placed in the path of the a rays of range 7'0 cm., 

 the resulting range of the « particles acting on the gas is 

 3' 5 cm., and the total range of the N or atoms measured 

 from the source should be 3*5+ f x3'5 = 8"0 cm. instead of 

 9*0 cm. Experiments of this kind were made with an 

 aluminium and a gold screen of stopping powers 3*7 and 

 1*2 cm. respectively, but were not altogether satisfactory 

 on account of the inequalities of the films already referred 

 to. They showed, however, that no appreciable number of 

 scintillations could be detected beyond 8 cm. The results 

 indicated that the scintillations were due to atoms of N and 

 and not to a particles from the source. This was further 

 confirmed by experiments with mica screens of stopping 

 power 7*0 cm. The number of bright scintillations which 

 resembled a particles were less than half the number observed 

 in air or oxygen gas under similar conditions, but the 

 presence of numerous H atoms from the mica interfered 



