210 SCIENCE PROGRESS 



possibility that in some of these intense and intimate collisions 

 the a particles disintegrate the N atom, causing an ejection from 

 it of an H atom or an atom of mass 2. The evidence is briefly 

 this : Using the apparatus already referred to, with the air 

 exhausted from the box, and absorbing foils of aluminium 

 being employed to stop the various flying particles instead of 

 air, one finds scintillations far beyond the range of the a par- 

 ticles. The swift atoms causing these scintillations carry a 

 positive charge, are deflected in a magnetic field, and have 

 about the same range and energy as the swift H atoms pro- 

 duced by the passage of a particles through hydrogen as already 

 described. These " natural " scintillations are believed, as 

 Marsden suggested, to be due to swift H atoms from the radio- 

 active source, but it is difficult to decide whether they are 

 expelled from the radio-active source itself, or are due to the 

 action of a particles on occluded hydrogen. If dried oxygen 

 or carbon dioxide is admitted into the vessel, there is a diminu- 

 tion in the number of scintillations of an amount to be expected 

 from the stopping power of the column of gas. When dry air 

 is admitted, however, the surprising effect is observed of an 

 increase in the number of scintillations ; in fact, at a distance 

 from the source equivalent to 19 cms. of air the number is about 

 doubled. As already pointed out, in air swift N and O particles 

 arise with a possible range of 9 cms. ; but the increased number 

 of scintillations referred to above is observed at much greater 

 distances than 9 cms., and cannot be ascribed to that cause. 

 A number of control experiments were made ruling out the possi- 

 bility of water vapour, or even the minute traces of hydrogen 

 in the air, as being the source. Pure dry nitrogen, when intro- 

 duced into the vessel instead of the dry air, still further increased 

 the number of scintillations, as was to be expected if they are 

 due to nitrogen. These long-range scintillations appear to be 

 due to the collision of a particles with nitrogen atoms, the 

 particle causing scintillation being a hydrogen nucleus or atom 

 of mass 2 expelled from the N atom. Such a revolutionary 

 conclusion, of course, will require careful investigation, and 

 further experiments are suggested to test by various methods 

 whether these particles are really H atoms, and are derived from 

 the N atoms. One anomaly in the third paper would receive 

 an explanation if this were so. In the case of swift N and O 

 particles, they appear to have a common maximum range of 



