242 Prof. Marsden and Mr. Lantsberry on the 



H particles might be expected to be absorbed. Further, on 

 interposing sheets of aluminium between the source and 

 screen, the curve of absorption was the same, within the 

 experimental error, as the curve for H particles given in 

 the previous paper. These results show that H particles 

 are given off from the source, and that, unlike a particles, 

 their velocities are not uniform, but are distributed in the 

 same manner as those of H particles produced during the 

 ordinary transmission of a particles through hydrogen. 



These results would be explained if we were to assume 

 that there is sufficient hydrogen either in the gas inside the 

 a-ray tube, or in the material of the glass of the tube, or in 

 any water- vapour or hydrogen occluded in the surface of 

 the glass. 



An estimate was therefore made of the amount of hydrogen 

 which would be required to produce the number of H par- 

 ticles observed. A film of wax of thickness about 10//- 

 (stopping power about 1*2 cm. air) was placed round the 

 a-ray tube and comparative measurements were made of the 

 number of H particles with and without the wax. It was 

 found that the presence of the wax practically doubled 

 the number of H particles, showing that about the same 

 amount of hydrogen must be associated with the a-ray tube 

 as with the wax. The mass per unit area of the wax was 

 about 1*0 x 10~ 3 gm. and its formula approximately C 27 H 56 . 

 Consequently the amount of hydrogen per unit area was 

 about 0*15 xlO -3 . The weight of the glass of the a-ray 

 tube per unit area was about 1*6 x 10~ 3 . Consequently, as 

 the glass does not ordinarily contain as much even as 

 0*5 per cent, of hydrogen, we cannot look to the glass as 

 the source of H particles. The mass of hydrogen required 

 represents about 1*6 cm. thickness at N.T.P. This was also 

 verified by actual comparison with the number of H particles 

 produced in a tube containing hydrogen, the results of 

 which showed that the hydrogen associated with the a-ray 

 tube would be equivalent to about 2'0 cm. This amount 

 of hydrogen cannot be contained in the a-ray tube, particu- 

 larly as the emanation was sparked with excess of oxygen 

 before condensation. There remains only the occluded 

 hydrogen or water vapour in the glass, unless we are to 

 assume that the H particles arise in some way from the 

 interior of the radioactive atoms. 



A strong source of radium C deposited on nickel was next 

 used, the nickel being heated to about 150° C. to remove 

 water-vapour. In this case also, H particles were observed, 



