538 Sir E. Rutherford on Collision of 



between filled with compressed hydrogen. It was found that 

 the number of H scintillations fell off approximately according 

 to an exponential law when absorbing screens of matter were 

 interposed, and the relative absorption of metal foils was in 

 good accord with the square root law observed by Bragg for 

 a particles. 



In a second paper, Marsden * showed that the a-ray tube 

 itself gave rise to a number of scintillations like those from 

 hydrogen. Similar results were observed with an a-ray tube 

 made from quartz instead of glass, and also with a nickel 

 plate coated with radium C. The number of H scintillations 

 observed in all cases appeared to be too large to be accounted 

 for by the possible presence of hydrogen in the material, 

 and Marsden concluded that there was strong evidence that 

 hydrogen arose from the radioactive matter itself. Further 

 experiments were interrupted by the departure of Mr. Marsden 

 to New Zealand early in 1915 to fill the Professorship of 

 Physics in Victoria College, Wellington. The quantity of 

 radium available there was too small to continue observations, 

 while the possibility of further work was precluded by the 

 return of Professor Marsden to Europe on Active Service. 



We have seen that Marsden in his second paper had some 

 indications that the radioactive matter itself gave rise to swift 

 H atoms. This, if correct, was a very important result, for 

 previously the presence of no light element except helium 

 had been observed in radioactive transformations. 



It was thought desirable to continue these experiments in 

 more detail, and during the past four years I have made a 

 number of experiments on this point and on other interesting- 

 problems that have arisen during the progress of the work. 

 The experiments recorded in this and subsequent papers have 

 been carried out at very irregular intervals, as the pressure 

 of routine and war- work permitted, and in some cases 

 experiments have been entirely dropped for long intervals. 



§ 2. Source of the scintillations from active matter. 



Marsden had observed that the number of H scintillations 

 from a nickel plate, coated with radium C, w^as considerably 

 greater than for a corresponding quantity of emanation — 

 measured by <y rays — from an a-ray tube. It thus seemed 

 possible that H atoms might arise from the disintegration of 

 radium C, for it is well known that this product is trans- 

 formed in an anomalous manner. In order to test this 

 point, observations were made on the variations of the 

 * Marsden, Phil. Mag. xxx. p. 240 (1915). 



