2 o6 SCIENCE PROGRESS 



PHYSICS. By James Rice, M.A., University, Liverpool. 



Prof. Sir Ernest Rutherford has contributed four papers 

 to the June Phil. Mag. y dealing with the collision between a 

 particles and light atoms. These papers contain results for 

 hydrogen, oxygen and nitrogen atoms, and in the case of the 

 last, evidence is advanced in favour of the view that in certain 

 cases disintegration of the nitrogen atom results from collision 

 with a particles. 



In the experiments the source of radiation was a small brass 

 disc which had been exposed for a few hours, with an assisting 

 electric field, to radium emanation in the usual manner. This 

 renders the disc active, and twenty minutes after removal from 

 the emanation, the a radiation arises entirely from radium C, 

 and is homogeneous with a range of 7 cms. in air. Such an 

 active disc was mounted in a tube which could be exhausted 

 and filled with the gas under consideration. A small opening 

 at one end was covered with a thin plate of a metal such as 

 silver, aluminium, or iron. A zinc sulphide screen was mounted 

 just outside this cover, about 1 mm. away, and viewed by a 

 microscope, so that scintillations on the screen could be noted 

 and counted in the usual way. 



In order that none of the scintillations on the screen should 

 be due to impact of the a particles themselves on the zinc 

 sulphide crystals, it was necessary to ensure that the screen was 

 beyond the range of these particles — i.e., that the combined 

 stopping power of the gas in the tube and the thin metal plate 

 covering the opening should be at least equivalent to 7 cms. of 

 air. Under these circumstances scintillations still exist, as was 

 shown by Dr. Marsden, one of Prof. Rutherford's co-workers 

 (Phil. Mag., xxvii, 19 14). These scintillations are due to the 

 nuclei of light atoms set into swift motion by intimate collisions 

 with a particles, which thus acquire velocities that carry them 

 in some instances into impact with a luminescent screen placed 

 far beyond the range of the a particles. Thus Marsden was 

 able to detect scintillations presumably due to hydrogen nuclei 

 (set in motion by the a particles from radium C) which had 

 travelled over 100 cms. in that gas from the place of impact, 

 a distance nearly four times as great as the range of a particles 

 in hydrogen, viz. 28 cms. There was evidence also in Marsden's 

 opinion that hydrogen nuclei are ejected by the radio-active 



