RECENT ADVANCES IN SCIENCE 185 



from their range of penetration and from the brilliancy of the 

 scintillations they produce on a zinc sulphide screen, seem to 

 be hydrogen atoms. The proof of this fact was not complete 

 in 1 91 9, for the crucial test provided by the deflection which 

 the particles undergo in a strong magnetic field had failed to 

 give definite results. However, during the past year the very 

 serious difficulties presented by this experiment have been 

 overcome, and the identity of these particles with hydrogen 

 atoms has been definitely established. 



Success has been attained by viewing the zinc sulphide 

 screen with objectives of wide aperture (whereby the brilliancy 

 of the scintillations is increased and counting made less difficult), 

 and by passing the stream of a-particles through wide slits, 

 so that the number of scintillations due to the gas, as compared 

 with the number due to particles originating at the source, is 

 much increased. By comparing the deflexion of the particles 

 derived from nitrogen with that of hydrogen atoms set in 

 motion by the impact of a-rays travelling through a mixture 

 of hydrogen and carbon dioxide having the same stopping 

 power as nitrogen, it has been shown that the mass of the par- 

 ticles is certainly less than 2 and, within the limits of experi- 

 mental error, equal to i. Thus it is to be inferred that the 

 positively charged atom of hydrogen is one of the components 

 of which the nitrogen nucleus is composed. 



This method of preparing hydrogen is not, of course, likely 

 to form the basis of a manufacturing process, for Rutherford 

 estimates that, if the whole of the a-radiation from i gram 

 of radium was absorbed in nitrogen gas, only about a two- 

 millionth of a cubic centimetre of hydrogen would be pro- 

 duced per year. 



Since the liberation of these particles from nitrogen is a 

 purely atomic phenomenon, similar particles should be emitted 

 from nitrogen compounds in number proportional to the amount 

 of nitrogen present. This has been verified by bombarding 

 a number of compounds rich in nitrogen which had been 

 carefully prepared so as to exclude the presence of hydrogen 

 in any form. The nitrides of boron, sodium, and titanium 

 were used for the experiments, and also para-cyanogen. Of 

 these sodium and titanium nitrides gave the estimated results ; 

 but boron nitride and para-cyanogen gave 1-5-2 times the 

 number of long-range (hydrogen) particles expected. This, 

 of course, might have been due to the presence of hydrogen in the 

 specimens employed, in spite of the care taken to exclude it ; 

 but there is also the possibility that boron itself may emit 

 hydrogen atoms. 



In addition to the long-range hydrogen atoms liberated from 

 nitrogen, the passage of a-particles through oxygen as well as 



