a Particles with Light Atoms. 585 



pressure, we may consequently conclude that only one long- 

 range atom is produced for every 12 close collisions giving 

 rise to a swift nitrogen atom of maximum range 9 cm 



It is of interest to give data showing the number of long- 

 range scintillations produced in nitrogen at atmospheric 

 pressure under definite conditions. For a column of nitrogen 

 3*3 cm. long, and for a total absorption of 19 cm. of air from 

 the source, the number due to nitrogen per milligram of 

 activity is *6 per minute on a screen of 314 sq. mm. area. 



Both as regards range and brightness of scintillations, the 

 long-range atoms from nitrogen closely resemble H atoms, 

 and in all probability are. hydrogen atoms. In order, how- 

 ever, to settle this important point definitely, it is necessary 

 to determine the deflexion of these atoms in a magnetic field. 

 Some preliminary experiments have been made by a method 

 similar to that employed in measuring the velocity of the 

 H atom (see paper II.). The main difficulty is to obtain a 

 sufficiently large deflexion of the stream of atoms and yet 

 have a sufficient number of scintillations per minute for 

 counting. The a rays from a strong source passed through 

 dry air between two parallel horizontal plates 3 cm. long and 

 1*6 mm. apart, and the number of scintillations on the screen 

 placed near the end of the plates was observed for different 

 strengths of the magnetic field. Under these conditions, 

 when the scintillations arise from the whole length of the 

 column of air between the plates, the strongest magnetic 

 field available reduced the number of scintillations by only 

 30 per cent. When the air was replaced by a mixture of 

 carbon dioxide and hydrogen of the same stopping power for 

 a rays, about an equal reduction was noted. As tar as the 

 experiment goes, this is an indication that the scintillations 

 are due to H atoms ; but the actual number of scintillations 

 and the amount of reduction was too small to place much 

 reliance on the result. In order to settle this question 

 definitely, it will probably prove necessary to employ a 

 solid nitrogen compound, free from hydrogen, as a source, 

 and to use much stronger sources of a rays. In such expe- 

 riments, it will be of importance to discriminate between 

 the deflexions due to H atoms and possible atoms of atomic 

 weight 2. From the calculations given in paper III., it 

 is seen that a collision of an a particle with a free atom of 

 mass 2 should give rise to an atom of range about 32 cm. in 

 air, and of initial energy about '89 of that of the H atom 

 produced under similar conditions. The deflexion of the 

 pencil of these rays in a magnetic field should be about *6 of 

 that shown by a corresponding pencil of H atoms. 



Phil. Mag. S. 6. Vol. 37. No. 222. June 1919. 2 S 



