March 3, 1922] 



SCIENCE 



231 



was about 107°. This fact, together with the 

 observed relative length of the spnr and the 

 track of the del^eeted a particle seems to show 

 that the spin' was due to an oxygen atom re- 

 coiling imder close imisact with the alpha par- 

 ticle. The Shimizu branclied ti'acks, however, 

 appear to be similar to wliat one would expect 

 to get, on the basis of Darwin's calcidations, 

 in a closed collision between an a particle and 

 the nucleus of a helium atom. 



This idea naturally suggests that we have in 

 the Shimizu branched tracks examples ot the 

 disruption of nuclei with the liberation of 



+-(■ 

 He^ or alpha particles. If this conjecture 



should turn out to be correct it would indicate 

 that a particles can exist as definite units 

 within the nuclei of atoms of one or more of 

 the gases which make up air. It would be of 

 interest to see if the Shimizu tracks can be ob- 

 tained in pure nitrogen and also in pure 

 oxygen and otiier simple gases. Since a par- 

 ticles are Icnown to exist at definite units 

 within the nuclei of the atoms of the I'adio- 

 aetive elements, it would not be surprising to 

 iind their occurrence in the nucleus of an 

 element such as oxygen. It would be of special 

 interest, however, to find out tlie lightest atom 

 other than that of helium in the nucleus of 

 which the a particle exists as a unit. 



(fc) Models of atomic nuclei. 



It is difficult with the present state of our 

 knowledge to go into details I'egarding the pos- 

 sible structure of the nuclei of even the lighter 

 and presumably less complex atoms. It would 

 seem, however, that there is strong evidence 

 for the view that among the possiI)le units or 



Tig. 3 



Helium Nucleus 



Fig. 4 

 Carbon Nucleus 



structural bricks out of which nuclei are con- 



+ 

 strueted are protons (H^ ) and a particles 



++ 

 (He^ ). There is also some evidence that the 



++ 

 particle (He^ ), i. e., the nucleus of a tripro- 



tonic isotope of helium can exist as a distinct 

 elementary unit in the nuclei of some types of 

 atom. With such or somewhat similar combining 

 units, attempts have been made by Harkins'''^ 

 to work out a constitutional formula applica- 

 ble to the nuclei of all the elements. The 

 validity of such generalizations can be firmly 

 established only through elaborate and varied 

 experiments, but in the meantime they can at 

 least serve as guides in arranging schemes of 

 attack for iDrospective experimental work. 



A rather suggestive set of models of the 

 atomic nuclei of helium, carbon, nitrogen, and 

 oxygen, based on the ideas of Rutherford is 



shown in Figs. 3, 4, 5, and G. In these, the 



+ ++ ++ 



particles Hj , He„ and He^ are utilized as 



constituent units. Similar models can be easily 



made for the nuclei of the atoms of other 



elements. From these models one would 



expect to find He,, particles released by the 



1 • n ^ ++ + 



disruption of carbon atoms, He, and H^ 



particles when nitrogen atoms are broken up 

 and Hbj as well as He^, particles when oxy- 

 gen nuclei are disintegrated. It will be seen 

 that the models provide the requisite masses 

 and resultant electric charges for the nuclei 

 they represent. In so far as the nuclei of 

 helium, nitrogen and oxygen atonrs are con- 

 cerned the constitution presented Vi'ould seem 



Harkins, Phys. Sev., Vol. IC 



1920. 



Fig. 5 



Nitrogen Nucleus 



Fig. 6 



Oxygen Nucleus 



