PRESIDENTIAL ADDRESS SECTION B. 55 



required for a complete shell. The analogous cases amongst the 

 higher elements will be discussed later on. 



Disintegration experiments. It hasbeen stated above that the 

 particle which hits a heavy nucleus full-on is reflected back. This 

 is not the case, however, when the nucleus in question is small. In 

 that case the nucleus is carried on in front of the impinghr 

 a-particle and the atom is thus disintegrated. The simplest case is 

 that of hydrogen gas or of organic compounds containing much 

 hydrogen, e.g,, wax; when these are bombarded with a-particles, 

 hydrogen nuclei (i.e., hydrions) are expelled. They are recognised 

 by tiavelling far further than the a-particles themselves can travel 

 against a resistance. Their weight is 1 with a positive charge 1 : 

 their initial velocity is ^/4 times that of the a-particle. Their 

 diameter is said to be about 3 x 10 -13 cm, that of the a-particle 

 being about 5 x 10 -13 cm. Now Rutherford 2 or 3 years years 

 ago made the astonishing discovery that the same kind of particle 

 could be obtained by bombarding nitrogen gas or certain solid 

 compounds of nitrogen with a-particles. We conclude that they 

 came from the nucleus of the nitrogen atom, which thus contains 

 hydrogen nuclei, in a sufficiently separate form for them to be 

 capable of receiving a direct impact from an a particle. Check 

 experiments with C0 2 and 2 as gases, and with silica and graphite 

 as solids gave negative results. A mixture of l\°/ hydrogen gas 

 with 92^% C0 3 or oxygen behaved towards the a-particle exactly 

 like nitrogen gas, giving the same number and intensity of 

 "hydrogen particles." By means of magnetic deviation they were 

 shown to have mass 1 and charge 1. The infrequency of the direct 

 impact required to generate "hydrogen particles" is shown by 

 Rutherford's estimate that only one in 300,000 of the a-particles 

 hits in the required way. Of the other elements tried, boron, 

 fluorine, phosphorus, sodium, and aluminium were also found to 

 contain removable hydrogen, but Li, Be, C, O, Mg, Si, S, gave 

 negative results. It is probable, however, that all the elements 

 are made of hydrogen but contain it in the form of local con- 

 densations of hydrion and electrons which are themselves so stable 

 that they do not break up on impact with an a -particle ; there are 

 two of these, viz., H 3 ++ and H,++. 



Rutherford's discovery of the disintegration of nitrogen was 

 so important that he had to publish it at once without waiting to 

 ascertain what the rest of the nitrogen atom was made of. He 

 therefore announced that the atomic weight 14 belonging to 

 nitrogen probably consisted of two hydrogen atoms combined with 

 three helium atoms. On continuing his experiments, however, he 

 soon found that the hydrogen particles were accompanied bv 

 another set of characteristic particles 5 to 10 times as many in 

 number which differed from any previously known. From the range 

 and mag-netic deviation they were shown to possess mass 3 and 

 charge 2 and thus to be similar to the a-particle with only f of the 

 latter's mass. Nitrogen of mass 14 was thus inferred to consist 

 of 4 particles of mass 3 instead of 3 particles of mass 4 in addition 



