ELECTRICAL STRUCTURE OF MATTER RUTHERFORD 177 



Of particles are emitted and six electrons, and it seems clear that the 

 nucleus of a heavy atom is built up, in part at least, of helium 

 nuclei and electrons. It is natural to suppose that many of the 

 ordinary stable atoms are constituted in a similar way. It is a 

 matter of remark that no indication has been obtained that the 

 lightest nucleus, viz, that of hydrogen, is liberated in these trans- 

 formations, where the processes occurring are of so fundamental a 

 character. At the same time, it is evident that the hydrogen nucleus 

 must be a unit in the structure of some atoms, and this has been 

 confirmed by direct experiment. Mr. Chadwick and I have ob- 

 served that swift hydrogen nuclei are released from the elements 

 boron, nitrogen, flourine, sodium, aluminium, and phosphorus when 

 they are bombarded by swift a particles, and there is little room 

 for doubt that these hydrogen nuclei form an essential part of the 

 nuclear structure. The speed of ejection of these nuclei depends 

 on the velocity of the a particle and on the element bombarded. 

 It is of interest to note that the hydrogen nuclei are liberated in all 

 directions, but the speed in the backward direction is always some- 

 what less than in the direction of the a particle. Such a result 

 receives a simple explanation if we suppose that the hydrogen nuclei 

 are not built into the main nucleus but exist as satellites probably 

 in motion round a central core. There can be no doubt that bom- 

 bardment by a particles has effected a veritable disintegration of the 

 nuclei of this group of elements. It is significant that the liberation 

 of hydrogen nuclei only occurs in elements of odd atomic number, 

 viz, 5, 7, 9, 11, 13, 15, the elements of even number appearing quite 

 unaffected. For a collision of an a particle to be effective, it must 

 either pass close to the nucleus or actually penetrate its structure. 

 The chance of this is excessively small on account of the minute 

 size of the nucleus. For example, although each individual a par- 

 ticle will pass through the outer structure of more than 100,000 

 atoms of aluminium in its path, it is only about one a particle in a 

 million that gets close enough to the nucleus to effect the liberation 

 of its hydrogen satellite. 



This artificial disintegration of elements by a particles takes place 

 only on a minute scale, and its observation has only been possible by 

 the counting of individual swift hydrogen nuclei by the scintilla- 

 tions they produce in zinc sulphide. 



These experiments suggest that the hydrogen nucleus or proton 

 must be one of the fundamental units which build up a nucleus, and 

 it seems highly probable that the helium nucleus is a secondary 

 building unit composed of the very close union of four protons and 

 two electrons. The view that the nuclei of all atoms are ultimately 

 built up of protons of mass nearly one and of electrons has been 

 strongly supported and extended by the study of isotopes. It was 



