536 SCIENCE PROGRESS 



decidedly smaller, 6^ cms. as against 90 cms. In the case of 

 nitrogen this backward ejection was scarcely observable, being 

 certainly less than 2 per cent, of the forward. Similar experi- 

 ments on the remaining four elements have not yet been made. 

 In considering why some of the elements should show this 

 effect and others in neighbouring places of the periodic table 

 should not, it is a striking fact, as Rutherford points out, that of 

 the elements so far examined only those whose atomic mass is 

 given by 4w + 2 or 4M +3, where « is a whole number, give rise 

 to ejected H atoms. The explanation offered is that the nuclei 

 of these elements are built up of helium nuclei of mass 4 

 and hydrogen nuclei. The study of radio-active transforma- 

 tions has already brought forward powerful evidence for the 

 view that the helium nucleus is an important unit of atomic 

 structure, in the case of the very heavy elements at all events. 

 In the case of elements of atomic mass 4M, there are no extra 

 hydrogen nuclei present in the nucleus of the element, unless 

 the helium nuclei themselves are composed of 4 hydrogen nuclei 

 and 2 electrons ; but if so, the structure of the helium nucleus 

 is so stable that even the intense bombardment by swift 

 a-particles is unable to break it up, and so no H atoms are ejected 

 from such atoms. Where, on the other hand, there are present 

 extra hydrogen nuclei, these can be broken away from the main 

 nucleus and endowed with a considerable portion of the mo- 

 mentum and energy of the impinging a-particles. Of course, 

 statistically very few indeed of the atoms of the bombarded 

 substance suffer such a direct blow as to have their structure 

 so shattered. Thus it appears that two a-particles only in one 

 million are able to liberate a swift H atom. Rutherford regards 

 the H atoms as being " satellites " to the main part of the nucleus 

 consisting of a whole number of helium nuclei. We are, of 

 course (as reference to the earlier work mentioned in the first 

 paragraph will show), concerned with distances of separation of 

 the order of the size of an electron or a nucleus itself, viz. io~^3 cm.; 

 at such distances the ordinary law of force no longer holds ; 

 and these positively-charged nuclei are actually assumed to 

 attract, so that the H satellites execute orbits around the main 

 nucleus from which they are ejected by an a-particle which 

 chances to come near enough, and for which the direction of 

 ejection is in consequence somewhat fortuitous. In a former 

 number of Science Progress (50, Oct. 191 8) will be found in 

 this section a reference to some work of Prof. Nicholson on the 

 modification of the law of force between two minute electric 

 charges which, while conforming to the law of inverse squares 

 for large enough distances, would give an actual inversion of 

 sign followed, perhaps, by complete coalescence of two similar 

 charges if brought sufficiently near. 



