ii8 



NATURE 



[September 23, 1920 



interest in this work is that the new particles come 

 off with greater energy than that of the bombard- 

 ing o-particles. If we carry over the language of 

 chemistry to this new domain we should say that 

 the reaction is exothermic, and nitrogen in a 

 metastable state. 



We have so far discussed the special properties 

 of particular elements. The next evidence is very 

 much more general, and holds out great promise 

 of extension. This is the recent work of Aston 

 with the "positive rays." It was undertaken in 

 order to detect whether ordinary elements were 

 composed of isotopes. He found that this was 

 true of the majority of those he examined (his 

 numerical results are given in the table in the 

 first article), but the important result from our 

 present point of view is that in all his elements 

 the atomic weight of each isotope was almost 

 exactly an integer. This will be slightly qualified 

 in the next paragraph. Aston has so far examined 

 eighteen elements and found the rule true, and 

 it is scarcely credible, therefore, that it should not 

 be universal. But if so, it can only mean that all 

 nuclei are constructed out of hydrogen nuclei. 

 That would satisfy the conditions as to mass, and 

 in order to make the atomic number correct we 

 must then suppose that these hydrogen nuclei are. 

 cemented together by electrons. For example, 

 chlorine has atomic number 17. Its atomic 

 weight, determined chemically, is 35-5, and Aston 

 finds that it is composed of two isotopes at 35 

 and 37. Therefore three-quarters of the chlorine 

 atoms have nuclei composed of 35 hydrogen 

 nuclei and 18 electrons, while for the remaining 

 quarter the numbers are 37 and 20 respectively. 

 This is the revived form of the famous hypothesis 

 of Prout. 



Aston 's experiments have not hitherto been 

 sufficiently accurate to detect any departures from 

 the whole number rule, except in the case of 

 hydrogen. In this case his work confirms the 

 chemical result that the atomic weight is i-oo8 

 instead of being i. .\t first sight this would 



appear to dispose entirely of the validity of our 

 interpretation of the whole number rule, but that 

 is not so, and by the use of arguments based on 

 the work of Einstein on relativity, the departure 

 from whole numbers can be made to yield valuable 

 information. Einstein's theory implies that all 

 energy must have mass (and also that all mass 

 must have weight). Thus the mass of a helium 

 nucleus (at. no. 2 and at. wt. 4) will not be simply 

 the sum of the masses of its four a-particles and 

 two electrons, but will also include the mass of 

 its energy of formation. As the helium is lighter 

 than the sum of the weights of these particles, we 

 conclude that this energy is negative- — that is to 

 say, an atom of helium has less energy than its 

 separated particles. Lenz has calculated the 

 difference and finds that it corresponds to the 

 energy of about three of the fastest o-particles. 

 So helium is an exceedingly stable substance, and 

 it need create no surprise that its nuclei, when 

 projected as o-particles, can undergo the most 

 violent collisions without disruption. It also 

 makes it not unnatural that in radio-active disin- 

 tegration it is helium, and not hydrogen, that is 

 emitted. Moreover, it is well known that the 

 atomic weights of most elements are nearer whole 

 numbers when helium is taken as standard at 4 

 than when hydrogen is taken as i. This indicates 

 that in most atoms the majority of the hydrogen 

 nuclei are bound together in helium sub-groups. 



It will thus be seen that a very promising 

 beginning has been made in the study of the 

 nucleus. We know that it is built up of hydrogen 

 nuclei and electrons, and that there is a strong 

 tendency to form helium sub-groups. We know 

 the number of hydrogen nuclei and electrons in the 

 nuclei of many of the elements, and may expect 

 to know those of the majority at no distant date. 

 Most encouraging of all, we may hope that it will 

 not be very long before a definite theory of the 

 structure of nuclei is made, based on exact know- 

 ledge of the energy of formation of the various 

 elements. 



Obituary. 



The Right Hon. Sir William Mather. 



WE deeply regret to record the death of 

 Sir William Mather, at his residence in the 

 New Forest, on Saturday last, September 18. 

 Sir William was born in Manchester in 1838, and 

 educated at private schools, 'and his studies were 

 continued- under one of his English tutors at 

 Dresden. Afterwards he entered his father's en- 

 gineering firm, well known as the Salford Iron 

 W^orks, and worked the usual hours of apprei^tices, 

 in the evenings attending lectures at the Owens 

 College, Manchester. In 1862, when he was 

 twenty-four years of age, he became solely respon- 

 sible for the management of the business. The 

 firm enjoyed great repute as makers of bleaching, 



NO. 2656, VOL. I06I 



dyeing, finishing, and other textile machinery, ex- 

 porting its products abroad, especially to Russia 

 and the United States, in which countries Sir 

 William travelled extensively. He took the 

 greatest interest in the well-being of his em- 

 ployees, his firm being among the first to establish 

 the eight hours' day, and providing for his young 

 workers, more than forty years ago, the means 

 of continued instruction in the fundamental prin- 

 ciples of engineering science, at the hands of his 

 chief technical officers, and with striking results, 

 many of the students gaining ^^'hitworth and 

 other science scholarships. 



Sir William Mather took a deep interest in local 

 affairs and movements, was a member of the 



