2-, 2 



NATURE 



[November 6, igig 



was elucidated by means of the X-rays, these 

 very models were used to represent it — a striking 

 proof, surely, of the basis of physical reality 

 underlying the conceptions of stereo-chemistry. 



But these triumphant vindications of what only 

 a generation ago were described as purely hypo- 

 thetical and unverifiable conceptions have been to 

 some extent overshadowed and eclipsed by the 

 startling progress made since the discovery of 

 radio-activity in 1896 and its almost immediate 

 interpretation as due to the explosive disintegra- 

 tion of the atoms of the radio-elements. This 

 subject is being treated by Sir Ernest Rutherford 

 in another article, and need be only briefly alluded 

 to here. The change is attended by the liberation 

 of energy a million times greater than is liberated 

 in any previously known change of matter, and so 

 it has come about, as for example in the spin- 

 thariscope, that the effect of each individual atom 

 disintegrating can be perceived by the senses. 

 The counting of the number of atoms disinte- 

 grating per minute has become one of the regu- 

 larlv used methods of investigation, whereas it 

 requires, at least, some 25,000 times as many 

 atoms as there are people alive in the world before 

 an element can be detected by the spectroscope. 

 The condensation of moisture on the columns of 

 ions, lying in the tracks of the fragments of the 

 atom after its explosion — both of the o- and 

 ^-particles, which may be likened to projectiles 

 fired from a gun, and of the recoiling residue of 

 the atom or gun itself — has in the hands of 

 C. T. R. Wilson enabled the individual atomic 

 explosions to be photographed. These permanent 

 records, of extraordinary interest and value as 

 they are as confirmatory evidence, yet revealed 

 nothing new. Every detail of the whole pheno- 

 menon had been correctly comprehended and 

 established without such direct aid. In particular, 

 the photographs show well the almost rectilinear 

 flight of the a-particle through the myriads of gas 

 atoms in their path, and their rare and occasional 

 wide-angle deviation when perchance they pass 

 near enough to the heart of the atoms penetrated, 

 which is the experimental basis for the present 

 provisional representation of the internal structure 

 of atoms. 



The atom is regarded now as a solar system, 

 but the massive central sun, comprising all but 

 a negligible fraction of the whole mass, is an 

 exceedingly minute positively charged nucleus, 

 attended by numerous rings or shells of the almost 

 mass-less electrons. In spite of its relatively 

 great mass, the nucleus is so minute that the 

 chance of an a-particle — which itself is the nucleus 

 of a helium atom — in its passage through the 

 atom approaching or colliding with the central 

 nucleus, is exceedingly small. Mass and radio- 

 activity alone seem to depend directly upon this 

 hitherto unsuspected and all-important 'nucleus. 

 The chemical and physical properties, including 

 the light spectrum, are governed probably by the 

 outermost shell or ring of valency electrons, 

 which alone are variable in number. The coming 

 and going of these seem to constitute chemical 

 NO. 2610, VOL. 104] 



change and to give rise to ordinary light radiation. 



Barkla's various series of X-rays characteristic 

 of each element probably originate in the suc- 

 cessive completed rings or shells of electrons 

 surrounding the nucleus. 



All the properties of the atom, practically, save 

 mass and radio-activity, depend solely upon the 

 numerical value of the positive charge of the 

 nucleus, which is equal to the number of the sur- 

 rounding negative electrons. This number, which 

 is known as the atomic number, increases unit by 

 unit in passing from one place of the periodic 

 table to the next. From numerical relationships 

 between the wave-lengths of the characteristic 

 X-rays, Moseley was able to determine or infer 

 this atomic number for all the elements. So he 

 called the roll of the elements for the first time • 

 and found between hydrogen, the first, and 

 uranium, the last and ninety-second element in 

 the table, only five still missing. 



In the course of successive radio-active changes 

 the radio-element expels from its nucleus an c- 

 or y3-particle, so losing two positive charges, or, 

 relatively, gaining one, and shifting back two 

 places or moving forward one in the periodic tabh;. 

 The expulsion of one a- and two /3-particles pro- 

 duces an isotope of the parent, chemically and 

 spectroscopically identical with it, but of atomic 

 mass four units less. The ultimate products of 

 uranium and thorium have been identified as iso- 

 topes of lead of atomic mass 206 and 208 respec- 

 tively, and this has been confirmed by an exam- 

 ination of the atomic weight of the lead derived 

 from uranium and thorium minerals. Of all 

 strange consequences of the atom changing, this 

 is perhaps the most subtle and hitherto un- 

 suspected, for now nothing is more certain than, 

 that the analysis of matter into chemical elements 

 depends on a superficial identity of the outer shell 

 of the atom, and that the same type of outer shell 

 may contain internal nuclei of different mass and 

 different constitution. 



Naturally, the many, at first separate and inde- 

 pendent, lines of evidence which have led to the 

 present results cannot all be even mentioned in 

 an article of this length. The significant fact is 

 that all the new and powerful methods of attack 

 developed by physics and chemistry during the 

 last quarter of a century are converging success- 

 fully on the problem of the internal constitution 

 of the atoms. The prospects of successful accom- 

 plishment of artificial transmutation brighten 

 almost daily. The ancients seem to have had 

 something more than an inkling that the accom- 

 plishment of transmutation would confer upon 

 men powers hitherto the prerogative of the gods. 

 But now we know definitely that the material 

 aspect of transmutation would be of small import- 

 ance in comparison with the control over the in- 

 exhaustible stores of internal atomic energy to 

 which its successful accomplishment would in- 

 evitably lead. It has become a problem, no longer 

 redolent of the evil associations of the age of 

 alchemy, but one big with the promise of a veri- 

 table physical renaissance of the whole world. 



