November 10, 1916] 



SCIENCE 



663 



particle it not only changes to a different 

 element, but the atomic weight of the ele- 

 ment is reduced by the weight of the alpha 

 particle, which is 4 units. As the alpha 

 particle carries two units of charge, its ejec- 

 tion removes two units of charge from the 

 nucleus of the atom, which changes the 

 valency of the element and shifts it two 

 places to the left in the periodic system. 

 The emission of a beta particle produces no 

 appreciable change in the atomic weight, 

 but adds one unit of positive charge to the 

 nucleus, which changes the valency and 

 shifts the resulting element one place to the 

 right in the periodic system. For example, 

 uranium in group VI. with an atomic 

 weight of 238 emits an alpha particle and 

 becomes uranium-Xj in group IV. with an 

 atomic weight of 234; this emits a beta 

 particle and becomes uranium-X 2 in group 

 V. with an atomic weight of 234; this 

 emits a beta particle and becomes uranium- 

 2 in group VI. with an atomic weight of 

 234; this emits an alpha particle and be- 

 comes ionium in group IV. with an atomic 

 weight of 230; this emits an alpha particle 

 and becomes radium in group II.; this 

 emits an alpha particle and becomes ra- 

 dium-emanation in the zero group; and so 

 on through the A, B, C, D, E and F prod- 

 ucts. The latter product, which is polo- 

 nium, emits an alpha particle and becomes 

 the end product, which is probably lead. 

 The thorium series and the actinium series 

 pass through cycles similar to that of ura- 

 nium. 



When these elements are arranged in the 

 periodic system, it happens in some cases 

 that several elements occupy a single posi- 

 tion in the table. A large amount of work 

 has been done on these elements, and it has 

 been found that in every case, where sev- 

 eral elements occupy a single position in 

 the periodic system, they are, so far as 

 known, chemically identical and non-sep- 



arable. Soddy has suggested the term 

 "isotopes" for such elements. 



The behavior of the radio-elements con- 

 firms to a remarkable degree the theory that 

 the chemical and physical properties of an 

 element depend more on the nuclear charge 

 of the atom than on the atomic weight. The 

 determination then of the atomic number 

 which represents the magnitude of the 

 nuclear charge of the atom becomes an im- 

 portant problem in chemistry. 



This brief but incomplete outline shows 

 that the first great advance in the determi- 

 nation of the nature of the atom has been 

 made. Much work is now being done, but 

 much remains to be done before we can as- 

 sume a definite structure to the atom. 

 Various hypothetical structures have been 

 suggested, especially by Bohr and Nichol- 

 son, who have accounted in a remarkable 

 manner for certain series of spectral lines. 

 Various theories have been suggested to ac- 

 count for the stability of atoms with ro- 

 tating electrons. These theories are based, 

 both on the arrangement and the manner 

 of rotation of the electron, and on the man- 

 ner in which an electron radiates energy. 

 A more accurate knowledge of the nature 

 of the atom will probably be necessary be- 

 fore its stability can be satisfactorily ex- 

 plained. 



In the disintegration of the radio-ele- 

 ments we have definite evidence of the 

 changes of various elements into other 

 elements. These transformations have 

 brought into prominence again the prob- 

 lem of how the various chemical elements 

 have been built up, and the problem of 

 transmutation again becomes a legitimate 

 problem for the chemist to investigate. 

 When we consider the unparalleled amount 

 of potential energy associated with the 

 atom, and the intimate relation of radiant 

 energy and electricity to atomic structure; 

 and when we consider that the supply of 

 energy is the most fundamental problem 



