290 BELL SYSTEM TECHNICAL JOURNAL 



smaller than this. I will later speak more fully of the corresponding 

 data for the few other kinds of atoms for which such studies have been 

 made. In the meantime the reader may think of 10~^^ and 10~^' cm. 

 as reasonable guesses for the radii of atom-nuclei. They agree in 

 order-of-magnitude with the value usually assigned for the radius of 

 the electron, and are ten or a hundred thousandfold smaller than the 

 radii of the atoms; so that, as many a writer has remarked, the nucleus 

 and electrons bulk about as large in the atom which they make up as 

 flies in a very great cathedral. 



Small as it is, an atom-nucleus cannot be regarded as an elementary 

 and an ultimate particle. No sooner had the physicists of a generation 

 ago divided the "indivisible atom" of the nineteenth century mentally 

 into electrons and a nucleus, than they found themselves obliged to 

 go on w^ith the division. The electron so far has escaped this surgery, 

 but the nucleus has been resolved — mentally, again — into as many 

 parts as the rest of the atom itself. The arguments are two. In 

 respect of their masses, the nuclei of the many kinds of atoms which 

 are known are so related among one another as to suggest that all of 

 them are aggregates of diverse numbers of particles of a very few 

 fundamental kinds, all those of a kind having quite the same charge 

 and almost the same mass wherever they appear. Moreover, particles 

 sometimes spring out of atoms — from certain elements spontaneously, 

 from others only under the bombardment of such missiles as alpha- 

 particles — which are of such a nature that their source must be sought 

 in or about the nuclei of the atoms whence they come. The two 

 arguments coalesce when it is noticed that the particles which must 

 be postulated for the one are some of those w^hich are observed in the 

 phenomena on which the other is based. The masses of atom-nuclei 

 imply that they are built out of certain kinds of bricks, and bricks of 

 these very kinds are indeed observed at times, falling or plunging or 

 being violently hurled out of disintegrating atoms. 



The study of the nucleus therefore involves, to begin with, the 

 measurement of its mass — the measurement of the masses of all the 

 known kinds of nuclei, amounting by now to several hundreds. This 

 seems to be the same as the basic task of chemistry, the task of 

 measuring atomic w^eights. Yet in spite of the indescribable labor 

 which numberless chemists have lavished upon atomic weights, their 

 data are seldom of value in modern nuclear physics. This is because 

 the atoms of most elements are of two or more different kinds (isotopes) 

 with different masses. Chemical methods yield an average of their 

 weights, but the student of the nucleus wants the mass of each kind 

 separately; and this nearly always requires a physical method of 



