RADIOACTIVITY— ARTIFICIAL AND NATURAL 311 



which thereupon eject an alpha-particle apiece. The arrow pointing 

 straight up signifies a process in which silicon is bombarded by neu- 

 trons; some are absorbed by nuclei Si^^, which instantly throw out 

 protons. With no fewer than five ways of making a single radioactive 

 type at his command, the physicist is in a position of power which 

 seems all the more remarkable when one recalls that as lately as five 

 years ago he had not (knowingly) made any radioactive substance by 

 any way whatever. 



Consider now the arrow pointing away from the solitary star in 

 Fig. 11, and the arrows pointing away from the many stars in Figs. 2 

 and 3. These signify what is really meant by calling an isotope 

 "radioactive." A radioactive nucleus is one which spontaneously 

 changes itself into a nucleus of another element by emitting a charged 

 particle. (Usually it lasts an appreciable time before it does so, and 

 this delay is to be mentioned in a complete definition of the word 

 "radioactive.") The arrows pointing away from the stars will serve 

 to specify these changes. All in these figures are vertical; every one 

 of these unstable isotopes transforms itself by emitting a particle of 

 which the mass is very small (compared to the mass-unit which we 

 are using) while the charge is + e or — e, according as the transfor- 

 mation is to the element preceding or to the element following. These 

 particles are positive and negative electrons. All of the man-made 

 unstable nuclei are radioactive after this fashion, being electron- 

 emitters; and so are more than half of those which are found in Nature. 



What decides whether it shall be a positive or a negative electron 

 which a given nucleus- type emits? Physicists cannot explain this as 

 yet, in any adequate sense of the verb "to explain " ; but we can readily 

 see the law which governs the choice by examining the pictures. In 

 Figs. 2 and 3 it will be seen that from each star the arrow points in 

 whichever sense — upward or downward — it finds a circle to point at. 

 Becoming completely animistic for the moment, I may say that the 

 unstable nucleus wants to be stable— knows that one of its two 

 neighbors, of identical mass-number but greater or lesser charge, is 

 stable — -knows which of the two is stable — and deliberately proceeds 

 to identify itself with its stable neighbor by emitting an electron of 

 the necessary sign. Putting the situation more drily: each of these 

 unstable nucleus-types tends to transform itself into its adjacent 

 stable isobar. Here "isobar" is a technical term for "nucleus of 

 the same mass-number," and "adjacent" is a short way of saying 

 "belonging to the preceding or the following element." 



Suppose the star has circles both above and below it, i.e. that both 

 of the adjacent isobars are stable (and prove themselves so by existing 



