NEW FRONTIERS IN THE ATOM — LAWRENCE 165 



TRANSMUTATION OF THE ELEMENTS 



These considerations reduce the age-old problem of alchemy to 

 simple terms. For we see to change one element into another is sim- 

 ply to change the nuclear charge, i. e., the number of protons, in the 

 nucleus. The subject of transmutation of the elements has recently 

 received a great deal of attention in the laboratory. All sorts of 

 transmutations have been produced on a minute scale — helium has 

 been made from lithium, magnesium from sodium, and even mercury 

 has been turned into gold. The day may come when we will indeed 

 possess the philosopher's stone and will be able to transmute the ele- 

 ments on a grand scale. But interesting as these developments are, I 

 should like to draw your attention to two other subjects, artificial 

 radioactivity and the question of tapping the vast reservoir of energy 

 in the nucleus of the atom. 



ARTIFICIAL RADIOACTIVITY 



One of the early results of atomic bombardment was the discovery 

 that neutrons could be knocked in or knocked out of the nucleus 

 to produce radioactive isotopes of the ordinary elements. Thus, for 

 example, the nucleus of the ordinary sodium atom contains 11 neu- 

 trons and 12 protons, 23 particles in all, and so it is called sodium 23 

 (or Na") ; and by bombardment it was found that a neutron could 

 either be added to make sodium 24 or subtracted to make sodium 22, 

 both isotopic forms not occurring in the natural state. The reason 

 that these synthetic forms are not found in nature is that they are 

 energetically unstable. They are radioactive and in the course of time 

 blow up with explosive violence. Sodium 24 has a half-life of 14.5 

 hours, i. e., it has an even chance of disintegrating in that time, 

 turning into magnesium by the emission of an electron. Sodium 22, 

 on the other hand, has a half-life of 3 years and emits positive elec- 

 trons to turn to stable neon 22. 



These artificial radioactive isotopes of the elements are indistin- 

 guishable from their ordinary stable relatives until the instant they 

 manifest their radioactivity. This fact deserves emphasis, and it 

 may be illustrated further by the case of chlorine. Chlorine consists 

 of a mixture of two isotopes, 76 percent of Cl^"^ and 24 percent of 

 CI", resulting in a chemical atomic weight of 35.46 which is the 

 average weight of the mixture. By elaborate technique, to be sure, 

 it is possible to take advantage of the extremely slight difference in 

 chemical properties and bring about separation of these isotopes, but 

 in ordinary chemical, physical, and biological processes, the clilorine 

 isotopes are indistinguishable and inseparable. The artificial radio- 

 active isotopes CI" and Cl^* are likewise indistinguishable. In fact, 



