RADIOACTIVITY— ARTIFICIAL AND NATURAL 317 



In F'ig. 15. notice that all of the members of the thorium series have 

 mass-numbers divisible by 4, or equal to 4w with various integer values 

 given to n. This is accordingly called the "4w series," and one 

 readily sees what is meant by calling the radium and the actinium 

 series by the names of "4n + 2" and " 4w + 3" series respectively. 

 One begins at once to wonder whether there is not a "4n + 1 " series. 

 Such a series was long sought after in vain, and no member of it has 

 yet been discovered in Nature; but in the laboratory of the Curies in 

 Paris thorium has lately been strongly bombarded by neutrons, and a 

 new sequence of radioactive bodies has thus been engendered which 

 has already been followed through several steps, and is in all probability 

 the series so long missing. 



As to the remaining feat — the creation of elements beyond uranium 

 — it is now beyond doubt. Fermi and his school at Rome, Hahn and 

 Meitner in Berlin, Curie and Joliot in Paris have all borne witness to 

 it. In one way it seems the most romantic of all the feats of transmu- 

 tation, for Nature had apparently set 92 as the limit of nuclear charge, 

 and now man has transgressed it. The process is begun by exposing 

 uranium to bombardment by streams of neutrons. It appears that 

 when a uranium nucleus has captured a neutron, it finds itself not 

 strongly enough charged to hold together, and proceeds to emit one 

 negative electron after another in its search for stability. Each 

 emission transfers the nucleus to an element one step higher, without 

 afifecting its mass-number; and the authorities agree that there are at 

 least four consecutive emissions, after the last of which the atomic 

 number is 96! This addition of four new elements to the Periodic 

 Table opens a new field to chemists, one which they can scarcely 

 have expected ever to be able to enter. The four have no proper 

 names as yet, a curious circumstance in view of the fact that dis- 

 coverers of new elements have thus far been in great haste (sometimes 

 too great haste) to name them. Mendeleieff long ago used to denote 

 an expected but undiscovered element by prefixing "eka" to the name 

 of the element just above the vacant place in the Periodic Table; 

 these new four are sometimes called eka-rhenium, eka-osmium, eka- 

 iridium and eka-platinum, but on looking at such words one is inclined 

 to prefer the atomic numbers. 



Now to summarize. The world as we knew it before the days of 

 transmutation was constructed out of some two hundred and fifty 

 kinds of atoms, each consisting of a nucleus surrounded by a family of 

 electrons. Of these 250 kinds of nuclei the great majority were stable 

 and perpetual, but some forty were unstable — ^doomed to perish in 

 due time, by ejecting either alpha-particles or negative electrons. 



