464 Sir Oliver J. Lodge [Feb. 2S, 



The number of positive atoms in the nucleus was counted, for 

 several elements, by Rutherford ; and the number of negative cor- 

 puscles in the orbit was counted by Moseley : the two numbers 

 agree. Xormal atoms are therefore electrically neutral, so that their 

 external electric attraction at any reasonable distance is nil ; but it 

 is supposed that at atomic or molecular distances the outer or orbital 

 electrons, which can interlock with those of others, determine the 

 atom's chemical affinity and all the chemical behaviour of the sub- 

 stance. An atom with one or two outlying planets — let us surmise — 

 would be an active chemical element, a monad or dyad perhaps. An 

 atom with a close-grouped self-contained system would be an inert 

 element of the argon neon helium series. These might exhibit 

 chemical properties, perhaps, under enormous pressure. The heavier 

 atoms contain the most particles, and must have the most complicated 

 structure. There is every grade ; from the simplest, hydrogen, with 

 one electron, to the most complex, uranium, with ninety-two. There 

 is room for ninety-two elements in the series, and no more All 

 of these are actually known except five or six. There are only these 

 few unfilled gaps in the chemical series of elements as thus planned. 



Radioactivity. 



A complicated atom has a certain amount of instability and may 

 fall down occasionally into the next simple grouping, flinging away 

 one or more of its units. When this happens there is a sort of 

 atomic cataclysm or explosion, a projectile and some quanta of energy 

 are emitted. This is the phenomenon of radioactivity. Uranium 

 after three (or possibly four) such eruptions becomes the element three 

 (or four) steps down the series, viz. radium. Radium after five more 

 explosions becomes apparently the well-known and stable element 

 lead, or at least something chemically indistinguishable though perhaps 

 of slightly different atomic weight— what has recently been called an 

 '' isotope " of lead. That is the kind of statement that without too 

 much rashness can be cautiously and tentatively made. 



At every serious cataclysm an a particle or atom of helium is 

 emitted from the nucleus, accompanied by a /3 particle or negative 

 corpuscle from somewhere, usually from the planetary system. A 

 sympathetic etherial gush of y rays accompanies the eruption. 

 A definite unit of energy — a quantum or a simple multiple of it — is 

 emitted at each explosion ; and the remaining electrons then settle 

 down into their new orbits, the element changing in character and 

 chemical properties accordingly. 



A catastrophe of this kind can be ju'oduced by a sufficiently 

 ra])id projectile, an a or /3 particle shot off say by radium ; and a 

 minor catastrophe or emission of a (3 particle can also l)e produced 

 by the accumulated energy of properly attuned X-rays. When an 

 X-ray or ray of ultra-violet light agrees in frequency with the orbital 

 frequency of an electron, we can suppose (not without a little difii- 



