Some Contemporary Advances in Physics IX 

 The Atom-Model, Second Part' 



By KARL K. DARROW 

 G. RlXAIMTLLATION OF THE FaCTS TO BE EXPLAINED 



EVERY atom-model that is worthy of notice was designed in 

 view of a certain Hmited group of facts. That is to say, every 

 \alual)le atom-model is the invention of somebody who, being ac- 

 quainted with certain of the ways in which matter behaves, set himself 

 to the devising of atoms of which an assemblage should behave like 

 matter in those ways. Of course, it would be a most wonderful 

 achievement to conceive atoms, of which assemblages should behave 

 like matter in all ways; but this is too e.xalted an ambition for this 

 day and generation, no man of science bothers with it. Each atom- 

 model of the present is partially valid, not universally; and nobody 

 can rightly appreciate any one of them, unless he knows the facts for 

 which it was designed. I might add that he should also know the 

 relative importance, in the world and in life, of the facts for which it 

 was designed. But this also is too exalted an ambition; we do not 

 know much, if anything, about the relative importance of facts sub 

 specie crternitalis, and can hardly refrain from regarding with an 

 especial favour the facts which happen to have been successfully 

 explained. At all events it is clear that every account of an atom- 

 model should be preceded by an independent account of the things it is 

 meant to explain. F"or the favorite atom of these days, the atom 

 of Rutherford and Bohr, I have provided this preliminary account 

 of the facts in the First Part of the article. Let me give a brief outline 

 of the most important among them, before entering upon tlie task of 

 constructing an atom-model to reproduce them. 



First and foremost, the elements are very definite things; each of 

 the ninety of them is distinguishable from the other eighty-nine, 

 not in one respect only but in many, and in many cases the contrasts 

 are \ery severe. The atom designed for each of them must therefore 

 have defmiteness and fixity and a sharply-inarkt-d character. 



Next: although the atom must be <lctiniir, il must not he abso- 

 lutely immutable; it must be capable, under stress, of assuming 

 various distinct states or forms or configurations or whatever you 

 choose to call them. This is prescribed by that great and essential 

 fact of the Stationary States, to which so much of the First Part of 



' Devoted to Bohr's atom model for hydrogen and ionized helium. The models for 

 other atoms, as well as some general considerations, are reserved for the Third Part. 



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