Some Contemporary Advances in Physics — X 

 The Atom-Model, Third Part 



By KARL K. DARROW 



M. A Very Brief Rpxapitulation of What 

 Has Gone Before 



ABUNDANT evidence of many kinds exists to show that each and 

 ■^ every distinct sort of atom is especially adapted to possess 

 energy, not in any random quantity whatsoever, but in certain peculiar, 

 definite, characteristic amounts. An atom having energy in one of 

 these particular amounts apparently cannot add arbitrary quantities 

 to its store, nor yield up arbitrary quantities from it; whenever the 

 atom receives or whenever it gives energy, it receives or gives only 

 just so much as is exactly sufficient to raise or reduce its supply 

 to some one among the others of these distinctive quotas. For 

 each of the chemical elements there is a great system of these dis- 

 tinctive energy-values. They are determined chiefly by analyzing 

 spectra, and for most of the elements — the exceptions being those 

 of which the spectra are excessively complicated — many of them 

 have been evaluated very accurately and set down in tables. 

 The system of distinctive energy-values for any element is a very 

 important feature of that element; perhaps, indeed, the most im- 

 portant feature of all. 



It is customary to say that when an atom acquires or surrenders 

 energy, it passes from one into another state; the various states cor- 

 responding to its various distinctive energy-values are called its 

 "Stationary States." This is a name which suggests, and is doubtless 

 meant to suggest, that the energy-value of the atom is but one among 

 many of its features, all of which change when the energy-value 

 changes. This is a legitimate idea; theorizing about the atom consists 

 in speculating about just such features. But the reader will go far 

 and grievously astray if he lets the name signify to him that many 

 of them are directly and definitely known. In some few cases there is 

 good reason to believe that we know the magnetic moment of an 

 atom in its normal state. Beyond these the energy-values are all 

 that are known. If the reader chooses everywhere to replace "Sta- 

 tionary State" by "energy-value" he will be holding fast to the 

 physical reality, to the one thing not liable to be compromised by the 

 future trends of thought. 



An atom may pass from one Stationary State to another because 

 of colliding with an electron or another atom of the same or a ditTerent 



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