Some Contemporary Advances in Physics-X, The Atom-Model, Third Part, A Very Brief Recapitulation of What Has Gone Before

B U N D A N T 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 distinctive 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 important 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 corresponding 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.