54 Recent Advances in Stellar Astronomy
This reasoning furnishes a beautiful explanation ot the
manner in which an element such as iron shows relatively
few lines when heated just to its boiling point in an elec-
tric furnace, more lines when the furnace is heated several
hundred degrees hotter, and still more when the atoms
are subjected to the much more intense excitation of the
electric arc.
But this is not the whole story. If the first and most
easily movable electron—of which alone we have been
speaking so far—receives a sufficient supply of energy, it
may be pulled clear away from the atom, leaving a posi-
tively charged, or “ionized” residue. Then the game
begins again, a second electron being shifted, stage by
stage, and at last removed. During this process the
energy-steps are quite different (and, in general, greater)
so that we get an entirely new set of spectral lines, usu-
ally with more rapid vibrations, and shorter wave-lengths.
For some atoms, such as sodium, the second electron is
exceedingly hard to get off, and the corresponding radia-
tions are of such short wave-length that they can only be
observed with special apparatus. In others, as magnes-
ium, a second electron comes off fairly easily—about twice
as hard as the first—giving lines in the visible spectrum
and the ultra-violet. These are the characteristic spark
lines—or “enhanced” lines, as Sir Norman Lockyer called
them, because, even when present in the arc spectrum,
they are strengthened in the spark. For magnesium, the
third electron is very hard to get away, and the lines cor-
responding to its removal are difficult to observe. In
some other atoms, such as silicon, a third or even a fourth
electron may be removed with the production of visible
radiation; but, even then, a state is finally reached in
which only radiations of too short wave-length to be de-