.
The
lines are of course the ultimate lines of neutral sodium. It will be shown[74] in [Chapter IX] that the partial electron pressure in the region from which ultimate lines originate is probably between
at maximum. When 99 per cent of the atoms are ionized, the pressure rises by a factor of about 100, and the corresponding partial electron pressure becomes between
. As the total pressure is probably, at the solar temperature, about twice the partial electron pressure, the total pressure should be nearer to
.