The question of relative abundances of elements in the reversing layer is discussed[411] in Chapter XIII. It may be mentioned that the abundances there deduced depend upon estimates of marginal appearance. Probably all lines are unsaturated at marginal appearance, that is, there are not enough suitable atoms present completely to absorb all the incident light of the appropriate wave-length. Hence all suitable atoms present, as far down as the photosphere, where general opacity begins to render the gas hazy, are actually contributing to the line. At marginal appearance, then, all the intensity phenomena are probably due to pure abundance, and considerations of level are eliminated. The deduced abundances are therefore independent of effects such as are discussed in the present chapter, and the results of Chapter XIII may be cited as giving evidence that the stellar abundances, for all the atoms here to be considered except barium, have a range with only a factor of ten, which is negligible in comparison with the quantities to be discussed. The relative abundance of [Pg 136] different atomic species will therefore be neglected in what follows, although, with more accurate data than are now available, it should become a factor of importance. Fractional concentrations, as derived from the ionization formula, govern the effective level at which absorption takes place. Fowler and Milne, as was pointed out earlier, suggested that the higher the fractional concentration at maximum, the higher the level and the lower the partial pressure from which the line originates. They suggested that the pressure for a principal line at maximum is from to of the corresponding value for a subordinate line. The assumption now introduced is, in effect, that the absorbing efficiency of individual atoms is the same. The partial pressure at the level from which a line originates should then vary inversely as the fractional concentration at maximum. In other words, the product should be constant, when is deduced from the class at which the observed maximum occurs. The quantity depends primarily on the excitation potential, and varies but slowly with . It is given by the expression[iii] [iii] For notation, see Chapter VII, p. 106. For subordinate lines, is given by the expression and this quantity is extremely sensitive to change in .