(a) Ultimate Lines[390]—The physical tests of the Saha theory that have been made in the laboratory have all supported it strongly. The fact that the ultimate lines of an atom are the lines normally absorbed by the cold vapor has long been familiar. Indeed it is this fact that is tacitly assumed in the identification of lines of zero excitation potential in the laboratory with lines which are strongest in the low-temperature furnace spectrum. De Gramont[391] designated the ultimate lines “raies de grande sensibilité” for the detection of small quantities of a substance, because they are the last to disappear from the flame spectrum when the quantity of the substance is decreased.
(b) Temperature Class.—The effect, upon the absorption spectrum of a substance, of raising the temperature has also long been recognized as an increase in the strength of lines associated with the higher excitation potentials. The use of A. S. King’s “temperature class” in assigning series relations[392] involves a tacit admission of the validity of the theory of thermal ionization in predicting the relative numbers of atoms able to absorb light corresponding to different levels of energy.[393]
(c) Furnace Experiments.—King’s explicit investigation[394] of the effects of thermal ionization in the furnace has contributed valuable positive evidence for the theory. For example, the production of the subordinate series of the neutral atoms of the alkali metals by raising the temperature was an experimental proof of the principle mentioned in the last paragraph; and the suppression of the enhanced lines of calcium by the presence of an excess of free electrons, derived from the concurrent ionization of potassium, with an ionization potential 1.77 volts lower than that of calcium, and the similar results obtained for strontium and barium, fulfill the predictions of ionization theory in a striking fashion.
(d) Conductivities of Flames.—The conductivity of a flame may be used as a measure of the ionization that is taking place at the temperature in question, and the available data on flame conductivities have been discussed by Noyes and Wilson[395] from the standpoint of the theory of thermal ionization. The calculations based upon the conductivities imparted to a flame by the different alkali metals, and leading to an estimate of the ionization constant, were in satisfactory agreement with the theoretical predictions of the ionization constant from the known critical potentials. The theory of thermal ionization is, therefore, strongly supported by all the laboratory investigations which have so far been undertaken in testing it.
SOLAR INTENSITIES AS A TEST OF IONIZATION THEORY
Before proceeding to discuss the stellar intensity curves, it is proposed to review some of the solar evidence, which can be treated as an observational test of the predictions of the theory relating to the distribution of atoms among the possible atomic states at a given temperature.
In two papers, Russell[396] has given a discussion of the solar and sunspot spectra, showing that ionization theory offers a very satisfactory interpretation of most of the observed phenomena. Attention was called to the anomalous behavior of barium and lithium,[397] and it was suggested that the theory of thermal ionization, while taking account of the temperature of the reversing layer, omitted to consider the effect of the absorption of photospheric radiation. This omission might cause a deviation such as is observed for barium, but appears inadequate to account for the behavior of lithium. In the case of lithium, low atomic weight, and a consequent high velocity of thermal agitation, has been suggested as the cause of the anomaly. The question of the absorption of photospheric radiation has more recently been discussed by Saha,[398] in the form of a correction to his own ionization equations. It has been pointed out by Woltjer[399] that the correction introduced by Saha and Swe may also be derived from considerations advanced by Einstein[400] and Milne.[401] The correction can be evaluated, but appears in every case to be rather small. The effect of the photospheric radiation is certainly one that must be included in a satisfactory theory, but at present, observation is probably not of sufficient accuracy to demand such a refinement.
The work just quoted was qualitative. A more quantitative test of ionization theory in the solar spectrum can also be made[402] by comparing the intensities of solar lines corresponding to different excitation potentials, but belonging to the same atom. The atoms which give a large number of lines in the solar spectrum are those of the first long period of the periodic table, and these, as is well known, consist of multiplets, with components of very different intensities. It appears to be legitimate to select the strongest line associated with any energy level for the comparison; the strength of this line probably represents fairly well the tendency of the atom to be in the corresponding state.
| Atom | Excitation Potential | Intensity | Atom | Excitation Potential | Intensity | ||
|---|---|---|---|---|---|---|---|
| Calcium | 0.00 | 20 | Chromium | 0.00 | 10 | ||
| 1.88 | 15 | 0.94 | 5 | ||||
| 2.53 | 8 | 1.02 | 5 | ||||
| 2.70 | 5 | 2.89 | 2 | ||||
| 2.92 | 4 | Titanium | 0.00 | 5 | |||
| Iron | 0.00 | 40 | 0.82 | 4 | |||
| 0.94 | 30 | 0.90 | 3 | ||||
| 1.54 | 30 | 1.05 | 3 | ||||
| 2.19 | 8 | 1.44 | 3 | ||||
| 2.46 | 10 | 1.50 | 2 | ||||
| 2.84 | 8 | 1.87 | 1 | ||||
| 2.96 | 7 | 1.98 | 1 | ||||
| 3.25 | 7 | 2.08 | 0 | ||||
| 3.38 | 6 | 2.16 | 1 | ||||
| 3.64 | 8 | 2.24 | 2 | ||||
| 4.13 | - | 2.26 | 0 | ||||
| 4.23 | - | 2.28 | 0 | ||||
| 4.35 | - | 2.33 | 0 | ||||
| 4.40 | - | 2.39 | 00 | ||||
| 2.47 | - | ||||||
| 2.56 | 000 | ||||||
| 2.67 | 000 |
The atoms for which there are enough known lines of different excitation energies in the solar spectrum are those of calcium, chromium, titanium, and iron. The correlation between the excitation potential associated with a given line and the intensity of the line in the solar spectrum is illustrated by the preceding tabulation. Successive columns give the atom, the excitation potential, the computed fractional concentration, expressed logarithmically, and the observed intensity, taken from Rowland’s table.