Later inquiries have shown, however, that between the spectral lines of different substances there are probably no _absolute_ coincidences. "Basic" lines are really formed of doublets or triplets merged together by insufficient dispersion. Of Thalèn's original list of seventy rays common to several spectra,[655] very few resisted Thollon's and Young's powerful spectroscopes; and the process of resolution was completed by Rowland. Thus the argument from community of lines to community of substance has virtually collapsed. It was replaced by one founded on certain periodical changes on the spectra of sun-spots. They emerged from a series of observations begun at South Kensington under Sir Norman Lockyer's direction in 1879, and continued for fifteen years.[656]
The principle of the method employed is this. The whole range of Fraunhofer lines is visible when the light from a spot is examined with the spectroscope; but relatively few are widened. Now these widened lines alone constitute (presumably) the true spot-spectrum; they, and they alone, tell what kinds of vapour are thrust down into the strange dusky pit of the nucleus, the unaffected lines taking their accustomed origin from the over-lying strata of the normal solar atmosphere. Here then we have the criterion that was wanted--the means of distinguishing, spectroscopically and chemically, between the cavity and the absorbing layers piled up above it. By its persistent employment some marked peculiarities have been brought out, such as the unfamiliar character of numerous lines in spot-spectra, especially at epochs of disturbance; and the strange _individuality_ in the behaviour of every one of these darkened and distended rays. Each seems to act on its own account; it comports itself as if it were the sole representative of the substance emitting it; its appearance is unconditioned by that of any of its terrestrial companions in the same spectrum.
The most curious fact, however, elicited by these inquiries was that of the attendance of chemical vicissitudes upon the advance of the sun-spot period. As the maximum approached, unknown replaced known components of the spot-spectra in a most pronounced and unmistakable way.[657] It seemed as if the vapours emitting lines of iron, titanium, nickel, etc., had ceased to exist as such, and their room been taken by others, total strangers in terrestrial laboratories. These were held by Lockyer to be simply the finer constituents of their predecessors, dissociation having been effected by the higher temperature ensuing upon increased solar activity. But Father Cortie's supplementary investigations at Stonyhurst[658] modified, while they in the main substantiated, the South Kensington results. They showed that the substitution of unknown for known lines characterizes disturbed spots, at all stages of the solar cycle, so that no systematic course of chemical change can be said to affect the sun as a whole. They showed further[659]--from evidence independent of that obtained by Young in 1892[660]--the remarkable conspicuousness in spot-spectra of vanadium lines excessively faint in the Fraunhofer spectrum. Lockyer's "unknown lines" may probably thus be accounted for. They represent absorption, not by new, but by scarce elements, especially, Father Cortie thinks, those with atomic weights of about 50. The circumstance of their development in solar commotions, largely to the exclusion of iron, is none the less curious; but it cannot be explained by any process of dissociation.