aggregation than any
with which we are ordinarily acquainted.

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 Thalen'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 labor