plexity is substituted for that fundamental unity of
matter which has long formed the dream of speculators. And it is
extremely remarkable that Sir William Crookes, working along totally
different lines, has been led to analogous conclusions. To take only one
example. As the outcome of extremely delicate operations of sifting and
testing carried on for years, he finds that the metal yttrium splits up
into five, if not eight constituents.[664] Evidently, old notions are
doomed, nor are any preconceived ones likely to take their place. It
would seem, on the contrary, as if their complete reconstruction were at
hand. Subversive facts are steadily accumulating; the revolutionary
ideas springing from them tend, if we interpret them aright, towards the
substitution of electrical for chemical theories of matter. Dissociation
by the brute force of heat is already nearly superseded, in the thoughts
of physicists, by the more delicate process of "ionisation." Precisely
what this implies and involves we do not know; but the symptoms of its
occurrence are probably altogether different from those gathered by Sir
Norman Lockyer from the collation of celestial spectra.

A. J. Angstrom of Upsala takes rank after Kirchhoff as a subordinate
founder, so to speak, of solar spectroscopy. His great map of the
"normal" solar spectrum[665] was published in 1868, two years before he
died. Robert Thalen was his coadjutor in its execution, and the immense
labour which it cost was amply repaid by its eminent and lasting
usefulness. For more than a score of years it held its ground as the
universal standard of reference in all spectroscopic inquiries within
the range of the _visible_ emanations. Those that are invisible by
reason of the quickness of their vibrations were mapped by Dr. Henry
Draper, of New York, in 1873, and with superior accuracy by M. Cornu in
1881. The infra-red part of the spectrum, investigated by Langley,
Abney, and Knut Angstrom, reaches perhaps no definite end. The
radiations oscillating too slowly to affect the eye as light may pass by
insensible gradations into the long Hertzian waves of electricity.[666]

Professor Rowland's photographic map of the solar spectrum, published in
1886, and in a second enlarged edition in 1889, opened fresh
possibilities for its study, from far down in the red to high up in the
ultra-violet, and the accompanying scale of absolute wave-lengths[667]
has been, with trifling modifications, univer