Colours of Inorganic Salts

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PROF. SAHA'S suggestion (NATURE of Feb. 1, p. 163) that colour in inorganic, compounds depends upon the existence of an electronic state of the metal ion separated from the ground state by about 18,000 wave-numbers finds some support at least from work which has been begun here upon the general problem of crystal spectra. There is a mass of evidence for his theory from the study of the rare earths (see particularly Freed and Speeding, Phys. Rev., September 1929); but since the rare earths are exceptional because of the incompleteness of an inner quantum group with a consequent shielding of f electrons in it, it is more interesting to consider the spectra of chromium salts.

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