IN two papers brought before tbe Society rather more.tha,n a year ago (Trans., 1906, 89, 13, 17) attention was drawn to the remarkable change of colour observed when p-nitrobenzeneazo-4 : 6-dimethylcoumarin and p-nitrobenzeneazo-4-methyl-a-naphthacoumarin were dissolved in alkali. Both substances are reddish-brown in colour and they dissolve in neutral solvents with a similar shade; the former gives,
IT is a remarkable fact that whereas the tetrabromo-and tetraiododerivatives of fluorescein show marked fluorescence in alkaline solutions, the corresponding nitro-compound exhibits no trace of this property.This fact has already been commented on by Richard Meyer (Zeit. physihl. Chem., lS97,24,468) and by one of the authors of the present communication (Proc., 1900, 16, 3 ; Zeit. physikal. Chem., 1900, 34, 1). Meyer, noticing that tetraiodofluorescein fluoresces less than tetrabromofluorescein, and this, in turn, less than fluorescein itself, would have been inclined to attribute the decrease in the fluorescence t o an
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