The aldo-ketenes are colourless compounds which are not capable of autoxidation, are polymerized by pyridine or quinoline, and are inert towards compounds containing the groupings C :N and C :O.
We now proceed to discuss the types of aliphatic compounds; then, the characteristic groupings having been established, an epitome of their derivatives will be given.
Examination of the absorption spectra of coloured compounds shows that certain groupings displace the absorption bands in one direction, and other groupings in the other.
It follows on this theory that all coloured substances contain either of the groupings or the former being a para-quinonoid, the latter an ortho-quinonoid.
Chemie, 24, p. 468) submitted the view that fluorescence was due to the presence of certain " fluorophore " groups; such groupings are the pyrone ring and its congeners, the central rings in anthracene and acridine derivatives, and the paradiazine ring in safranines.