Colour and Chemical Constitution. 
187 
Similarly, the absorption-band of xanthone, the simplest fluorescent 
substance, in H2SO4 was found to be at A 410. 
Note on the guaiacol-phthaleins. — It having been recently shown that 
when guaiacol is brominated the bromine does not enter para to the — OH 
but is para to the — OCH3 group (Journ. Chem. Soc, Lond., 1917, p. 941), 
it becomes probable that guaiacolphthalein and phenolguaiacolphthalein also 
have the methoxyl group |?am to the central-linking carbon, and consequently 
have the free — OH group of guaiacol meta to this carbon. Experiment 
confirmed this, for when 5-bromoguaiacol was condensed with oxybenzoyl- 
benzoic acid and H2SO4 at a low temperature, the phthalein produced was 
phenolguaiacolphthalein, bromine having been eliminated. This could 
scarcely be imagined unless the linkage to a phthalein occurred at the same 
point as the bromine came off at. The dibromoguaiacolphthalein described 
in Part I, obtained by direct bromination, has, therefore, probably one bromine 
in each ring ortho to the — OH groups and also ortho to the central carbon. 
Note on quinolphthalein. — The ordinary formulation of this substance : 
H0\ \ I 'on 
\/ 
is not quinonoid and does not account for its colour. 
The present writer now suggests two (tautomeric) formulae for the salts 
of this substance, which are quinonoid. 
and j 
COoK 
These two formulae are almost identical when it is considered that the 
K atoms are ionised off the molecule. 
