12 
Transactions of the Uoyal Society of South Africa. 
uitro-compound — in that the 2-2'-oxo group distinguishing fluorescein from 
phenolphthalein must take part in a quinonoid change which totally alters 
the configuration of the molecule, just as the quinonoid change 
\0H^-/— \:0 
NO. : NO2H 
has completely altered the nature of dinitrophenolphthalein. 
Now, the only orthoquinonoid configuration which can be planned for 
fluorescein is 
\ 
^ \ / ^ — ^ 
\ / 
in which a phenolic acid group has combined with a basic oxonium group. 
It should be noted that alternative methods of making: this linkasre are 
excluded by the fact that resorcin-benzeine, which is fluorescein minus the 
COoH group, shows the same behaviour as fluorescein, and that fluorane in 
strong HoSO^ shows a similar colour and fluorescence which, I suggest, is. 
due to the linkasre 
// 
HSO4 • o 
\ 
CO.,H 
\ / 
If the still simpler substance of the annexed constitution 
/ \ 
/ 
C(OH)- 
\ / 
could be prepared, its behaviour in HoSOj. would, to some extent, settle the 
question of the cause of the colour and fluorescence of this class of sub 
stances. 
Phthaleins dissolved in Concentkated Sulphuric Acid. 
The author has discovered that the phthaleins and aurines when dis- 
solved in H3SO4 all give coloured solutions with definite absorption-bands. 
