Colour and Chemical Constitution. 
135 
is very unlikely, that this substance is a mixture of the isomers e-f-g-j-h and 
c-f-g-j-Jc (theories X601 and X594). 
Constitution of Phenolphtlialein. 
Having now exhibited the experimental evidence, I ask the reader to 
examine again the table on p. 130, which has been constructed from this 
evidence, and, confining his attention to the phthalic ring, particularly the 
positions h and c, to remember the result obtained in the study of 
benzaurine-derivatives (Part X). The latter were found to have a negative 
para- position : thus aurine, which is parahydroxybenzaurine, has X 534, as 
against X 553 for benzaurine ; so the paracarboxylic acid of benzaurine 
{ex terephthalic acid and phenol) has X 550, again showing a lowering 
caused by the para-position, its isomer, phenolphthalein having X554. Now, 
as the table on p. 130 shows, this lowering does not occur when phenolphtha- 
lein is substituted in the para-position, when 6-monobromophenolphthalein 
would have had X 546 instead of X 556 (colour-value = — 8 instead 
of + 1). 
I infer from this therefore that the customary formulation of phenol- 
phthalein (that on p. 130) is not the correct one. This formulation makes 
phenolphthalein to be simply the ortho- or A2-carboxylic acid of benzaurine, 
which, if true, should cause it to have a negative para-position as regards 
colour. In this connection also I may repeat what I have said before — that 
if phenolphthalein were really benzaurine-A2-carboxylic acid, it ought to 
exhibit the yellow colour of benzaurine derivatives when its alkaline solution 
is carefully neutralised (and then pass into a colourless lactone). Both 
phenolterephthalein and phenolsulphonephthalein change from pink to 
yellow on neutralising, and they are undoubtedly derivatives of benzaurine. 
The customary formula for alkaline phenolphthalein, which has been 
accepted for thirty-five years, having been shown to be unsatisfactory, we 
may next consider the one which was current before 1885, viz : 
^^CO_/~\-OH 
I I or when ionised 
\^\qo—(^ \— oh 
Phenolphthalein was then known as dioxyphthalophenone. 
Now these formulae were rejected about 1885 when it was discovered 
that diphenylphthalide had both phenyl rings on the same carbon- atom, 
and it was assumed, in some manner which I am unable to trace, that 
phenolphthalein is the dioxy-derivative of diphenylphthalide. 
I should like therefore to put forward tentatively some of the other 
possible schemes, e. g. : 
