214 
Transactions of tJie Boyal Society of South Africa. 
All the naphthalene derivatives of the classes already discussed can be 
brought into the scheme by allowing the value 53 to the grouping 
/ \ 
CH CH 
\ / 
CH: CH 
which when added to benzene turns it into naphthalene. 
Thus phenol-a-naphtholphthalein is 554 + 53 = X 607 and a-naphthol- 
phthalein is 607 + 53 = X 662. The tautomeric ring 
H - O - C^ ^C- 
has thus the value 219 + 58 = 272 and the para-a-naphthylamine-ring the 
value 276. The grouping 
when non- tautomeric is a combination of the C4H4 and the CoHo loads and 
its value is therefore 53 + 22 = 75. 
Colour of Indigo. 
The most probable tautomeric change in this substance is as shown below: 
/Oil 
This is ceo + CCC + CCC + CCN + CCN + CCO fully tautomerised, 
plus the load CgH^lSrH. Allowing 20 as an estimate from analogy for the 
latter the total is 610. This is evidently quite near the truth, but of course 
indigo cannot be observed in water solution, for which all the other data 
are calculated. Sulphindigotic acid in water has X 614. It is next desirable 
to discuss the nitro and nitroso-colours, which do not at once fit into the 
scheme. Unfortunately all the published data deal with substances in 
alcoholic solution, consequently in the case of alkaline solutions the molecule 
contains non-ionised sodium or potassium, and in acid solution an acid 
radicle, thus introducing an unnecessary complication due to varying load 
on the molecule. The first thing that is noticed is that the spectrum of 
an alkaline nitro-phenol is frequently about 100 units above that of the 
parent substance, which certainly on my theory suggests that it has one 
tautomerism more in the alkaline condition than when free. Examples from 
Baly's and Tuck's work are jD-nitrophenol X 308, K-salt x 405 ; p-nitroso- 
phenol X 311, Na-salt X 415 ; o-nitrophenol X 338, K-salt X 425 ; m-nitro- 
phenol X 328, K-salt about X 390, very broad, evidently imperfectly formed. 
