308 
Transactions of the Royal Society of South Africa. 
= 1'003. This practically proves that in the guaiacol colours the methyl is 
really para to the central carbon-atom : it is worth noting, however, that in 
this formulation of guaiacol-phthalein the compound becomes a meta-quinone, 
a type of comj)ound which has hitherto met with no acceptance. 
The 17th colour-factor is that of the a — SO.-H group in the so-called 
sulphonephthaleins or sulphureins. As pointed out in Part Y of this work, 
these substances are merely sulphonic acids of benzaurine— not phthaleins 
at all in the strict sense : for example, they are yellow when acid and do not 
combine with hydroxylamine. The value of this colour-factor is I'OIB. 
Since the factor for the COgH in phenolphthalein is 1*002, the factor relating 
any sulphonephthalein to the corresponding phthalein is 1'016, e.g. X 563 
for (di)phenolsulphonephthalein when phenolphthalein is \ 554. The case 
of catecholsulphonephthalein may be worked out from the general formula, 
and is instructive. This substance contains one a — SO3H (factor I'OIS) ; 
two y-hydroxyls (factor 0-9657) ; and two /S- or £-hydroxyls (factor 1-030). 
The calculated wave-length is therefore 593 x 1-018 x (0-9657)2 x (1-030)^ 
= 597-3. Observation (JSraHC03 solution) g'^^e X600. The agreement is 
quite good enough for such a complicated case, since there are five 
multipliers, each of which may be wrong by 0*0005. In the case of 
guaiacolsulphophthalein the observation was X 608 and the calculation 
593 X 1*018 X (0*973)2 x (1*030)2 z= 607. 
Sulphuric-acid Colours. — It should be noted that although the — SO.^H 
group in the molecule has a colour-factor, it is not possible to explain by 
means of a colour-factor the behaviour of coloured substances when dissolved 
in strong sulphuric acid, and it must be assumed that an oxonium-salt is 
formed outside the molecule. As shown in Parts I and YI, the sulphuric-acid 
colours obey the different law ^ =: ^| — 0*000695, in which Xq is the 
alkaline and X, the sulphuric-acid wave-length. 
I have now succeeded in extending this law, which is evidently one 
governing change of colour by change of ionisation at or near the colour- 
centre, to explain the fact that benzaurine-derivatives exhibit a third colour, 
viz. in dilute acid. (Thus benzaurine has X 553 in alkali [pink], X473 in 
II0SO4 [orange], and X493 [salmon] in dilute HCl, as well as the bandless 
yellow colour when neutral.) 
Calculation of Acid Colours of the Benzauri7ie Family. 
Name. 
Value of Ao. 
Calculated Aj. 
Observed in 
HCl. 
1 
Benzaurine ...... 
553 
494 
493 
2 
Phenolsnlpliophthalein 
563 
506 
507 
3 
Catecliolsulphophthalein 
600 
553 
557 
4 
Guaiacolsulphophthalein 
608 
564 
565 
5 
Thymolsulphophthalein 
604 
558 
552 
