184 Transactions of the Boyal Society of South Africa. 
(4) Phenolphthalein dimethyl ether. — This long-known substance, which 
gives no colour in alkali, gives a salmon colour in HgSOj^ with absorption- 
band centre at X 508. Phenolphthalein monomethyl ether, examined in the 
same way, showed its band-centre to be at X 503 : that of phenolphthalein 
itself in HoSO^ is at X 499. 
(5) Phenolphthalein monosuccinic acid, made by condensing oxy benzoyl- 
benzoic acid with o-oxyphenylsuccinic acid (itself made from coumarine), is 
purplish-pink in alkali with band-centre at X 564. In H2SO4 it is salmon- 
coloured with band-centre at X502. 
(6) Phenolphthalein- disuccinic acid, from the above acid with phthalic 
anhydride, could only be obtained in traces. Its band-centre in alkali is 
at X 575, the displacement of the band from that of phenolphthalein being 
about twice that of the foregoing monosuccinic acid. 
(7) Thijmolmetacresolphtlialein. — This substance, which is intermediate in 
constitution between ^«-cresol- and thymol-phthaleins, and thus enabled the 
influence of the o-isopropyl group to be studied, was made by condensing 
metacresol with a new acid, thyinoylbenzoic acid (2-mefhyl-5-isoprojpyl-4f- 
oxybenzoylbenzoic acid). The latter is made from thymolphthalein in the 
same way as oxybenzoylbenzoic acid is made from phenolphthalein (see 
Part II). This new phthalein is violet in alkali with its band-centre 
at X 590 almost exactly across the D line of the solar spectrum. In H0SO4 
it is pink with a broad band, the centre of which is at about X 530. 
(8) Thijmol-a-naphtholphthalein, from a-naphthol and thymoylbenzoic 
acid, is bluish-green in alkali, the centre of its absorption-band being 
at X 633. This forms an interesting example of the fact that the effects 
of the substitutions are additive, i. e. that the colour and position of the 
absorption-band of very complex phthaleins can be predicted from data 
obtained from simpler phthaleins. Thymolnaphtholphthalein can be looked 
on either as {a) thymolphenolphthalein, to which the butylidene group has 
been added (converting phenol into naphthol), or (h) as phenolnaphthol- 
phthalein, to which the methyl and isopropyl groups have been added 
(converting phenol — in the other ring — into thymol). Now, in case {a) the 
value of the butylidene group can be ascertained by comparing the spectrum 
of phenol-a-naphtholphthalein with that of phenolphthalein. The central 
wave-lengths of these substances are 607 and 554, so that the single 
butylidene group which makes the difference between them has raised the 
wave-length of the band by 9*5 per cent. Applying, therefore, a correction 
of +9-5 per cent, to the figure for phenol^%moZphthalein (previously 
ascertained to be X 578, see Part II), we get 1-095 x 578 =:X633 as the 
calculated value for thymol-a-naphtholphthalein. In case (h), in the same 
way, we take the experimentally ascertained value for ^^le^nolnaphthol- 
phthalein (viz. X 607) and multiply it by the ratio indicating the conversion 
of one phenol group into a thymol group (viz. 1*044 or 578 -^ 554), this 
