124 
Transactions of the Royal Society of South Africa. 
(2) HO— < 
.<^__\..C(OH)-/__\ 
+ CH; 
o CO 
HO 
+ HoO. 
Plienolparacresolplithaleiii dissolves in caustic alkali with an indigo 
colour, whereas the ortho compound is pinkish -violet in alkali. 
The absorption band in its spectrum is broad compared with those of 
the ordinary phthaleins, its centre lying- at ?^ 572, i. e. just in the same place 
as that of the above ordinary orthocresolphthalein. This remarkable fact is 
due to the breadth of the band, for while the violet-pink orthocresol com- 
pound has a spectrum showing only the loss of the yellowish-green light 
round about x 572, the indigo para-compound shows the loss of all the orange 
and part of the red as well as of much more of the green, even though the 
average absorption is also at a 572. 
This phenomenon can only occur at this part of the spectrum, since in 
other places a broadening of the absorption-band does not lead to the 
absorption of an essentially differeyit colour. The phenomenon is the same 
as that shown by a birefringent mineral plate under polarised light, when 
the plate is of such thickness as to produce " red of the first order," a colour 
which has much the same spectrum as the violet-pink phthaleins ; the 
slightest increase of the double-refraction shifts the absorption-band from 
the greenish-yellow into the orange and the " red of the first order " becomes 
indigo. 
Phenolparacresolphthalein is a very weak acid and is scarcely coloured 
by weak bases like ammonia. When dissolved in cone. HoSO.^ it gives an 
intense salmon-orange shade, which when examined in a sufficiently thin 
layer shows a broad absorption-band with its centre at about A 487. It is 
rather uncommon to find any orange colour which thus transnfiits the violet 
rays, nevertheless, another example is to be found in p-oxybenzoylbenzoic 
acid itself, which dissolves in H^SO^ with a yellow colour, the solution not 
absorbing the violet but having a narrow absorption-band with centre at 
A 459. Phenolphthaleinoxime, from which the above acid is derived, also 
gives this absorption-band in HgSO^. 
o-^-2)henolphthalein-7n-carhoxylic acid. — This is prepared in the same way 
as the paracresol-phthalein, using j>oxybenzoic acid as the phenolic con- 
