46 SECTIONAL ADDRESSES 
separation and purification, and it at once indicates, by its appearance or 
disappearance, the occurrence of a chemical reaction. Small wonder 
that the successful outcome of the investigation of many colourless sub- 
stances has awaited the discovery of some characteristic colour-reaction ; 
a noteworthy example being Vitamin A. Odour is a more specific 
property than colour as judged by the eye, and in a more limited field it 
has proved equally useful to chemists who prefer to follow their noses. 
However, we cannot yet resolve odours in a spectrum. 
Thus the pursuit of a fascinating object has been along a path of 
relatively low resistance and the pioneers have been richly rewarded. 
Like a list of best books, a catalogue of outstanding achievements 
invites destructive criticism. I do not fear this, however, in recalling 
the researches of Laurent, Kekulé, Baeyer and Heumann on indigo ; of 
Sir William Perkin, Hofmann, Otto and Emil Fischer, Meldola and many 
others on the basic dyes ; of Griess and his host of followers on the azo- 
compounds ; of Arthur Perkin and of Kostanecki on the flavones and 
flavonols ; of Willstatter on the respiratory pigments and the antho- 
cyanins ; and, not least, of Hans Fischer on the synthesis of the prosthetic 
group of the blood pigment. 
No attempt can be made to cover this vast field, but the mere mention 
of these topics serves to prove the immense theoretical and practical value 
of a study of organic colouring matters. ‘The work proceeds and a long 
chapter on the natural carotinoid pigments is even now being written by 
Karrer, Kuhn and others ; it is of great chemical and biological interest. 
Before dealing with the special group of the anthocyanins, some aspects 
of which have recently been studied at Oxford, attention may be directed 
to the analogies in constitution existing between natural colouring matters 
and artificial dyestuffs. 
As the result of the researches of Baeyer, indigotin is generally regarded 
as having the formula I, but a technical digression may be made to the 
effect that the formula II has not yet been completely disproved. 
vA JNEY_/CO 
Ges: & ae va | : 
7 Rg icin. C 
~ me \Y/ Ng aaa 
(I) (11) 
The oxidation of indoxyl to indigotin appears to favour I, but then 
Gabriel has shown that the oxidation of diketohydrindene (III) by means 
of alkaline persulphate furnishes dihydroxynaphthacenequinone (IV). 
OH 
JV SO O Sere 
2 CH: a 
(III) (IV) 
