SECTION B—CHEMISTRY. 
NATURAL COLOURING MATTERS 
AND THEIR ANALOGUES 
ADDRESS BY 
PROF. ROBERT ROBINSON, F.R:S. 
PRESIDENT OF THE SECTION. 
On taking the Chair of this Section I should like to express my thanks 
for the great honour which you and the Council of the British Association 
have conferred on me. 
Although the subject which I have selected for my address is necessarily 
somewhat technical, it occurred to me that the problem of flower colour 
is of general interest, and the gist of what I have to say is a contribution 
to the answer to the question: why are some flowers blue and others, 
containing the same pigment, ved? In the interests, too, of members of 
this audience who are not organic chemists I propose to allow the spoken 
to diverge from the written word, and I shall venture also to attempt the 
performance of a few simple experiments. 
In every country and throughout the ages emotions have been stirred 
and curiosity aroused by the display of colour in Nature, but it is perhaps 
not generally realised that the ready availability of artificial colouring 
matters suitable for every kind of tinctorial purpose, from boot polish 
to finger nails, is a comparatively recent development. We read of the 
ancient Tyrian purple, the purple of kings, and of the red cosmetic pig- 
ments of natives of the Orinoco, so rare that they were used as the basis 
of exchange ; in contrast, at the present time dyes of all shades may be 
indulged in to an extent controlled certainly by individual courage, taste and 
discretion, but hardly at all by limitations of purse or social status. It may 
be that this ‘ freedom of the hues ’ has been enjoyed for so brief a period 
that a state of equilibrium has not yet been reached and we are not using our 
privileges in this matter either as fully as possible or as wisely as possible. 
It is not, however, for an organic chemist to discuss such problems as 
that of masculine sartorial conservatism on the one hand, or to attempt an 
estimate of the esthetic value of the film-fan magazine cover on the other. 
The chemist has been attracted to the investigation of natural and 
artificial colouring matters for a variety of reasons, including not only 
colour-pleasure, the incentive of the knowledge that chlorophyll and 
hemoglobin perform some of the most important functions in vital pro- 
cesses, and the industrial importance of dyestuffs and pigments, but also 
on account of the fact that visible colour more than any other property 
facilitates the experimental study of organic substances whether by 
analysis or synthesis. It furnishes a standard of homogeneity or a measure 
of concentration, it is an invaluable guide in the search for methods of 
