278 Transactions of the Royal Society of South Africa. 
roughly intermediate between those of quinone and its dioxime. This 
gives a ratio : 0/ : NOH of 145 experimentally, whereas the ratio 
— 0— / —NOH— is 1 45-7-1 *2 or 1-21. Alternatively, quinonedioxime 
calculated from HO-NOH • C 6 H 4 ■ NOH • H, or from NO • C C H 4 • HNOH, 
should have A 412 instead of A 366, so that it is necessary to make some 
distinction between the two groups attached to the benzene ring. 
The old terms chromophore and chromogen involve two elements and a 
double bond and are now out of date, since, as I have shown in the first 
paper of this series, colour may arise merely from the attachment of two 
groups to a ring, the whole being oxidised. 
I propose the new term hapton * for the second element and its attach- 
ments, reserving the old term auxochrome for OH, NH 2 , NHAlk, and NAlk 2 
restrictively. The hapton is intended to be a general term for C, N, 
0, or S. 
Such a substance as sodium phenate, consisting of a ring and an auxo- 
chrome but no hapton, has not a strong absorption visible in great dilution 
(say N/ 40,000) as the true colouring-matters have. Technically it is a 
coloured substance with an absorption-band in the ultra-violet. Its penta- 
bromo substitution-product is yellow, the ' loading ' having raised the 
absorption-band from A 287 to about A 365, but the absorption-band is 
still not strong. 
Now let a hapton be added, giving in the unoxidised condition 
CH 3 C 6 H 4 0', NH 2 • C 6 H 4 • 0', HO ■ C 6 H 4 • 0', SH ■ C 6 H 4 • 0'. These still 
remain coloured substances, not colouring -matters. They are leuco-com- 
pounds, with only shallow absorption. 
Parahydroxybenzaldehyde is the simplest true colouring-matter giving 
a strong, sharp absorption-band even in high dilution. We thus require 
for a colouring-matter a ring, an auxochrome, and a hapton, and the whole 
must be oxidised. It is best to adopt an agnostic attitude as to the nature 
of the oxidation. The formula HO • C 6 H 4 • CHO refers to solid neutral 
hydroxybenzaldehyde, but the coloured aqueous alkaline solution may 
be 0' • C 6 H 4 • CHO, 0 : C 6 H 4 : CHO', 0 • C 6 H 4 • CHO' and half a dozen 
i i 
other formulge involving the valencies of the ring itself. 
Apparently the relation of the hapton to the auxochrome may be either 
ortho, meta, or para, but the bands of meta-compounds appear to be less 
pronounced. 
The term dye is restricted to colouring-matters which stick to fabrics 
or mordants, e.g. phenolphthalein is not a dye. 
* Cf. haptophore in bacteriology. 
