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COLOUK AND CHEMICAL CONSTITUTION. 
Part XIII. — The Calculation of the Colour of the Monocyclic 
Dyes. 
By James Moir. 
Readers of Part X of this work will remember that by assuming 
triphenylcarbinol (or its anhydride) to have a potential colour with \ 593 
for the position of its absorption-band, I was able to show that the colour 
of any dye derived from it could be calculated by successive multiplying 
of X 593 by factors characteristic of every substituting atom (or group) and 
position contained in the dye. 
Among the simpler members of the family, however, several exceptions 
were met with. Thus the salts of fuchsimonium or amino-triphenylcarbmol 
had a calculated X = 593 x 0 972= X 570, whereas observation gave the much 
lower value of about A 425 (in water). When Part X was written I 
suggested, as many other writers on colour have also done, that certain 
substances only exhibit harmonics of their calculated colour. Further 
experiment has now shown me that this hypothesis is not only unnecessary, 
but probably quite untrue. 
The true explanation is that the scheme described in Part X does not 
apply when the dye is " monocyclic," i. e. when it contains only one active 
or ionisable colour-ring. This means that the high colours (between X 470 
and X 760) of the commercial dyes are in practically every case due to a 
" dicyclic " constitution, namely, one in which two ionisable colour-rings 
co-operate. 
To make my meaning perfectly clear it will be sufficient to give a list 
of " ionisable colour-rings," viz. : 
— <^ ^NHj, — / ^>OH, — / ^> — NHMe, -/ V_ NMe 2 , 
and the other four in which the loose linkage is ortho, not para, to the 
ionised group. There are no doubt others in which thiophene, furfurane 
and pyrrhole are the rings. 
An example of a monocyclic dye is dimethylaminobenzhydrol Ph . CH 
(OH) . C 6 H 4 NMe 3 . Two analogous dicyclic dyes are: 
