( 273 ) 
COLOUR AND CHEMICAL CONSTITUTION. 
Part XVII. — The Azo Dyes and other Monocyclic Colours. 
By James Moir. 
Some preliminary work on the azo dyes appears in Part XII of this 
work, pp. 215-216, accompanied by the remark that the problem appeared 
hopeless of solution. Since then, by examining all the related simpler 
substances, I have discovered a method of calculating their colours, and 
thus uniting them to the monocyclic class already discussed in Part XIII. 
The examination of the simpler substances could only be done by 
photography. Other colour chemists will therefore join me in thanking 
Mr. E. R. Grills (of Hortor's Ltd., Johannesburg), who presented me with 
a diffraction-grating specially mounted on a thin cover-glass such as is 
used in microscopy : this instrument enabled me to photograph down to 
A 320 and thus find the absorption of many colourless substances which 
required to be examined in order to discover the theory of the azo dyes. 
The fact that I had to use a glass (not quartz) lens prevented a complete 
investigation further down in the ultra-violet. This I hope to attack 
later on. 
Remarkable to say, the very first photograph I took with the new 
apparatus gave a discovery. Alkaline paraoxyazobenzene in water was 
the substance the spectrum of which was photographed, and the negative 
showed not one broad band at A 420, as previously supposed from visual 
observation (Tuck, J.C.S., London, 1907, diagram on p. 450, and Moir, 
loc. cit.), but two distinct although overlapping bands at AA 433 and 395. 
Further investigation showed that all the members of the family show this 
phenomenon : sometimes, as in acid butter-yellow, both bands are very 
distinct, and sometimes they are so overlapped that only a faint luminosity 
appears in the middle and the band looks at first like a single very broad 
one ; in such cases the estimate of the centres of the two bands is quite 
uncertain, being in doubt by more than five units. 
Again, all the substances are indicators and change colour if made 
acid or alkaline, thus giving three kinds of spectrum according to the 
