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COLOUR AND CHEMICAL CONSTITUTION 
Part XII. — The Calculation of Colour from the TAJjTO^^^^tfh p ^ \ ^ ., r^<y\cS-'J^y' 
Theory. 
By James Moir, 
The scheme suggested in Part X of this work for calculating the colours 
of the triphenylcarbinol dyes, while no doubt of considerable practical 
importance, has two great drawbacks from the point of view of chemical 
theory, viz. : (1) that the parent-substance, f uchsene, of wave-length 593, may 
be wholly imaginary, and (2) that no light is thrown on coloured substances 
of smaller complexity or of different chemical families. 
In the search for a general and fundamental theory of colour I have 
examined all that have been put forward in the last forty years, an excellent 
summary of which appears in E. R. Watson's Colour in Relation to 
Chemical Constitution (Longmans, 1918). It is evident first of all that 
no one theory fits all cases, from diazomethane to indigo for example. The 
most promising from a quantitative point of view (i. e. leading to the 
possibility of calculating colour with some exactitude), appeared to be the 
Watson-and-Meek modification of Hewitt's suggestion that depth of colour 
depends on length of continuous tautomeric change or alternation of single 
and double bonds in the molecule (/. c, pp. 87-91). 
I have therefore analysed my observations of the wave-lengths (absorp- 
tion-centres) of all the common dyes to see whether numerical values in 
wave-length could be assigned to each possible kind of tautonierism, which 
could then be added up to give a result approximating, to that observed in 
the dye. This I found to be the case, and the main part of this paper will 
be found to consist of calculations of colour, with the observed figure 
appended for comparison. It will be seen that the theory embraces many 
other classes of coloured substances than the " aniline dyes." 
The additive property of colour has been sufficiently demonstrated in the 
earlier sections of this woi'k ; it now remains to give it a physical basis. 
The reader has first to remember that frequency of vibration is the inverse 
of periodic time, and that wave-length is inverse to frequency. It follows 
that wave-length is directly proportional to periodic time. The simplest 
assumption therefore is that the observed wave-length (absorbed by a 
highly- dilute coloured solution) is physically caused by an electron travelling 
through a sinuous orbit, which is completed (on the average) in a periodic 
time during which light would travel through that distance (e. g. 602 yu/> 
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