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COLOUR AND CHEMICAL CONSTITUTION. 
Part IX. — An Empirical Law of Change of Colour. 
By James Moir, M.A., D.Sc, F.I.C. 
In Part I of this series (March, 1917) it was pointed out that the change 
of colour caused in phthaleins and fluoresceins by the introduction of 
substituting groups is not explicable in terms of the atomic weight — for 
example, tetrabromo- and tetriodo-phenolphthalein are very nearly the same 
in spectrum. 
A special investigation of the halogen-derivatives of the two parent- 
substances has now shown that the laws connecting the change of colour 
with the number and nature of the halogen substituents are quite simple 
and regular, with the result that the whole of the experimental results can 
be calculated (and the spectrum of unknown substances predicted) from a 
single simple algebraical formula. This formula is : 
in which n is the frequency (V^.) of the halogenated derivative, rio the 
frequency of the parent-substance (18*05 for phenolphthalein and 20-27 for 
fluorescein), m the number of halogens, and N the atomic number of the 
halogen (17 for CI, 35 for Br, and 53 for I). The table on p. 226 exhibits 
the close agreement between theory and observation. 
An examination of the algebraical formula shows that the factor involving 
the atomic number is small ; if we neglect it we arrive at the additive law 
enunciated in the previous sections of this work, addition of wave-length 
being the same as subtraction of frequency. It is very important to note 
that the formula holds for fluorescein derivatives as well as for phthaleins. 
In the case of the dihalogen-derivatives, two isomers are possible and 
were made and examined. The observation marked (a) in each case refers 
to the 2-2' derivative,* in which each phenol ring contains one chlorine (or 
bromine) atom, while that marked (b) refers to the 2-6 derivative, in which 
both halogens are in the same ring, the other phenol ring being unhalo- 
genated. The 2-2' derivative is obtained either from phthalic acid and 
^ The hydroxyl is counted as 1 in the numbering. 
