I 
Colour and Chemical Constitution. 11 
events, a similar order appears to reign among the other classes of dyes 
which have been similarly investigated. 
The following are some of the deductions from these observations : 
(1) The weight of the substituting material is of little importance com- 
pared with its volume ; thus the addition of 8 bromine atoms has not much 
more effect than the addition of 2 o-methoxyl groups, and 4 iodine atoms 
have the same effect as 2 hydroxy 1 groups. This agrees with the usually 
accepted view that the atomic volumes of bromine and iodine are not much 
greater than that of hydrogen, whilst those of carbon, nitrogen, and oxygen 
are larger. 
(2) The position of the substituting group, whether ortho- or meta- to 
the phenolic hydroxyls, is important ; thus the meta-position has more than 
twice the effect of the ortho-position ; in addition, it may be noted that 
groups actually attached to a phenolic hydroxyl have very little effect. 
(3) The two first conclusions may l^e put in one by supposing that the 
shifting effect of substituting groups on the absorption-band of phenol- 
phthalein is essentially due to mutual interference of the two phenol rings ; 
such groups as project much from the ring, or are nearer to the junction 
(the central carbon atom), cause a great effect — an effect which is heightened 
when the group is unsaturated as in naphtholphthalein, the explanation 
being that the unsaturated ring is more rigid and causes greater interference 
to rotation than any combination of simply attached groups could do. 
(4) Groups in the phthalic ring have comparatively little effect, which is 
also the case with acidic groups in the phenolic rings. 
(5) Methoxyl groups have a remarkably large effect. This suggests 
that the oxygen atom is comparatively large in volume, and seems to support 
the notion suggested by the author in 1906 that oxygen is CXg where X is 
the unknown element (? halogen) of atomic weight 2. 
(6) The differences for the same substituent are not always similar — for 
example, in the different bromo-derivatives — a fact which seems to suggest 
that there is a mutual action between the groups themselves tending to 
distort the quinonoid ring which is the cause of the colour. Thus it is 
striking that the isomeric dibromothymol-, and dibromocarvacrol-phthaleins 
have almost the same spectrum, whereas there is a slight difference between 
thymol- and carvacrol-phthaleins lying in the opposite direction. 
(7) The effect of the bridge oxygen in fluorescein and its derivatives is 
profound, and must be ascribed to the prevention of rotation of the phenol 
rings. Contrary to expectation, however, the frequency of the absorption- 
band is raised — viz. from 18'05 to 20*05 — in passing from phenolphthalein 
to fluorescein, whereas the usual effect of mere loading of a molecule is to 
lower the frequency, as seen in the 39 derivatives of phenolphthalein 
described in Table I. It is probable, therefore, that the case of fluorescein 
is similar to that of the exception in the phenolphthalein series — viz. its 
