Colour and Chemical Constitution. 
163 
about 0-990. This is completely discrepant with the phenolphthalein 
figures. 
C. The chloro-derivatives follow as before from the bromo-derivatives 
1499 
by multiplying by as often as the change from bromine to chlorine 
is made. 
D. The nitro-derivatives are not materially different from the bromo- 
derivatives. The a-, d-, g-, and j-nitro colour-factors are all about 1-016, 
and the c-, f-, and k-nitro colour-factors are all about 1-012. 
E. The methyl colour-factors are considerably smaller than the bromo 
ones, the methoxyl colour-factors are smaller still, say 1-006 for the biggest 
of them, viz. the a-methoxyl, and the hydroxy colour-factors are smallest 
of all, practically unity. Indeed, when caustic alkali is employed all the 
oxy-derivatives of fluorescein have their absorption band in the same place 
as that of fluorescein. Gallein and oxyquinolphthalein constitute excep- 
tions to this rule. 
Two examples of the method of calculation may be given : — 
1. Octobromofluorescein : X x = 493-5 x 1-0162 x 1-0091 x 1-0162 x 1-0122 
X 1-0157 x 1-0122 x 1-0157 = 543-0. The observed value was 544. 
/1499 
2. Octochlorofluorescein : A r = A for octobromofluorescein x 
J x \1500 
This is 540-0. The observed value was 537. Further experiment may 
show that the ratio CI/Br is not so near to unity, the error only showing 
in this extreme case. 
B. — Derivatives of Resorcin-benzeine. 
Resorcin-benzeine is phenyl-2-7-dihydroxy-xanthhydrol and is fluores- 
cein minus the C0 2 H group. The same numeration is therefore used, with 
m for the position occupied by the new hydrogen. Resorcin-benzeine in 
alkaline solution appears identical with fluorescein in colour and intensity 
of fluorescence. The absorption-band is at A 492. 
