Colour and Chemical Constitution. 
307 
nearly the same effect on colour, their colour-factors being 1-013 ± 0*001 
(9th to loth of the known factors) ; also that in the ^ position the effect is 
twice as great, the colour-factor being 1'025 ± •002. 
Before finishing with the phthaleins it may be noted that the behaviour 
of the para-position in the phthalic ring both of phenolphthalein and of 
fluorescein is usually abnormal — a point which is discussed in a separate 
communication. Another exception is oxydiphenylphthalide, which being 
fuchsonecarboxylic acid should have X 574, whereas observation gave X 560. 
It is very difficult to obtain this substance free from phenolphthalein, and it 
is excessively easily bleached by alkali, so much so that the American writers 
on colour describe it as colourless in alkali. 
The 14th colour-factor, that of y-methoxyl, depends on two observations 
only, and is therefore somewhat in doubt: its value is 0'973. This makes 
the y-CH3 in phenolphthalein-monomethylether have a factor value of 
1-007, whereas the e-CH.^^ h as , as mentioned just above, the value 1-012. 
The a-CHg and the S-CH3 both appear to have a value about 1-024, which 
is practically the same as the N-CH3 given at the beginning (1-0245), 
so need not be counted as a new factor. 
The 15th and 16th colour-factors are those of e-hydroxyl and e-methoxyl: 
their values are comparatively large, viz. 1"030 and 1-037 respectively. It 
is very significant that e-OH and y-OH are practically the reverse of one 
another (1-030 x 0-9657 = 0-9946). An example of this is the substance 
catechaurine (/i.^-trioxyaurine), made by condensing catechol with oxalic 
acid. The systematic name is iSg-y^-hexaoxyfuchsene, and its theoretical 
absorption is therefore 593 x (0-9946)-^ or 583J, which agrees with observa- 
tion (in dilute NaHCO^ solution). It should be noted, however, that these 
polyhydroxyl-compounds from catechol are capable of exhibiting a second 
colour when dissolved in excess of NaOH. This is always a very high colour, 
generally green, and the colour-factor is then about 1-085 for the b or jS 
hydroxyl — really the £ or /3 ionised oxygen O'. For example, di-catechol- 
phthalein in NaOH shows X 652 for its absorption-band, and by theory 
593 X (0-9657)2 x (l-085)-^ x 1-002 = X 653. 
It should also be noted that when guaiacol combines to a phthalein or 
aurine, it is probable from the evidence stated in Part TV that the — OCH3 
group is para- and the —OH group meta- to the centre instead of vice versa. 
If then the 16th colour-factor just mentioned as being equal to 1'037 and 
derived from these guaiacol compounds is not f-methoxyl in presence of 
y-hydroxyl but y-methoxyl accompanied by /3- (or £-) hydroxyl, the value 
of that combination, previously ascertained, should agree with 1-037 X y- 
hydroxyl. Now y-OH is 0-9657, /3-OH is 1-030, and y-OCHs is about 
0-973. Combining these we find 1-030 x 0*973 = 1-003 and 1-037 x 0*9657 
* The distinction between the positions (6 and £ is only valid when y is not 
occupied by OH, and does not exist in aurine compounds. 
