160 
Transactions of the Royal Society of South Africa. 
phenolphthalein with bromine for hydrogen in the a, b, c, and d positions) „ 
we have A^ = A 0 (554) x 1-018 x 1-002 x 1-0145 x 1-0180 = 583-6. 
Again, it was shown in Part IX, p. 225, that the factors for chlorine 
and iodine are almost the same as those for bromine. Putting in the 
numerical values into the algebraical formula given on that page, we find 
that the colour-factor for chlorine differs from that of bromine by 
18x0-000037, or 1 part in 1500, and that the colour-factor for iodine is 
greater than that of bromine by a like amount. Hence, for example, the 
calculated colour of phenol-tetrachlorophthalein is 583-6 (that of phenol- 
/1499\ 4 
tetrabromophthalein just calculated) multiplied by , chlorine having 
v 1500/ ' 
replaced bromine four times. This is 582-1. This agrees with observation,, 
viz. A 581 and A 582 from two specimens. I am greatly indebted to 
Professors Partington of London and Mackenzie of Dundee for specimens 
of tetrachlorophthalic acid which they sent me after I had vainly tried to 
purchase it all over the w r orld for three years. 
/1500 V 1 
\1499/ 
= 585-2, but I have not been able to make (or obtain from its discoverer) 
this compound. 
In the same way, the calculated colour of ordinary (fgjk) tetrabromo- 
phenolphthalein is 554 x (1-0145) 2 x (1-0125) 2 = A 584-0, and the calculated 
/1499\ 4 
Similarly, phenol-tetraiodophthalein should have A=583-6x( 
colours of tetrachloro- and tetriodophenolphthalein are A 584-0 x 
/1500X 4 
and A 584-0 X | ) respectively. These agree with observation. 
\1500/ 
A. — Derivatives of Fluorescein. 
Employing the same numeration of the positions as was used for phenol- 
phthalein in Part XI, i.e. assuming that fluorescein is e-Z-oxophenol- 
phthalein, it is to be expected that the same colour-factors will hold good. 
Unfortunately, the experimental difficulties have in some cases proved 
