114 Transactions of the Royal Society of South Africa. 
frequency of vibration by 4'1 per cent., and the entrance of a second pair 
only brings the depression to 7*2 per cent. The same law is true of all the 
others, and may be stated as follows — that the entrance of the first 
grouping causes a change equal to about 58 per cent, of the total change 
due to two groupings. 
This phenomenon is easily explained in terms of the interference of the 
rings, or rather of their projecting substituents, with one another (see 
Part I). It is obvious that the entrance of one — OCH3 group into one 
ring of phenolphthalein will cause a great interference with the other empty 
ring and that the entrance of — OCH3 into the other ring will cause a 
second interference, but not so great. 
The Sulphone-phthaleins (so-called Sulphtjreins). 
I have discovered that some of these can be easily made by heating 
phenols with " saccharin " (the artificial sweetening agent) and a little 
sulphuric acid, followed by boiling the product with dilute acid to split off 
ammonia. They form excellent indicators. In my opinion, phenolsulphone- 
phthalein is the best general indicator yet discovered for sensitiveness and 
definiteness of colour. Its constitution is that of phenolphthalein with the 
group CO replaced by SO2 ; it is therefore, chemically speaking, the ortlio- 
sulphonic acid of benzaurine (see Part I). The following table gives the 
spectra of those examined so far • 
Table II. 
No. 
Name. 
Colour in 
Alkali. 
Wave-length 
absorbed in 
Alkali. 
Wave-length 
abBorbed in 
iVHCl. 
Wave-length 
absorbed in 
cone. H3SO4. 
1 
Phenolsiilphonephthalein . 
Purple- 
563 
507 
500 (practi- 
pink 
cally same 
as phenol- 
phthalein). 
2 
Thymolsulphonephthalein 
Blue 
604 
? 
About 570 
3 
a-naphtholsulphonephtha- 
Olive- 
About 730 
? 
About 720 
lein 
green 
and 500 
4 
Guaiacolsulphonephthalein 
Blue 
608 
565 
570 
5 
Eesorcinolsulphonephtha- 
Salmon 
497 (and 
576 
{Cf. fluores- 
lein 
(fluorescent) 
522 faint) 
cein). 
In all these cases the neutral colour is a shade of yellow or brown, not 
colourless as in the case of the ordinary phthaleins, but none of them when 
neutral gives a spectrum containing an absorption band ; loss of more or less 
of the violet is all that can be observed. No satisfactory explanation of 
this coloured neutral phase of the sulplionephthaleins has yet been put 
