TRANSACTIONS OF SECTION B. 753 
given by human urine when its acidity is low, and is in this case due to the exist- 
ence together of the dihydric and monohydric phosphates, MH, PO, and M, HPO,, 
of which the former acts as an acid and the latter as an alkali towards litmus. 
The actual colour resulting is violet. No satisfactory explanation has been offered 
of the nature of the change in the litmus when the violet is produced. Heintz? 
supposed that litmus was of the nature of a diabasic acid, the red pigment con- 
taining two atoms of displaceable hydrogen, which in the blue litmus were replaced 
by a metal. He supposed that the monohydric and dihydric phosphates displaced 
only one atom of hydrogen or of metal, forming a body analogous to an acid salt— 
the violet litmus. He seems to have overlooked the fact that the violet litmus is 
only produced when both phosphates act together—alone they produce red and 
blue litmus. 
The object of the present communication is to show that the violet litmus is a 
mechanical mixture of the red and the blue. It can be shown on theoretical 
erounds that when both phosphates are present in nearly equal proportions they 
must each affect the litmus in their own special way, the exact amount of blue and 
red produced being determined by the mass action of the phosphates. If red 
litmus is provisionally represented by the formula LH the reactions may be repre- 
sented as follows :— 
nM,H PO, +” M H, PO, +2L=(n—1) M,HPO,+(n+1) MH,PO,+LH+LM. 
n M,H PO, +nMH, PO, +2 LM=(n+1) M,HPO, + (x—1) MH, PO,+LM+ LH. 
The violet litmus is shown to be a mixture of the red and the blue by observing 
the light transmitted through its solution by the eye and by the spectroscope. 
This light is, in all respects identical with that transmitted through the red and 
the blue successively. Cochineal behaves in an analogous manner. 
It can be shown that acetic acid behaves in a manner analogous to litmus in 
presence of the two phosphates. A small quantity of sodium acetate is added to 
a strong solution of NaH, PO, and Na,H PO,. On distilling the solution free 
acetic acid comes over. The solution is then taken to dryness, the residue dis- 
solved in water and acidulated with a few drops of dilute sulphuric acid. On 
distilling a further quantity of acetic acid comes over. Thus it appears that 
sodium acetate behaves in a manner analogous to blue litmus in the amphoteric 
solutions. To sum up :— 
1. There is no evidence to show that any special modification of litmus is pro- 
duced by amphoteric solutions, 
2. The violet litmus is a mixture of the red and the blue. 
3. The amount of the two forms of litmus in the amphoteric solution is deter- 
mined by the mass action of the two kinds of phosphate. 
7. Constitution of Sun Yellow or Curcumine, and Allied Colowring 
Matters. By Artuur G. GREEN and ANDRE WAHL. 
When caustic soda or caustic potash is added to a hot concentrated solution of 
paranitrotoluene-ortho-sulphonate of soda the liyuid becomes at first bluish red, 
then changes to orange and deposits a thick orange-yellow precipitate (Walter, 
* Bull. Soc. Mulhouse,’ 1887, 99), The yellow colouring-matter thus formed, which 
is known in commerce as curcumine, sun yellow, &c., and dyes unmordanted 
cotton orange-yellow shades of considerable fastness to light and other agents, 
appears to consist for the most part of a body to which the constitution of an 
azoxy-stilbenedisulphonic acid 
Sk : CH 
N z 
Ren: > OuHa(80,Na) 
OK 
0) 
ms, Heintz, Wirzburger med. Zeitschr., 2, 230, 1861; Journ. fiir prakt. Chem., 85, 24, 
C,H,(SO,Na) 
