132 
The N.Z. Journal of Science and Technology. [June 
is very rapid at first, and then diminishes. Plotting from the curve, we 
obtain, — 
Addition of- — 
8 c.c. NaOH, 
making solution XNa 2 S 
12 c.c. ,, 
? ? 
XNa 2 S 
16 c.c. ,, 
5 ? 
XNa 2 S 
20 c.c. ,, 
? 5 
XNa 2 S 
24 c.c. ,, 
,. 
XNa 2 S 
f NaOH 
4 
| NaOH 
^ NaOH 
X NaOH 
0-370 
0-505 
0-620 
0-695 
TO T3 
?-l 
O ^ 
0-760 ' 
K 
* This point obtained by interpolation. 
That is, the addition of caustic soda in amount equal to a quarter of the 
percentage of Na 2 S present increases the solubility of the HgS by 37 per 
cent. ; if half the percentage is added the increase is 70 per cent. ; while 
if the same percentage of NaOH is added the solubility is doubled. 
To confirm this, an excess of mercuric sulphide was treated with a 
4-per-cent, solution of sodium sulphide, without the addition of soda, and 
with the addition of amounts equivalent to 1, 2, 3, and 4 per cent. The 
results were,—- 
Experiment. 
Na 2 S, 4 per Cent, 
taken. 
' •• V 
NaOH present. 
Grammes HgS 
dissolved. 
Per Cent. Increase 
on A. 
A 
16 
Nil 
0-64 
B 
16 
1 per cent. 
0-84 
31-3 
C 
16 
2 „ 
1-04 
62-5 
D 
16 
3 
1-16 
81-2 
E 
16 
4 
l 26 
96-9 
