262 ALBERT D. BROKAW 
of 11.02 N/1o acid. The manganese in solution was then con- 
verted to manganese dioxide by a standard method* and redis- 
solved in the standard mixture of oxalic and sulphuric acids. 
Essentially the same loss of acid was obtained (10.89 c.c.), proving 
the first precipitate to be manganese dioxide. 
The equation for the precipitation may be written as follows: 
2AuCl,+3MnCl,+12KOH= 2Au+3Mn0,+6H,0+12KCl, 
or, ionically: 
2Auttt++3Mntt=2Aut+3Mntttt 
It is evident from the preceding experiments that the precipi- 
tation may start before all the acid is neutralized, depending on 
the concentration of gold and manganous salt in the solution. 
Consequently substances which can partially neutralize the acid 
ought to induce precipitation. To test this conclusion a mixture 
of gold and manganous chlorides? in solution was made up as before, 
and small crystals of calcite were added. After 3 days the crystals 
were entirely coated with a brown precipitate of manganese dioxide 
(hydrated) and particles of gold were distinctly visible on close 
inspection. The equation may be written: 
2AuCl,+3MnCl,+6CaCO;=2Au+3MnO0O,+6CaCl.+CO, 
b) Manganese carbonate.—In the similar way it was shown that 
rhodochrosite can cause the precipitation of gold. In this case 
the reaction doubtless involved an additional stage, as follows: 
MnCO,+2HClI=MnCl.+CO,+H.0, 
after which the reaction was analogous to the one obtained with 
calcite, with the substitution of MnCO, for CaCO,. In both cases 
carbon dioxide was evolved in noticeable quantity. Doubtless 
manganiferous calcite can react similarly. 
It must be admitted that the concentration of gold and manga- 
nous salts used in these experiments were greatly in excess of any 
reported mine waters, but higher concentrations were necessary 
in outlining the nature of the chemical reactions involved. It 
remains to be shown that similar reactions can take place in 
extremely dilute solutions. Accordingly a liter of N/500 manga- 
t Clowes and Coleman, Quantitative Analysis, p. 259. 
2 MnCl, was used instead of MnSO, to avoid the precipitation of gypsum. 
