A N T I M O N Y : ITS HISTORY, CHEMISTRY, MINERALOGY ...

THE ANTIMONY PREPARATIONS AND THEIR USES. 163
in excess; but when they are in excess the black precipitate disappears,
owing to its being changed into the soluble hyposulphite of
iron. The contents of the reservoir are then well stirred, and, if
necessary, sulphide of calcium is added, until the black precipitate
of sulphide of iron reappears and remains permanent. At the same
time a certain proportion of hyposulphite of iron should remain in
solution. This condition is easily fulfilled when we operate upon a
sufficiently large amount of materials. After the precipitate has
settled the liquor is decanted, and is a neutral solution of hyposulphite
of lime, with a certain proportion of hyposulphite of iron
and of chloride of calcium.
"We should carefully avoid, in this regeneration of the hyposulphite
of lime, leaving in an excess of sulphide of calcium, which
will impair the coloration of the vermilion by causing the formation
of the ordinary orange-yellow sulphide of antimony. Therefore, if
the solution of hyposulphite of lime be yellow and alkaline, a liquor
charged with sulphurous acid should be added, until complete
neutralisation of the alkaline reaction.
"This solution of hyposulphite of lime, like the first, is employed
in the preparation of a new quantity of vermilion of antimony. The
mother-liquors, charged with sulphurous acid, are again neutralised
in the large reservoir by a new proportion of sulphide and oxysulphide
of calcium, and so on, until the liquors become so much
loaded with chloride of calcium that it becomes necessary to throw
them away, or to reserve them for some other purpose. But this
takes* place only after twenty or thirty operations.
"It is even possible to save the sulphurous acid in these exhausted
mother-liquors, by saturating them with a milk of lime. This produces
a precipitate of oxide of iron and of sulphite of lime, and leaves
chloride of calcium only in the mother-liquors. The precipitate,
mixed with sulphide of calcium, is transformed by sulphurous
acid into the hyposulphites of lime and iron. And if the proportion
of iron be too great, it may be precipitated by a slight
excess of milk of lime.
" The precipitate of vermilion of antimony left on the bottom of
the first tank is received into a conical cloth filter, and the drained
liquors are added to those of the reservoir. The tank is then rinsed
with tepid water, which is made to pass through the filter.
" The washing of the vermilion should be done very carefully, and
it is often necessary to empty the contents of the filter into a large
volume of pure water and to wash the filtrate several times by
decantation. The red sulphide is afterwards filtered again and dried