, and much care should be taken to prevent any considerable
reduction from this cause. Subsequent ignition, with ready access
of air, reconverts the sulphide to sulphate unless a considerable
reduction has occurred. In the latter case it is expedient to add one
or two drops of sulphuric acid and to heat cautiously until the excess
of acid is expelled.]

[Note 6: Barium sulphate requires about 400,000 parts of water for
its solution. It is not decomposed at a red heat but suffers loss,
probably of sulphur trioxide, at a temperature above 900 deg.C.]

DETERMINATION OF SULPHUR IN BARIUM SULPHATE

PROCEDURE.--Weigh out, into platinum crucibles, two portions of about
0.5 gram of the sulphate. Mix each in the crucible with five to six
times its weight of anhydrous sodium carbonate. This can best be done
by placing the crucible on a piece of glazed paper and stirring the
mixture with a clean, dry stirring-rod, which may finally be wiped off
with a small fragment of filter paper, the latter being placed in the
crucible. Cover the crucible and heat until a quiet, liquid fusion
ensues. Remove the burner, and tip the crucible until the fused mass
flows nearly to its mouth. Hold it in that position until the mass has
solidified. When cold, the material may usually be detached in a lump
by tapping the crucible or gently pressing it near its upper edge. If
it still adheres, a cubic centimeter or so of water may be placed in
the cold crucible and cautiously brought to boiling, when the cake
will become loosened and may be removed and placed in about 250 cc.
of hot, distilled water to dissolve. Clean the crucible completely,
rubbing the sides with a rubber-covered stirring-rod, if need be.

When the fused mass has completely disintegrated and nothing further
will dissolve, decant the solution from the residue of barium
carbonate (Note 1). Pour over the residue 20 cc. of a solution of
sodium carbonate and 10 cc. of water and heat to gentle boiling for
about three minutes (Note 2). Filter off the carbonate and wash it
with hot water, testing the slightly acidified washings for sulphate
and preserving any precipitates which appear in these tests. Acidify
the filtrate with hydrochloric acid until just acid, bring to boiling,
and slowly add hot barium chloride solution, as in the preceding
determination. Add also any tests from the washings in which
precipitates have appeared. Filter, wash, ignite, and weigh.

From the weight o