The alkaline salts are
extracted with water, and filtered from the magnesia into a weighed
platinum dish. The solution is then evaporated with an excess of
hydrochloric acid, ignited at a low red heat, and weighed. The residue
consists of the mixed alkaline chlorides.
For substances (such as most silicates and similar bodies) not
completely decomposed by acids, Lawrence Smith's method is generally
used. This is as follows:--Take from 0.5 to 1 gram of the finely
powdered mineral, and mix, by rubbing in the mortar, with an equal
weight of ammonium chloride. Then mix with eight times as much pure
calcium carbonate, using a part of it to rinse out the mortar. Transfer
to a platinum crucible, and heat gently over a Bunsen burner until the
ammonic chloride is decomposed (five or ten minutes). Raise the heat to
redness, and continue at this temperature for about three quarters of an
hour. The crucible must be kept covered. Cool, and turn out the mass
into a 4-inch evaporating dish; wash the crucible and cover with
distilled water, and add the washings to the dish; dilute to 60 or 80
c.c., and heat to boiling. Filter and wash. Add to the filtrate about
1.5 gram of ammonium carbonate; evaporate to about 40 c.c., and add a
little more ammonic carbonate and some ammonia. Filter into a weighed
platinum dish, and evaporate to dryness.
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