DRY METHOD.
[Illustration: FIG. 47.]
Weigh up 5 grams, if the ore is rich, or 10 grams, if a poorer mineral.
Take a piece of combustion tube from 18 inches to 2 feet long, closed at
one end, and place in it some powdered magnesite, so as to fill it to a
depth of 2 or 3 inches, and on that a layer of an equal quantity of
powdered lime (not slaked). Mix the weighed sample of ore in a mortar
with 10 grams of finely powdered lime and transfer to the tube; rinse
out the mortar with a little more lime, and add the rinsings. Cover with
a layer of six or seven inches more lime and a loosely fitting plug of
asbestos. Draw out the tube before the blowpipe to the shape shown in
fig. 47, avoiding the formation of a ridge or hollow at the bend which
might collect the mercury. Tap gently, holding the tube nearly
horizontal, so as to allow sufficient space above the mixture for the
passage of the gases and vapours which are formed. Place the tube in a
"tube furnace," and, when in position, place a small beaker of water so
that it shall just close the opening of the tube. The point of the tube
should not more than touch the surface of the water. Bring the tube
gradually to a red heat, commencing by heating the lime just behind the
asbestos plug, and travelling slowly backwards. When the portion of the
tube containing the ore has been heated to redness for some time the
heat is carried back to the end of the tube.
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