Page 104 - My FlipBook

100 THE TECHNICAL PROCEDURES IN FILLING TEETH.

These general facts with regard to gold may readily be illus-
trated by a few simple experiments, which any one can perform.
Ammonia is strongly attracted to gold. Place a small quantity
of spirits of ammonia, or of aqua ammonia, in a large glass jar.
The ammoniacal gas from this will fill the space above the liquid.
Now take a rope of gold which has been annealed and the weld-
ing property of which is perfect, and swing it by a thread above
the liquid in the jar and replace the cork. In fifteen minutes
remove the gold and try its welding property. It will not weld
any more than so much tissue paper. If it is swung above
chlorine water the welding property will be completely destroyed
in two minutes. Now reanneal these ropes of gold ; the welding
property is completely restored.
How are we to know that this effect is produced by a
condensation of gas on the surface of the gold ? Place
the gold first in chlorine gas for ten minutes, and then transfer it
to ammonia for an equal time. Now, as these two gases unite
to form a volatile salt, ammonium chloride, which readily crys-
tallizes upon any cold substance, place the gold thus treated in
a long test tube, and heat it quickly over a bunsen burner.
Immediately white fumes begin to leave the gold, and these
crystallize in a white ring on the colder portion of the test tube.
Chemical examinations of these crystals show them to be ammo-
nium chloride. This could occur only by the condensation of
the gases on tha gold, and the amount formed shows this con-
densation to be in very considerable quantity. The experiment
may be varied by placing the gold first in the gaseous ammonia,
and then transferring to the chlorine, but in this case there will
not be so large an amount of the ammonium chloride formed,
for the reason that the ammonia is not condensed on the gold in
so large a quantity as the chlorine.
In this experiment the salt formed is volatile, and the gold is
readily cleaned by heat. But suppose the salt formed were a
fixed salt that we can not volatilize by the annealing temperature ;
then the welding property of the gold is permanently destroyed.
This is what often occurs when the gold is not well protected.
Now one of the principal reasons why the crystalline forms of
gold are more difficult to keep in good condition than foil, is the
fact that the crystals form a sponge that more readily takes up
and holds gases.