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GOLD. 271

length, and in width equal to the sheet of foil. Two little drawer-
knobs of ebony should be inserted into the centre of each of the pieces
of board. These act the part of handles for holding the appliance.
The gold is then placed upon the kid strip between the two pieces of
board, and by bringing the two surfaces of the kid in contact the foil is
rolled between them. The undressed surface of the kid should be the
•one upon which the gold is rolled. Ropes thus made may be cut in
lengths to suit the size of the cavity to be filled, and, as gold thus pre-
pared has great softness and ease of adaptation, it may be inserted in
quite large pieces if plenty of condensing force be applied to it.
Cohesive Gold Foil.—All gold which has been refined by any of
the ordinary methods and is in a pure state may be said to be cohesive.
Nor is absolute freedom from alloy an absolute necessity. It has been
shown that softness is dependent upon purity, but a foil may contain
quite a percentage of silver, copper, palladium, or zinc, and yet its
cohesion may not be impaired. It may also be alloyed or combined
with platinum and not lose its cohesive properties. It is, however,
desirable that cohesive gold be pure, since the smallest percentage of
alloy destroys its softness.
When two sheets or laminae of freshly annealed foil are brought into
contact and slight pressure is applied, they form a permanent union and
are practically inseparable. It is this property in gold to which the
term cohesive has been applied. But this property is soon lost by the
occlusion of gases or impurities of any kind which may be deposited
upon the surface of the gold.^
Experiments have demonstrated the fact that if the gold be sub-
jected to the fumes of ammonia, hydrogen, hydrogen carbid, hydrogen
phosphid, or sulfurous acid gas its cohesive property is quickly de-
stroyed, but this property may be restored by heat except in the case
of sulfur or phosphorus fumes. Hence the importance of excluding
the gold as much as possible from the atmosphere, especially during the
Avinter months when gases arising from the combustion of coal are most
liable to be present in the operating room.
Dr. Black has shown that ammoniacal gas has the power to prevent
the deleterious influence of other gases, and recommends that the foil
be subjected to the influence of carbonate of ammonia by keeping it in
a drawer with a bottle of that salt.
The advantages of cohesive foil cannot be overestimated. With its
introduction in 1855 began a new era in the possibilities of saving cari-
ous teeth. Operations which were deemed impossible by the use of
non-cohesive foil were made comparatively easy by the intelligent use
of cohesive foil. The restoration of broken-down or badly decayed
^ G. V. Black, Dental Cosmos, vol. xvii. p. 138.