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FILLING TEETH 29

also to have been subjected to the action of some
vapour, for, as above mentioned, they arc not so
absokitely non- cohesive when annealed, and heating
them causes a vapour to be given off, just as with
other makes of non-cohesive gold. This vapour vdll
only occasionally be noticed with any kind of non-
cohesive gold, and no doubt is due to an excess of
whatever has been deposited on its surface.
Intermediate between the non-cohesive and the
cohesive gold is a kind known as semi-cohesive.
The name semi-cohesive attached to a make of
gold is sometimes misleading, for it will almost
always be found necessary to anneal it before use
(as it is worked on the cohesive principle), and it
usually becomes cohesive when well heated. There
are, however, makes of true, semi-cohesive gold, of
which the semi-cohesive globe-foil was an excellent
example. This kind of gold when fully annealed is
still semi- cohesive.
If non-cohesive gold of the kind that becomes
cohesive when fully annealed is lightly annealed on
a sheet of mica an excellent semi-cohesive gold may
be produced, although it demands great experience in
order to hit the happy medium between under- and
over-annealing it. Non-cohesive gold is often termed
soft gold, while cohesive gold is often spoken of as
hard gold. Semi - cohesive gold is frequently