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308 PLASTIC FILLING MATERIALS.
a cinity inclosing it. This alloy contains, as stated, 90 per cent, of sil-
ver. The ap})earance of an expanded amalgam is similar to that of ice
at the mouth of an iron tube in which the water has been frozen.
Copper amalgam is the only alloy tested by Dr. Black which under-
went no change of form in hardening.
" Flow " ov Amalgam.—A property attributed to certain amalgams,
that of spheroiding, has been shown by Dr. Black to be without exist-
ence. The bulging of amalgams from the orifices of cavities was held
to be due to the tendency of the mass to assume a spheroidal form, hence
the term spheroiding. Tests showed the appearance to be delusive, the
phenomenon being due to expansion and not to a spheroidal tendency.
In addition to the properties of contraction and expansion the same
investigator has discovered the property, hitherto unsuspected in amal-
gams, that o^flow. The property of flow i. e. change of mass form, from
molecular motion under stress— had been observed in the majority of
metals, but as found in amalgams it has a unique expression. Instead
of being limited to a definite degree, proportioned by the stress applied,
it has been found that amalgams yield repeatedly to the same amount of
stress when applied at intervals, as in mastication, or yield continuously
when the stress is constant. The process appears to be without limita-
tions. It is at zero in copper amalgams ; next less in amount with alloys
containing 55-60 per cent, of silver with 5 per cent, copper and the
remainder tin. It will be readily seen that this property exercises a
great influence upon the integrity and adaptation of an amalgam filling.
The notes quoted from Dr. Black were compiled from studies made
of amalgams whose exact chemical composition had not been actually
tested by the investigator. Later experiments ^ made with alloys pre-
pared with the utmost care and exactitude by the investigator himself,
gave widely different results (particularly as to the effect of adding a
third or fourth metal to the basal alloy) in the direction of both flow
and shrinkage. The first series of experiments which appeared to show
an enormous increase of shrinkage and flow together with a lessening of
edge strength, by the addition of a third or fourth metal (except copper,
which the latest experiments still show to lessen flow and increase
rigidity) were not confirmed when Dr. Black experimented with alloys
made by himself, and an additional and unsuspected factor was taken
into consideration, viz. the influence of heat upon the alloy.
It has been noted by Dr. J. Foster Flagg^ that alloys which were
freshly cut possessed working properties different from the same alloys
when " old cut," or when aged. Dr. Black's observations appeared
to confirm this, and his later experiments were directed toward deter-
mining the cause underlying the change. Motion, which was said to
' Dental Cosmos, December, 1896. ^ Plastics and Plastic Fillings.
308 PLASTIC FILLING MATERIALS.
a cinity inclosing it. This alloy contains, as stated, 90 per cent, of sil-
ver. The ap})earance of an expanded amalgam is similar to that of ice
at the mouth of an iron tube in which the water has been frozen.
Copper amalgam is the only alloy tested by Dr. Black which under-
went no change of form in hardening.
" Flow " ov Amalgam.—A property attributed to certain amalgams,
that of spheroiding, has been shown by Dr. Black to be without exist-
ence. The bulging of amalgams from the orifices of cavities was held
to be due to the tendency of the mass to assume a spheroidal form, hence
the term spheroiding. Tests showed the appearance to be delusive, the
phenomenon being due to expansion and not to a spheroidal tendency.
In addition to the properties of contraction and expansion the same
investigator has discovered the property, hitherto unsuspected in amal-
gams, that o^flow. The property of flow i. e. change of mass form, from
molecular motion under stress— had been observed in the majority of
metals, but as found in amalgams it has a unique expression. Instead
of being limited to a definite degree, proportioned by the stress applied,
it has been found that amalgams yield repeatedly to the same amount of
stress when applied at intervals, as in mastication, or yield continuously
when the stress is constant. The process appears to be without limita-
tions. It is at zero in copper amalgams ; next less in amount with alloys
containing 55-60 per cent, of silver with 5 per cent, copper and the
remainder tin. It will be readily seen that this property exercises a
great influence upon the integrity and adaptation of an amalgam filling.
The notes quoted from Dr. Black were compiled from studies made
of amalgams whose exact chemical composition had not been actually
tested by the investigator. Later experiments ^ made with alloys pre-
pared with the utmost care and exactitude by the investigator himself,
gave widely different results (particularly as to the effect of adding a
third or fourth metal to the basal alloy) in the direction of both flow
and shrinkage. The first series of experiments which appeared to show
an enormous increase of shrinkage and flow together with a lessening of
edge strength, by the addition of a third or fourth metal (except copper,
which the latest experiments still show to lessen flow and increase
rigidity) were not confirmed when Dr. Black experimented with alloys
made by himself, and an additional and unsuspected factor was taken
into consideration, viz. the influence of heat upon the alloy.
It has been noted by Dr. J. Foster Flagg^ that alloys which were
freshly cut possessed working properties different from the same alloys
when " old cut," or when aged. Dr. Black's observations appeared
to confirm this, and his later experiments were directed toward deter-
mining the cause underlying the change. Motion, which was said to
' Dental Cosmos, December, 1896. ^ Plastics and Plastic Fillings.
