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Contraction and expansions of dental amalgams
8IO
CURRENT TOPICS.
[J. F. I.
Contraction and Expansions of Dental Amalgams.--The amalgams so extensively used in tooth fillingare required to have a slight expansion on setting so as to fillthe cavity tightly. If not sufficiently. expanded the space will be quickly invaded by the fluids and organisms of the mouth, and decay will occur. If the expansion is too great the tooth may be broken. A . W . GRAY communicates to the Physical Review (1921 , xviii, lO8) the results of an extensive study o.f the properties of amalgams prepared from commercial alloys, and finds much difference in the different samples. The experiments were ex:tended over considerable time. The general opinion is that tin causes contraction and silver expansion. A principal conclusion is as follows: The contraction found when amalgam from a properly adjusted dental alloy is packed under a very high pressure does not in any way prevent the making of a tight tooth filling, because packing hard en.ough to. cause contraction in such an amalgam will stretch the resilent den,tin more than enough to, make it follow the slight shrinkage of the filling. In fact, moderate contraction after very tight packing is an advantage, in that it relieves to some extent the straining of the tooth. The tighter the packing, the better the filling, because •heavy packing pressure not only adds to the strength of the amalgam, but also shortens considerably the time required for it to complete all its dimensional changes and become stable. Moreover, it secures much better adaptation of the filling to the cavity walls and, consequently, reduces liability to leakage. The pheno.mena described are to. be expected from a corLsideration of conditions that influence diffusion, solution, and crystallization. Accurate measurements of the dimensional changes, termed rea.ction expansions, ought to thro.w light on other problems o.f metallography and physical chemistry. H.L. Production of Organic Compounds by MicroSrganisms.--In his presidcntal-address entitled "The Laboratory of the Living Organism," DR. M. O. FORSTER, President of the Chemical Section, British Association for the Advancement of Science, pays tribute to the lowly yeast plant and its close relatives, certain bacteria and moulds. Yeast produces glycerol as well as alcohol. Bacillus maceraus produces acetone and acetic and formic acids. The following o.rganic compounds have also been obtained by the action of micro6rganisms : Acetaldehyde, dihydroxyacetone, butyl alcohol, butyric, oxalic, suecinic, fumaric, lactic, citric, and pyruvic acids. If the proper genus and species of micro6rganism be chosen and be given the proper food and the proper environment, it will produce the desired organic compound, and will work 24 hours per day. Certain of these microbiological processes are used on a commercial scale, for ins,tance in the manufacture of acetone and butyl alcohol. (Scientific M., 192I , xiii, 3Ol-3O8.) J.S.H.
CURRENT TOPICS.
[J. F. I.
Contraction and Expansions of Dental Amalgams.--The amalgams so extensively used in tooth fillingare required to have a slight expansion on setting so as to fillthe cavity tightly. If not sufficiently. expanded the space will be quickly invaded by the fluids and organisms of the mouth, and decay will occur. If the expansion is too great the tooth may be broken. A . W . GRAY communicates to the Physical Review (1921 , xviii, lO8) the results of an extensive study o.f the properties of amalgams prepared from commercial alloys, and finds much difference in the different samples. The experiments were ex:tended over considerable time. The general opinion is that tin causes contraction and silver expansion. A principal conclusion is as follows: The contraction found when amalgam from a properly adjusted dental alloy is packed under a very high pressure does not in any way prevent the making of a tight tooth filling, because packing hard en.ough to. cause contraction in such an amalgam will stretch the resilent den,tin more than enough to, make it follow the slight shrinkage of the filling. In fact, moderate contraction after very tight packing is an advantage, in that it relieves to some extent the straining of the tooth. The tighter the packing, the better the filling, because •heavy packing pressure not only adds to the strength of the amalgam, but also shortens considerably the time required for it to complete all its dimensional changes and become stable. Moreover, it secures much better adaptation of the filling to the cavity walls and, consequently, reduces liability to leakage. The pheno.mena described are to. be expected from a corLsideration of conditions that influence diffusion, solution, and crystallization. Accurate measurements of the dimensional changes, termed rea.ction expansions, ought to thro.w light on other problems o.f metallography and physical chemistry. H.L. Production of Organic Compounds by MicroSrganisms.--In his presidcntal-address entitled "The Laboratory of the Living Organism," DR. M. O. FORSTER, President of the Chemical Section, British Association for the Advancement of Science, pays tribute to the lowly yeast plant and its close relatives, certain bacteria and moulds. Yeast produces glycerol as well as alcohol. Bacillus maceraus produces acetone and acetic and formic acids. The following o.rganic compounds have also been obtained by the action of micro6rganisms : Acetaldehyde, dihydroxyacetone, butyl alcohol, butyric, oxalic, suecinic, fumaric, lactic, citric, and pyruvic acids. If the proper genus and species of micro6rganism be chosen and be given the proper food and the proper environment, it will produce the desired organic compound, and will work 24 hours per day. Certain of these microbiological processes are used on a commercial scale, for ins,tance in the manufacture of acetone and butyl alcohol. (Scientific M., 192I , xiii, 3Ol-3O8.) J.S.H.