When two metals are so situated in the mouth that the mucous membrane forms a connecting conductor and the fluids are capable of acting on one metal, galvanic action is established sufficient to decompose any of the binary compounds contained in these fluids; the liberated nitrogen and hydrogen form ammonia, which being exposed to the action of oxygen is decomposed and nitric oxid formed, resulting in nitric acid. We also have in the mouth air, moisture, and decomposing nitrogenous food to assist in the production of nitric acid. "Galvanic action is more likely to develop hydrochloric acid, for the chlorids of sodium and potassium are present in the normal saliva and mucus, and when decomposed their chlorin unites with the hydrogen derived from the water of the saliva." (Dr. George Watt.) The fact should also be noted that both nitric and hydrochloric acids are administered as medicine, and often assist in producing decay. When there is a battery formed in a mouth containing tin fillings and gold fillings, and the fluids of the mouth are the exciting media, tin will be the positive element and gold the negative element; thus when they form the voltaic pair, the tin becomes coated or oxidized and the current practically ceases. There is more or less therapeutical and chemical action in cavities filled with tin, and its compatibility and prophylactic behavior as a filling-material depends partly upon the chemical action which occurs. Some dentists fill sensitive cavities with tin, in order to secure gentle galvanic action, which they believe to be therapeutic, solidifying the tooth-structure. "Tin possesses antiseptic properties which do not pertain to gold for arresting decay in frail teeth; it not only arrests caries mechanically, but in chalky (imperfect) structure acts as an antacid element in arresting the galvanic current set up between the tooth-structure and filling-material." (Dr. S. B. Palmer.) If the metal is acted on, the tooth is comparatively safe; if the reverse, it is more or less destroyed. The galvanic taste can be produced by placing a piece of silver on the tongue and a steel pen or piece of zinc under it; then bring the edges of the two pieces together for a short time, rinse the saliva around in the mouth, and the peculiar flavor will be detected. "In 1820 attention was called to the injurious effects of the galvanic current on the teeth, and dentists were advised never to use tin and amalgam in the same mouth.