Where there is considerable decomposition of food which produces sulfuretted hydrogen, the sulfid of tin may be formed on and around the fillings; it is of a yellowish or brownish color, and as an antiseptic is in such cases desirable. To offset the discoloration, we find that the sulfid is insoluble, and fills the ends of the tubuli, thus lending its aid in preventing further caries. A sulfid is a combination of sulfur with a metal or other body. A tin solution acted on by sulfuretted hydrogen (H_{2}S) produces a dark-brown precipitate (SnS), stannous salt, which is soluble in ammonium sulfid (NH_{4})_{2}S_{2}; this being precipitated, gives (SnS_{2}) stannic salt, which is yellow. Brown precipitates are formed by both hydrogen sulfid and ammonium sulfid, in stannous solutions. Yellow precipitates are formed by hydrogen sulfid and ammonium sulfid in stannic solutions. The yellow shade is very seldom seen on tin fillings; the dark brown is more common. An oxid is a combination of oxygen with a metal or base destitute of an acid. In oxidation the oxygen that enters into combination is not sufficient to form an acid. The protoxid of tin (SnO) is black, and can be obtained from chlorid of tin, or by _long_ exposure of tin to the atmosphere. The oxygen in the saliva helps to blacken the tin, and the metallic oxid penetrates the dentin more or less, acting as a protection, because it is insoluble. Oxygen is the only element which forms compounds with all others, and is the type of electro-negative bodies; it combines with all metals, therefore with tin, and in many cases only the metal is discolored, and not the tooth. Steam boilers are made tight by oxidation. Where there is complete oxidation, the tooth is blackened to but a very slight depth, and the oxid fills the ends of the tubuli, thus affording an additional barrier to the entrance of caries. The filling itself will prevent caries, but oxidation acts as an assistant. "In the mouth, a suboxid is more likely to be formed than a protoxid, but both are black; sulfur and oxygen are capable of acting on tin under favorable circumstances, such as warmth, moisture, full contact, condensation of elements, and their nascent conditions; the first three are always present in the mouth. The protosulfuret of tin is black." (Dr. George Watt.) Others give the color as bluish-gray, nearly black. Experiments show that slight galvanic currents exist between fillings of dissimilar metals in the mouth, and practical experience demonstrates that these currents occasionally produce serious results. Direct galvanic currents do not decompose normal teeth by true electrolysis, but acids resulting from decomposition of food and fluids react upon the lime constituents of the teeth and promote secondary caries.