Manganese oxide is any of a variety of manganese oxides and hydroxides. Sodium hydroxide also reacts with acidic oxides, such as sulfur dioxide. Many inorganic substances are insoluble too, including most metal oxides, sulfides, and silicates. Acid rain is caused by the emission of nitrogen oxides and sulfur dioxide. Slag is usually a mixture of metal oxides and silicon dioxide. Upon reacting with oxygen, alkali metals form oxides, peroxides, superoxides and suboxides. Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides. These gases include oxidizers such as ozone and various oxides of nitrogen. Stainless steel welding is suspected of producing carcinogenic fumes from cadmium oxides, nickel, and chromium. Protactinium forms mixed binary oxides with various metals. Nitrogen can be fixed by lightning that converts nitrogen and oxygen into (nitrogen oxides). This design lowers the emissions of both nitrogen oxides (NO) and carbon dioxide (CO). High temperature also leads to production of nitrogen oxides. For this reason, EDTA is able to dissolve deposits of metal oxides and carbonates. Both cuprous and cupric oxides are known.