The combustion of coal, as a heat source, in the production of electricity releases two very acid producing gases. Sulfur dioxide and nitrous oxide will combine with water vapor already present in the atmosphere to form sulfuric acid and nitric acid. These two acids are the principal ingredients when we talk about acid rain. Acids are substances that dissociate, upon mixing with water, and release hydrogen ions (H+) that can be and are harmful to the environment at large. Acid rain upsets the balance of forestry soils, destroys existing vegetation, and acidifies the pH of rivers and streams, killing fish and other wildlife associated with aquatic environements. Lakes and ponds may have their pH corrected by "liming" a process of adding lime to the water to pull the pH back to the 7 mark, which is neutral on the pH scale.