Acid rain is a rain or any other form of precipitation that is unusually acidic meaning that it has elevated levels of hydrogen ions. It can have harmful effects on plants aquatic animals and infrastructure. Acid rain is caused by emissions of sculpture dioxide and nitrogen oxide which react with the water molecules in the atmosphere to produce acids. Some governments have made efforts to reduce the release of sculpture dioxide and nitrogen oxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes and sculpture dioxide is produced by volcanic eruptions. Acid rain has been shown to have adverse impacts on freshwater and soils killing insect and aquatic life forms. It also causes paint to peel corrosion off steel structures such as bridges and weathering of stone buildings and statues as well as having impacts on human health. Acid rain is a popular term referring to the deposition of a mixture from wet and dry components. Once carbon dioxide is removed distilled water has a neutral pH of seven. Unpolluted rain can also contain other chemicals which affect its acidity level. A common example is nitric acid being produced by electric discharge in the atmosphere. The corrosive effect of polluted and acidic city air on limestone and marble was noted by John Evelyn who remarked upon the poor condition of the Arundel marbles. Since the Industrial Revolution emissions of sculpture dioxide and nitrogen oxides into the atmosphere have increased. Robert Angus Smith was the first to show the relationship between acid rain and atmospheric pollution in Manchester England. The term acid rain was coined by Robert Angus Smith. Harold Harvey was among the first to research a dead lake. At first the main focus in research lay on local effects of acid rain. Wildman Christopher was the first to acknowledge long distance transportation of pollutants crossing borders from the United Kingdom to Norway. The problem of acid rain has not only increased with population and industrial growth but has become more widespread. The use of tall smoke stacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. Often deposition occurs downwind of the emissions on a considerable distance with mountainous regions tending to receive the greatest deposition. The principal cause of acid rain is sculpture and nitrogen compounds from human sources such as electricity generation or factories and vehicle emissions. Electrical power generation using coal is among the greatest contributors to gaseous pollution responsible for acidic rain. The gases can be carried hundreds of kilometers in the atmosphere before they are converted to acids and deposited. In the past factories had short funnels to let out smoke but this caused many problems locally so factories now have taller smoke funnels. However dispersal from these taller stacks causes pollutants to be carried farther causing widespread ecological damage. Adverse effects may be indirectly related to acid rain like the acid effects on soil or high concentration of gaseous precursors to acid rain. High altitude forests are especially vulnerable as they are often surrounded by clouds and fog which are more acidic than rain. Effect on food crops is minimized by the application of lime and fertilizers to replace lost nutrients.