Acid rain was at one time an important point of contention between the United States and Canada. Much of this acid was the result of the emission of sulfur oxides by coal-fired electricity-generating plants in southern Indiana and Ohio. These sulfur oxides, when dissolved in rainwater, formed sulfu- ric acid and hence "acid rain." How many metric tonnes of Indiana coal, which averages 3.5% sulfur by weight, would yield the H2S04 required to pro- duce a 0.9-in. rainfall of pH 3.90 precipitation over a 104 square mile area