Nitric acid is a strong inorganic acid that has many industrial uses. Its principal use is for the production of fertilizers and explosives. Large quantities of nitric acid are produced during thunderstorms and fall to the earth in rain. The rain falls as a very weak solution of nitric acid. The production of nitric acid during thunderstorms allows nitrogen from the air to become part of the soil in a form that plants can use (see Nitrogen cycle). Nitric acid was one of the first acids known. Many alchemists of the Middle Ages used it in their experiments.