[Salt; and Gk. petra, a rock.] A kind of whitesalt made up of nitric acid and potash, often found oozing from rocks. It is bitter in taste, and is called potassium nitrate. It is found in caves or got from the soil in Egypt, Persia, and India. Its principal use is in making gunpowder and in preparing nitric acid, sulphuric acid, in making fireworks, and medicinally for rheumatism. Chili saltpetre is a sodium nitrate, which cannot be used in making gunpowder, but from which nitric acid is got.