Gunpowder was developed in the 13th century, as a result of the discovery of how to purify saltpetre. This led to the isolation of amyl nitrite, which turned out to be an effective treatment for angina pectoris. Nitroglycerine is not only a high explosive but also effective in treating angina. The search for an understanding of how it works in the body has led to the astonishing discovery that nitric oxide gas is actually produced in the body and is involved in the control of a number of important functions, including blood pressure.