The nitrates are all soluble salts, possessing the colours of their metallic ions. As a rule, they crystallise easily. They are all decomposed by heat; those of the metals of the alkalies into nitrite and oxygen ; and all others into the oxide of the metal and nitric oxide and peroxide, NO arid NO9. They are formed by dissolving the metal, the oxide, or the carbonate in dilute nitric acid. All metals are attacked by nitric acid, except gold and the metals of the platinum group. The chief nitrates are those of potassium, KNO3, saltpetre or nitre ; of sodium, NaNO3, Chili saltpetre ; ammonium nitrate, NH4NO3, from which nitrous oxide, N2O, is obtained on heating ; lead nitrate, Pb(NO3)2, and silver nitrate, AgNO3, still known by its old name " lunar caustic," the word " lunar " referring to the ancient alchemical connection between silver and the moon. It is used as a caustic for removing growths and warts ; metallic silver is deposited, blackening the place rubbed.