In the Middle Ages and the early modern period, chemical warfare advanced along with the development of chemistry. In 1456, during the Siege of Belgrade, an alchemist created poison clouds by burning rags that may have contained chlorine gas. In the fifteenth century, Leonardo da Vinci designed explosive shells filled with arsenic and sulfur for use against ships, and in the nineteenth century, Thomas Cochrane advocated the use of burning sulfur as a naval weapon. Later in the nineteenth century, the Union Army in the American Civil War devised plans to attack Confederate trenches with hydrochloric acid and sulfur acid, but failed to carry out its intentions.