Geber made this 'strong water', Nitric acid, by distilling green vitriol, with saltpetre and alum. When pure it is a colourless liquid, but it is often tinged brown by dissolved nitrogen oxides, which also give it a peculiar smell.
It reacts vigorously with most metals, but Gold will withstand its action. It is also a powerful oxidising agent, and will react with most substances. It is used to separate silver from gold in ored like electrum, making silver nitrate (lunar crystals).