Characters and Reactions. - A colourless and strongly acid solution, which gives a yellow precipitate with solution of potash added in excess (mercuric oxide). If a crystal of sulphate of iron be dropped into it, in a little time the salt of iron, and the liquid in its vicinity, acquire a dark colour (nitrate).

Uses. - It is a powerful caustic, and is used as such in lupus. It is to be applied with a camel's-hair brush to the extent of a crown piece over the ulcers, tubercles, and scars which are soft and ready to break. The part is then covered with lint moistened in the solution. It soon becomes white, a kind of erysipelatous inflammation sets in around it, and it falls oif as a yellow scab. The solution is also applied to the os uteri when there are large ulcers with flabby unhealthy granulations upon it. It has been used in cancer and in chancres, condylomata, syphilitic and scrofulous ulcers, favus, and obstinate psoriasis. If applied often it may cause mercurialism, and indeed salivation has occurred after one application to the os uteri. To prevent this it should be washed off immediately after being applied.