The _strong acid_ varies in colour from a pale yellow to a deep orange. The colour is due to admixture with peroxide of nitrogen. On _cloth_ and _articles of dress_ it produces _yellow stains_, which are darkened by the application of an alkali. If poured on copper cuttings, reddish fumes of nitrous acid are given off. _Dilute nitric acid_ is a colourless acid liquid, not precipitated by nitrate of barium or nitrate of silver, showing absence of sulphuric and hydrochloric acids. All its alkaline salts are soluble in water. _Symptoms._--The symptoms have been before described, and are similar to those produced by sulphuric acid, though not quite so severe. The _vapour_ of this acid has caused death in eleven hours by congestion of the bronchial tubes and lungs; care should, therefore, always be taken not to inhale the fumes given off by the acid in the manufacture of gun-cotton, &c. _Post-mortem Appearances._--Same as the mineral acids generally, but the tissues touched are turned yellow, and if bile be in the stomach it is turned green. The stomach is rarely perforated. _Chemical Analysis._--Nitric, like sulphuric, acid will be examined under four heads; but being a volatile acid, easily decomposed, and also having its nature changed by contact with organic substances, its presence on clothes may fail to be detected after a few weeks. The colour of the stain on cloth may also remain, although the acid has been entirely removed by washing. 1. Strong, concentrated acid. 2. Dilute acid. 3. Mixed with organic liquids, &c. 4. On the clothes of the person injured. I. Concentrated Acid 1. _Volatility._--When exposed to the atmosphere, strong nitric acid gives off colourless or orange-coloured acid fumes. Heated in a watch-glass, it is evaporated without residue. 2. _Action on Organic Matter._--The acid leaves on woollen clothes a _yellow-coloured stain_, which is darkened by the addition of an alkali. The colour of the stain is due to the formation of picric acid. 3. _Action on Metals._--Gently heated in a test tube with copper filings, orange-coloured fumes are given off, which redden but do not bleach litmus paper. Starch paper treated with iodide of potassium becomes purple. 4. _Solution of Gold._--If a small portion of gold leaf be put into the acid, no effect is produced; but on the addition of concentrated hydrochloric acid, the metal is rapidly dissolved. II. Dilute Acid 1. Absence of sulphuric and hydrochloric acids, proved by no precipitate being formed with nitrate of barium and nitrate of silver. 2. It does not char paper when the paper is heated, as is the case with sulphuric and hydrochloric acids. 3. If a piece of filtering paper be dipped into a solution of the acid saturated with carbonate of potash, dried and ignited, it will burn like touch-paper.