Treatise on Poisons by Sir Robert Christison

4. The best liquid reagent is _Hydrosulphuric acid_. In a solution

containing only an eighth part of a grain per ounce, it strikes an orange-red colour, which, when the excess of gas is expelled by heat, becomes an orange-red precipitate; and if the proportion of salt is greater, the precipitate is thrown down at once.—The colour of the precipitate is so peculiar as to distinguish it from every other sulphuret; but if any doubt regarding its nature should occur, it may be known by collecting it, dissolving it with the aid of gentle heat in hydrochloric acid, and adding water to the solution; which will then yield a white precipitate, the sesquioxide of antimony in union with a little chlorine.