Mercury is a liquid silvery metal, from which its name is derived, i.e. hydrargium, meaning water silver. The extreme toxicity of this element and its compounds must be emphasised. It is absorbed as fumes and by skin contact. Its effects are cumulative. Neurological symptoms are shown by people with lifetime toxic levels. Treat with great caution. Compounds of mercury are among the most poisonous compounds we use in histotechnology. Many laboratories now exclude mercury compounds altogether, using non-toxic or less toxic alternatives.
Mercuric chloride is one of the best fixing agents available. It shrinks tissues evenly and leaves them in a state which promotes staining with both acid and basic dyes. Trichrome staining methods in particular give excellent results without further pretreatment, and some may even require extended treatment with it. It leaves an artifact in tissues, mercury pigment, appearing as black or dark brown granules. This is removed by treatment with an iodine solution, such as tincture of iodine or Gram's, Lugol's, etc., followed by bleaching with sodium thiosulphate and washing in water.
Mercuric oxide has been used as an oxidant on Harris' hematoxylin, although it is now deprecated due to its toxicity. Other mercury compounds are used for the demonstration of sulphydril and disulphide groups in cystein and cystine (mercury orange). Mercury does not appear free in tissues and is not usually demonstrated.