Calomel

Calomel is a mercury chloride mineral with formula Hg2Cl2 (see mercury(I) chloride). The name derives from Greek "kalos" (beautiful) and "melas" (black) because it turns black on reaction with ammonia. This was known to alchemists.
Calomel occurs as a secondary mineral which forms as an alteration product in mercury deposits. It occurs with native mercury, amalgam, cinnabar, mercurian tetrahedrite, eglestonite, terlinguaite, montroydite, kleinite, moschelite, kadyrelite, kuzminite, chursinite, kelyanite, calcite, limonite and various clay minerals.
The type locality is Moschellandsburg, Alsenz-Obermoschel, Rhineland-Palatinate, Germany.
History.
The first time that the substance that would come to be known as calomel was documented was in Ancient Persia by the Persian medical historian, Rhazes in year 850; however, only a few of the compounds he mentioned could be positively identified as calomel as not every alchemist disclosed what compounds went into their drugs. Calomel first entered Western medical literature in 1608, when Oswald Croll wrote about the drug's preparation in his "Tyroncium Chemicum", although it was not called calomel until 1655 when the name was created by Théodore de Mayerne. In 1618, Mayerne was the first to publish the preparation and formula for calomel in “Pharmacopoeia of Londinensis.” 
By the 19th century, calomel was viewed as a panacea, or miracle drug, and was used to cure almost every disease. Some of these diseases included: syphilis, bronchitis, cholera, ingrown toenails, teething, gout, tuberculosis, influenza, and cancer. Although during the 18th and early 19th century pharmacists used calomel sparingly, by the late 1840s calomel was being prescribed in heroic doses. This was, in part, due to the research of Benjamin Rush who coined the term heroic dose to mean about 20 grains taken four times daily. This stance was supported by Dr. Samuel Cartwright who believed that taking large doses was the “gentlest” on the body. As calomel rose in popularity, more research was done into how it worked.
J. Annesley was one of the first to write about the different effects of calomel when taken in small or large doses. Through experimentation on dogs, Annesley concluded that calomel acted more like a laxative on the whole body rather than acting specifically on the vascular system or liver as previous physicians believed. In 1853, Samuel Jackson described the harmful effects of calomel on children in his publication for Transactions of Physicians of Philadelphia. He noted that calomel had harmful effects causing gangrene on the skin, loss of teeth, and deterioration of the gums. On May 4, 1863, William A. Hammond, the United States’ Surgeon-General, stated that calomel would no longer be used in the army as it was being abused by soldiers and physicians alike. This caused much debate in the medical field, and eventually led to his removal as Surgeon-General. Calomel continued to be used well into the 1890s and even into the early 20th century. Eventually calomel’s popularity began to wane as more research was done, and scientists discovered that the mercury in the compound was poisoning patients.
Calomel was the main of the three components of the pill number 9 of the British army during the First World War. 
Electrochemistry.
Calomel is used as the interface between metallic mercury and a chloride solution in a saturated calomel electrode, which is used in electrochemistry to measure pH and electrical potentials in solutions. In most electrochemical measurements, it is necessary to keep one of the electrodes in an electrochemical cell at a constant potential. This so-called reference electrode allows control of the potential of a working electrode.
Chemical properties.
Calomel is a powder that is white when pure. When it is exposed to light or contains impurities it takes on a darker tint. Calomel is made up of mercury and chlorine with the chemical formula Hg2Cl2. Depending on how calomel was administered, it affected the body in different ways. Taken orally, calomel damaged mainly the lining of the gastrointestinal tract. Mercury salts (such as calomel) are insoluble in water and therefore do not absorb well through the wall of the small intestine. Some of the calomel in the digestive system will likely be oxidized into a form of mercury that can be absorbed through the intestine, but most of it will not. Oral calomel was actually the safest form of the drug to take, especially in low doses. Most of the calomel ingested will be excreted through urine and stool.
Powdered forms of calomel were much more toxic, as their vapors damaged the brain. Once inhaled, the calomel enters the bloodstream and the mercury binds with the amino acids methionine, cysteine, homocysteine and taurine. This is because of the sulfur group these amino acids contain, which mercury has a high affinity for. It is able to pass through the blood brain barrier and builds up in the brain. Mercury also has the ability to pass through the placenta, causing damage to unborn babies if a pregnant mother is taking calomel.
Calomel was manufactured in two ways - sublimation and precipitation. When calomel first started being manufactured it was done through sublimation. Calomel made through sublimation tends to be a very fine white powder. There was some controversy over the sublimation of calomel. Many argued that the more times calomel was sublimed, the purer it got. Opponents believed that the repeated sublimation made calomel lose some of its therapeutic ability. In 1788 pharmacist Carl Wilhelm Scheele came up with the mechanism to make precipitated calomel. This became rapidly popular in the pharmaceutical industry because it was both a cheaper and safer form of production. Precipitation also tended to form very pure calomel salts.
Medicine.
Calomel was a popular medicine used during the Victorian period and was widely used as a treatment for a variety of ailments during the American Civil War. The medication was available in two forms, blue pills and blue masses. The blue pill was an oral form of calomel containing mercury that was often mixed with a sweet substance, like licorice or sugar in order to be taken by mouth. The blue mass was a solid form of calomel in which a piece could be pinched off and administered by a physician or other medical provider. Neither form of the medication came with a standardization of dosing. There was no way of knowing how much mercurous chloride each dose contained.
Uses.
Calomel was marketed as a purgative agent to relieve congestion and constipation; however, physicians at the time had no idea what the medication’s mechanism of action was. They learned how calomel worked through trial and error. It was observed that small doses of calomel acted as a stimulant, often leading to bowel movements, while larger doses caused sedation. During the 19th century, calomel was used to treat numerous illnesses and diseases like mumps, typhoid fever, and others—especially those that impact the gastrointestinal tract, such as constipation, dysentery, and vomiting. As mercury softened the gums, calomel was the principal constituent of teething powders until the mid-twentieth century. Babies given calomel for teething often suffered from acrodynia.
Side effects.
It became popular in the late 18th century to give calomel in extremely high doses, as Benjamin Rush normalized the heroic dose. This caused many patients to experience many painful and sometimes life-threatening side effects. Calomel, in high doses, led to mercury poisoning, which had the potential to cause permanent deformities and even death. Some patients experienced gangrene of the mouth generated by the mercury in the medicine, which caused the tissue on the cheeks and gums inside the mouth to break down and die. Some patients would lose teeth, while others were left with facial deformities. High doses of calomel would often lead to extreme cramping, vomiting, and bloody diarrhea; however, at the time, this was taken as a sign that the calomel was working to purge the system and rid the disease. Calomel was often administered as a treatment for dysentery; the effects of calomel would often worsen the severe diarrhea associated with dysentery and acted as a catalyst in speeding up the effects of dehydration. One victim was Alvin Smith, the eldest brother of Joseph Smith, founder of the Church of Jesus Christ of Latter-day Saints. It was also used by Charles Darwin to treat the severe gastrointestinal infection that presumably began the inductive phase of his documented Crohn's disease. Eventually, it was determined that calomel was causing more harm than good, as the side effects were often worse than the illness it was being used to treat; because of this, calomel was removed from medical supply shelves.