Calomel (hydrargyri chloridum mite), the mild chloride of mercury, HgCl, is a bland or unirritating heavy powder, completely insoluble in water. It has few chemic affinities, but is decomposed by alkalies. When it is added to a solution of sodium carbonate, it turns gray with the formation of the carbonate, oxide, or hydroxide of mercury, a change which takes place, as shown by Schaefer, when the calomel passes from the stomach into the duodenum. Some of the salt goes into solution, for the filtrate contains mercury (Schaefer, MacCallum). This gray salt of mercury has more chemic affinities than calomel, is irritant locally, and is antiseptic, and it is upon this chemic change that the value of calomel in the alimentary tract largely depends. This suggests the advisability of dividing the large doses, so that not too much is passed into the duodenum at one time. After cathartic doses mercury is found in the urine.

The result of the irritation is increased peristalsis beginning in the duodenum and extending through the whole length of the bowel. In addition, there is a mild antiseptic action, though many more bacteria are carried out by good catharsis than can be killed by an antiseptic. Calomel is not a very powerful colon stimulant, so if the dose is too small, the movement may not be copious; however, if the dose is too large, there may be griping, rectal irritation and tenesmus, and numerous small stools.

At times, if the action is not severe enough, the bowels are not thoroughly cleaned out, and the result is autointoxic headache and lassitude. The explanation of this is that the calomel hastens the undigested food through the small intestine to the colon, where the putrefactive bacteria are located. The raw proteins, 9 not being carried out, furnish pabulum from which these bacteria generate an extra amount of poisons of the indol type (Herter). Because of this not infrequent sequence to calomel, it is the custom to follow the dose in about eight hours with a saline cathartic to insure a thorough washing out of the colon.

The calomel stools may be gray in color from the presence of the mercurous oxide or other mercurous salts; occasionally they are green from the presence of unchanged biliverdin, this being due either to the rapid carrying of the bile through the intestines, or to the prevention of the usual reduction of the bile-pigment. This prevention may result - (1) From the direct chemic action of the mercury salt on the pigment; (2) from an antiseptic effect upon the bacteria which cause the changes in bile-pigment, or (3) from an interference with the oxidases. Frequently repeated in large amounts, as in the Lambert treatment for morphinism, calomel results in copious bile stools, and would seem to have a "cholagogue" action. But it may be merely that the increased activity of the duodenum favors the outflow of stored bile from the gall-bladder and liver.

Calomel may be given in the form of a powder or tablet triturate (compressed tablets and pills are not recommended, as the calomel is insoluble), in amounts of 1-3 grains (0.06-0.2 gm.) in divided doses, say 1/4 grain (0.015 gm.) every fifteen minutes for 6 doses. If the stomach is irritable, even smaller amounts may be given at a time. The smaller doses insure complete conversion of the calomel. A teaspoonful has been taken without any violent effects, presumably because it passed through the intestine mostly unchanged (Schaefer). It is quite a common practice to give tablets of 1/10 grain (0.006 gm.) each until 1 or 2 grains have been taken; but this requires too many doses to be watched, and spreads the dosage over too long a time.

At one time it was taught that calomel should be given with sodium bicarbonate to prevent the hydrochloric acid in the stomach from changing it to the poisonous and corrosive bichloride. But it has been shown that even in highly acid gastric juice the calomel does not change to bichloride; and it is obvious that a few grains of sodium bicarbonate could have little if any effect in neutralizing the acid of the gastric juice during the whole time the calomel remains in the stomach. If the stomach needs sodium bicarbonate, the patient may feel better after such a dose, but he is not protected from poisoning.

Diekman, 1899, mixed 0.5 gm. of calomel with solutions of ammonium chloride, sodium chloride, potassium chloride, and citric and tartaric acids, and found the loss through the formation of a soluble salt to be not more than 1 to 3 mg., an amount that if converted into mercuric chloride would not be toxic.

A very extensive research by T. W. Schaefer, 1910, establishes beyond doubt that there is no bichloride formed from sodium chloride, hydrochloric acid, or hydrochloric acid and pepsin. He found that the administration of calomel to dogs, and sprinkling calomel on the mucous membrane of the stomach, intestines and common bile-duct, or mixing it with the bile and pancreatic juice produced no bichloride. When sprinkled on the duodenal mucous membrane of living dogs there was an immediate change to a gray color which rapidly darkened, and the ultimate soluble salt formed was found to be mercurous oxide (2HgCl+2Na2-Co3+H2O = Hg2O+2NaCl+2NaHco3). Sprinkled lower down in the small intestine the change was very slow, and in the common bile-duct and in bile itself was almost absent.

Therapeutics

The chief uses of calomel as a cathartic depend upon its combined cathartic and antiseptic powers. It is employed:

1. At the onset or during the course of acute illnesses.

2. In plethoric conditions, such as are usually associated with habitual overeating (so-called "sluggish liver").

3. In intestinal auto-intoxication, whether associated with liver insufficiency or not, and in food or ptomain-poisoning.

4. In fermentative conditions of stomach and bowels.

5. In hyperacidity and "biliousness."

Each compound cathartic pill contains about one grain of calomel. The other mercurial cathartics are the mercury or blue pill (massa hydrargyri), dose, 5 grains (0.3 gm.); and the mercury with chalk (hydrargyrum cum creta), dose, 5 grains (0.3 gm.), in both of which metallic mercury is in a state of fine subdivision. The Pharmacopoeia requires that these preparations shall not contain more than traces of the mercury oxides. Metallic mercury in bulk, when administered by mouth, may act mechanically, passing out of the intestines unchanged; but poisoning has occurred from its ingestion.

Mercurials used for other purposes are occasionally cathartic, e. g., the protoiodide given for syphilis; in such cases bismuth or opium is sometimes administered with them.