Properties : Mild mercurous chlorid is a white impalpable powder, becoming yellowish-white on trituration with strong pressure, odorless, tasteless and permanent in the air. It is practically insoluble in water, alcohol or ether. It undergoes changes when exposed to the action of light or under the influence of alkaline chlorids, bromids or iodids, by which mercuric salts are more or less rapidly formed. The mercuric salt enters into solution in combination with the salt of the alkali metal present. Alkaline hydroxids convert it into mercurous oxid; ammonia forms with it a mixture of mercury and mercuric ammonium chlorid.

Incompatibilities: Calomel is incompatible with alkalies, with oxidizing acids like nitric acid and also with soluble bromids and iodids. The fear that non-oxidizing acids like hydrochloric acid will form mercuric chlorid from it is unfounded. Calomel is not incompatible with such acids.

Action and Uses : Mild mercurous chlorid is not irritating to the mucous membrane of the mouth, esophagus and stomach, but it provokes bowel movements by a slow action. This action has been thought to be due to a partial change into a mercuric salt or a protein compound. The absorption of the mercuric salt may produce symptoms of subacute mercurial poisoning. The chief indications of toxic effects are pain in the abdomen, loose passages, salivation, loosening of the teeth, swelling, soreness and ulceration of the gums, foul breath and general malaise.

Calomel was formerly supposed to have a cholagogue action, but it does not increase the quantity of bile secreted, although by its cathartic action it may increase temporarily the amount poured into the intestine. The stools resulting from the action of calomel are frequently greenish, resulting from changes in the bilirubin; this is partly because the bilirubin in the intestinal contents, being hurried through the colon, fails to undergo the change into urobilin which normally occurs. The change in color may also be due in part to the formation of mercuric sulphid.

Calomel is used for a special effect on the gastrointestinal tract which is sometimes attributed to an antiseptic action. It is thought to be of special value as a cathartic in gastroenteritis and at the beginning of mild catarrhs of the stomach and intestines. It is also frequently used to empty the bowels in conjunction with the more active salines in cases of infection, or toxemia. It is sometimes useful in small doses to check vomiting and is frequently administered as a laxative when the stomach is irritable because it is retained better than other cathartics. It is useful as an intestinal antiseptic.

Calomel is sometimes an excellent diuretic in cardiac dropsy. It is of much less value in other forms of dropsy.

It is sometimes applied externally to sluggish ulcers, and is used by insufflation on the cornea for ulceration or opacities, phlyctenular conjunctivitis, etc.

Dosage: Only a small portion of the calomel is absorbed, so that minute doses are generally effective. From 0.005 to 0.02 gm., or from 1/10 to 1/3 grain may be given every half hour or hour until from 0.1 to 0.2 gm., or from 1 to 3 grains have been given. The calomel should be followed in a few hours or the next morning by a saline cathartic. When calomel is used externally, care should be taken that no iodids are administered internally at the same time, because the presence of iodids in the secretions, for example, tears, may cause the formation of a mercuric salt and induce great irritation.