8. (Precipitated calomel; Calomelas præcipitatum, L.) Digest pure quicksilver, 9 parts, in nitric acid (sp. gr. 1·02 to 1·25), 8 parts, until no more metal will dissolve, applying heat as the effervescence ceases; then mix the hot liquid quickly with a boiling solution of sodium chloride, 8 parts, dissolved in water (slightly acidulated with hydrochloric acid), 64 parts; lastly, well wash the precipitate in boiling distilled water, and dry it. The product, when the process is skilfully managed, is perfectly white and pure.
Prop. A heavy, white, tasteless powder; insoluble in water, alcohol, and cold dilute nitric acid; volatilises at a temperature below redness, and yields a white or yellowish-white sublimate; hot nitric acid oxidises and dissolves it; alkalies, the alkaline carbonates, and lime water, decompose it, with the production of the black oxide; ammonia converts it into a dark slate-grey coloured powder (BLACK PRECIPITATE—Kane). Sp. gr. 7·14 (Boullay; 7·156—Pelouze and Fremy; 7·176—Ure).
Pur. Calomel is frequently contaminated with small quantities of corrosive sublimate, which may be detected by digesting a little in alcohol, decanting the clear portion, and testing it with a drop or two of potassium hydrate, when a reddish precipitate will be formed if any mercuric chloride be present. It is pulverulent, whitish, and sublimes entirely by heat. It becomes black on the addition of potassium hydrate, then, heat being applied, it runs into globules of mercury. Neither silver nitrate, lime water, nor sulphuretted hydrogen, being added to the water in which it has been washed, or boiled throws down anything.
Uses. Calomel is one of the milder mercurials, and in this respect takes its position immediately after blue pill, mercury with chalk, and the grey oxide; but it probably ranks before all the other salts of mercury. Universal experience appears to show it to be a most valuable alterative when judiciously administered. With this intention it is given in doses of 1⁄4 to 1 gr., generally combined with antimonials, as in Plummer’s pill, and repeated every night, or every other night, for some time, followed by a mild saline aperient in the morning. As a purgative, 2 to 5 gr., either combined with or followed by other purgatives, as jalap, rhubarb, senna, colocynth, Epsom salts, &c. As a vermifuge, 2 to 5 gr., over-night, followed by a sufficient dose of castor oil next morning. Combined with opium it is frequently used in various complaints to produce salivation, or bring the system under the influence of mercury. It is also employed as a sedative and errhine, and in a vast number of other indications. It is, indeed, more frequently used, and in a greater variety of complaints, than probably any other medicine.
Obs. Of the two methods of preparing calomel, that by precipitation is not only the best, but the most economical. That by sublimation is, however, the one generally adopted in England. Mr Brande states that “a small portion of sodium chloride is apt to remain combined with it, which might affect its medical uses.” Such a contamination is not found in carefully prepared precipitated calomel, although we doubt whether the quantity of it which exists in any of the samples we have met with (being merely a trace) would at all interfere with its therapeutical action; more especially when it is recollected that alkaline chlorides are present in, not merely the primæ viæ, but also in every part of the animal body. The late Mr Fownes, a chemist who was unsurpassed in the accuracy of his researches, and the caution and delicacy with which he expressed his opinions, once assured us that calomel was more easily and cheaply prepared of the best quality by precipitation than by sublimation, and that if, from careless manipulation, it occasionally contained a minute quantity of common salt, this was of much less importance than the contamination
of corrosive sublimate, which was frequently present in samples of sublimed calomel.
To produce a superior article of calomel in the dry way is a somewhat difficult task, and the process frequently fails in the hands of inexperienced operators. The solution of the mercury is best made in an iron vessel, and the sublimation should be conducted (preferably) in an earthenware retort with a short but very wide neck, and fitted in a spacious receiver, having a large flat bottom, also of earthenware, and containing a little cold water. On the small scale the heat may be applied by means of a sand bath. The form above given for calomel, by precipitation, produces a large product, perfectly free from corrosive sublimate and mercurous nitrate, and is consequently free from the objections frequently raised against that mode of preparing it.
“The form in which calomel sublimes depends much upon the dimensions and temperature of the subliming vessels. In small vessels it generally condenses in a crystalline cake, the interior surface of which is often covered with beautiful quadrangular prismatic crystals, transparent, and of a texture somewhat elastic or horny. In this state it acquires, by the necessary rubbing into powder, a decided yellow or buff colour, more or less deep, according to the degree of trituration it has undergone. If, on the contrary, the calomel be sublimed into a very capacious and cold receiver, it falls in an impalpable and perfectly white powder, which requires only one elutriation to fit it for use; it then remains perfectly colourless.” (Brande.)
The long-continued action of steam on calomel in a state of minute division is attended by the formation of a small quantity of corrosive sublimate. (Righini.) Boiling water, hot air, and light, also produce a like effect.
Mercurous Iodide. HgI. Syn. Subiodide Of mercury, Green iodide of mercury, Mercurous iodide; Hydrargyri iodidum viride (B. P.), Hydrargyri subiodum, H. iodidum (Ph. L.), H. i. viride (Ph. D.), L. Prep. 1. (B. P.) Mercury, 1 oz.; iodine, 278 gr.; rectified spirit, a sufficiency. Rub the iodine and mercury in a porcelain mortar, moistening occasionally with a few drops of spirit, and continue the trituration until the mass assumes a uniform green colour, and no metallic globules are visible.