All the salts of mercury possess the diuretic property to some extent, but the chlorides, especially calomel, are the most efficient. In cardiac dropsy calomel becomes one of the most important means of relief. When the action of the heart is rapid and the tension of the vessels high, and the breathing is embarrassed by the accumulating fluid, the exhibition of a daily dose of two or three grains each will often procure considerable discharge of urinary water, and by unloading the portal circulation will in this way also rid the system of water. It has long been known that calomel has diuretic effect, but it was prescribed in fractional doses with squill and digitalis. Graves, of Dublin, and G. B. Wood, of Philadelphia, and before them Rush, were advocates of this practice, but the use of larger doses daily is a distinctly modern method. The experimental evidence showing the effect of mercury on the kidneys is conclusive. Jendrassik was probably the first to demonstrate the action; afterward Cohnstein showed that the hypodermatic injection of a solution of mercury increases diuresis, probably by an action on the epithelium, and by raising the pressure in the renal vessels, the mercury itself appearing in the urine. Dr. Pal gives a summary of the observations made experimentally, which show that mercury raises the blood pressure in the renal system, that it acts on the renal epithelium, and may even cause nephritis.

In these physiological actions we have an adequate explanation of the nature of the curative power of mercury in dropsy. As regards its action in the cardiac form of dropsy, the effect may be in part referred to its influence on the hepatic secretions—to the formation of urea and the conversion of uric acid; to the mechanical and reflex results of depletion of the portal circulation. The effect on the heart is shown in a diminution of its beats and in a lowering of the general vascular tension; the renal effect consists in an increase of the urinary secretion through stimulation of the tubular epithelium, and rise of tension in the kidney vessels.

The good effects of mercurials, of calomel especially, in dropsy are not confined to the cardiac form. It is often highly useful in dropsy strictly of renal origin. In the acute form of tubular nephritis, with desquamating epithelium and scanty urine, a purgative dose of calomel may have a prompt and powerful effect in relieving the head symptoms and starting up again renal excretion.

The preparations of mercury must be used with caution in cardiac dropsy when coincident with renal disease, or in nephritis when a special susceptibility to the mercurial action exists. Very severe ptyalism has occurred from the exhibition of even small doses of calomel in cases of renal albuminuria. While caution is necessary, it is also true that mercury is often most useful in renal dropsy. The dose of calomel as above given is rather under than above the quantity now considered necessary. From three to six grains a day in single doses may be regarded as the most efficient mode of administration.

The most important application of mercury therapeutically is in the treatment of syphilis. The reaction which set in against its use a few years ago has certainly led to important modifications in the mode and quantity in which mercury should be given; but the fact has been conclusively established that mercury in a certain sense is antidotal to syphilis. As mercury exerts a germicide action on the organism of syphilis, this agent should be used with the earliest manifestations of the specific character of the infecting sore. Mercury is not indicated in chancroid, or non-infecting chancre, and should not be used. If the chancre have the characteristic quality of the infecting sore, small doses of one of the mercurial preparations should be begun and continued steadily until all induration has disappeared. The important point is, not to induce ptyalism. It is now conceded that the danger of a relapse will be very much lessened by continuing the mercurial treatment for some time—for several months—after local manifestations have ceased. The ill effects of a mercurial course may be prevented by the use of small doses, by careful attention to hygiene, and by lessening the dose, or discontinuing the remedy entirely, whenever soreness of the jaws can be developed by smartly closing the teeth. By the adoption of these precautions, a mercurial course may be continued without important interruptions until the period of incubation has entirely passed, and through the so-called secondary or constitutional stage.

Various methods are resorted to for the introduction of mercury Into the organism:

1.  Inunction.—Before practicing inunction, the patient should take a warm bath, or, at least, wash the part to be operated on with soap and water. From fifteen to thirty grains of mercurial ointment is the quantity required for each inunction. The oleate of mercury in proportion of fifteen to thirty per cent in the solution may be substituted for the ointment, but the oleate is to be applied with a brush, and not be rubbed in. Sigmund, the great advocate for the inunction method, has prescribed certain rules, which should be followed. The ointment should be rubbed in with moderate friction by the palm of the hand: on the first day on the legs; on the second day on the thighs; on the third day on the abdomen and sides of the chest; on the fourth day on the back; on the fifth day on the arms. Mercurial inunctions are not borne equally well by all patients. Some are easily salivated, and others suffer from eczema or erythema. Moreover, the inunction treatment is filthy and troublesome, and it should, therefore, be restricted to those cases in which mercurials are badly borne by the stomach.

2.  Fumigation.—Various mercurial preparations may be used—the sulphuret, the iodide—but calomel is the best. The apparatus consists of a spirit-lamp, a plate to hold the calomel, surrounded by a shallow vessel containing water, a blanket large enough to cover the patient and the apparatus. The calomel is volatilized by the heat of the lamp, and is deposited, together with the vapor of water, on the skin of the patient. About fifteen minutes is the time required for the bath, and the quantity of calomel used ranges from eight to fifteen grains. The method of fumigation is especially adapted to cases of the constitutional, or secondary, and to the tertiary, with ulcerations, when the state of the patient is such as to forbid the internal administration of mercurials.

3.  Hypodermatic Method.—This consists in the introduction under the skin of corrosive chloride, or albuminate of mercury, Rx Hydrarg. chlor. cor., gr. j; glycerini, 3 j; aquae destil., 3 j. M. Sig.: Ten minims a dose once a day. This method is cleanly, quick in results, and more successful than any other in preventing relapses.

The peptonate of mercury, as advocated by Bamberger, has been much employed lately for the treatment of syphilis by the subcutaneous method; but the albuminate alone, without pepsin, is both more effective and less likely to set up local inflammation. The following formula for the preparation of albuminate of mercury, recommended by Gourgues, is probably the best now available for this purpose: Bichloride of mercury, gr. xv; distilled water, 3 v. To this solution is added five drachms of white of egg, and, after thorough shaking the mixture is incorporated with the following: Chloride of sodium (common salt), gr. xxx; distilled water, f oz ij. This mixture is then well agitated, and afterward filtered, after which sufficient distilled water is added to make the solution up to four fluid ounces. The dose of this solution for hypodermatic injection is from τη xv— 3 ss. When finished, the solution is not clear, but rather semi-transparent; but it remains longer and better in solution than most of these preparations. The peptonate of mercury is not only variable in composition, but easily undergoes change, and is liable to produce violent local irritation. The albuminate prepared as above suggested is readily administered, is not painful, and is rarely followed by unpleasant accidents at the site of the injection. As regards the results of the administration on the progress of the disease, they are remarkable (Gourgues). According to Martineau, the hypodermatic mercurial treatment exercises on syphilis in its various manifestations an action much more effective, more energetic, and more rapid, than any other method hitherto proposed. The subcutaneous areolar tissue of the back is the best place for the injections. The number of them and the quantity of fluid used will depend on the stage of the disease, the severity of the manifestations, and the condition of the patient.

Besides the preparations above mentioned, other solutions of corrosive chloride have been proposed. Matthés recommends the following: Bichloride of mercury, gr. xv; chloride of sodium, gr. xxx; distilled water, f oz iij. Make a solution. Matthés had observed the action of this during two years at Auspitz's polyclinic. The usual dose was a Pravaz syringeful—fifteen to twenty minims—every two days. No irritation is produced beyond some tumefaction, and, in general, twenty to thirty injections sufficed to effect a cure.

Recently, Liebreich has brought forward the formamide of mercury as the most suitable preparation for the hypodermatic method. It is neutral in reaction, readily soluble in water, and does not coagulate albumen.

4. Internal. This method, in the nature of things, must be most frequently resorted to. Various mercurial preparations are used by different syphilographers. The Ricord school prefer the green iodide; Sigmund, mercurial-ointment inunctions; Foerster, the yellow iodide; Berkeley Hill, the red iodide; Tilbury Fox, the cyanide; Bumstead advises mercurial pill, the green iodide, the bichloride, according to circumstances, but his preference is for the use of calomel by fumigation. That preparation of mercury is to be preferred which best agrees with the patient, is the rule.

The indications for the use of iodine preparations have already been stated, but it may be useful, now, to place in juxtaposition the comparative utility of mercury and iodine: mercury for the primary infecting sore; mercury for the affections of the skin, especially macular and papular exanthemata; iodine for the tertiary symptoms: gummata; tubercular syphilides; serpiginous ulcers; affections of the bones and periosteum, and nervous diseases.

Hydrargyrum cum creta is usually preferred for the treatment of congenital syphilis. Mr. Marshall recommends the twenty-per-cent ointment (the size of a pea) of the oleate of mercury, to be placed in the axilla night and morning for five or six days.