There are three principal forms in which this medicine may be exhibited. Blue pill contains the metal itself in a finely divided state, as well as a small quantity of the oxide. Calomel is an insoluble Chloride of Mercury. From the great similarity that exists between the action of these two it seems likely that they are reduced by the gastric fluid to the same condition. Both must be rendered soluble (p. 88) before they can be absorbed. Probably they are both absorbed in combination with the acid of the stomach.
Bichloride of Mercury is soluble in water, and probably absorbed unchanged. It differs from the other two as a medicine, partly, but not entirely, on account of its solubility. The dose required is smaller, for it is much more powerful. It is also much more irritant, being in large doses a corrosive poison, and often producing soreness of the throat and of the urinary passages. Thirdly, it is less likely to produce salivation. It is more adapted for chronic than for acute diseases.
As Hæmatic medicines, Mercurials have a double action. They counteract inflammation in general, and the poisons of Syphilis in particular. They thus belong to the first and second orders of Catalytics.
Mercury deteriorates the blood, diminishing in it the amount of fibrine and corpuscles. As an anti-inflammatory agent, it may be thus compared with Antimony and Blood-letting. The immediate effect of Blood-letting is mechanical; that of Antimony, nervous; that of Mercury, hæmatic. Blood-letting weakens the force of the heart by diminishing the pressure on the vessels; Antimony diminishes the pressure on the vessels by weakening the force of the heart; and Mercury does both of these things, by impoverishing the blood. Thus all of them favour absorption, and counteract effusion; but, from its nature, the action of Mercury is slower than that of the others, and for the same reason more lasting. To produce this action on the blood, the Mercurial should be continued until some effect on the mouth is perceived, but not so as to cause copious salivation. This symptom is a sign that the blood is sufficiently saturated with the medicine. This point will be sooner reached if the Mercury be conjoined with Opium, so as to prevent it from passing out directly by the bowels. Any ill result is less likely to occur if the patient be kept warm and quiet while under the influence of the medicine. On account of the durable and effectual nature of its action, Mercury is of great use in preventing the process of effusion, and in causing the absorption of effused products. It is thus employed with advantage in Pleurisy, and in other membranous inflammations. Next to these, it is most useful in inflammations of the liver and brain. It is inferior to Antimony in fevers and rapid inflammations, because slower in operation, and without any direct action on the nervous system.
In cases of primary Syphilis, Mercury is by far the best medicine with which we are acquainted. It should be used in all cases except where there is deep-rooted scrofula, or marked debility, or a sloughing and irregular condition of the primary sore. (Dr. Pereira.) It should always be given in Iritis. In Periostitis, and secondary eruptions, Iodide of Potassium is generally preferable.
Mercury, being unnatural to the blood, passes at length out of the system through the glands, and acts as an Eliminative. Like Antimony it tends to increase all the secretions in the body. But whereas Antimony acts especially on the secretions of the skin and pulmonary membrane, Mercury tends particularly to excite the functions of the liver and bowels, being Cathartic and Cholagogue. Of its true eliminative action we have better proof than has yet been obtained in the case of Antimony; for Mercury has been found to pass into the alvine excretions, by M. Lehmann; and discovered in the bile of dogs to whom it had been administered, by M. Buchheim.
For the purpose of acting upon the liver and bowels, Mercurials are frequently used in disordered digestion and cases of hepatic derangement. In Intermittent disorders, debility, Gout, Rheumatism, and Scrofula, small doses of Mercury are often of service. I have endeavoured to show (p. 143) that in these cases they may prove indirectly tonic, by restoring to the system some of the wanting elements of bile, which are normally secreted by the liver for the purpose of being reabsorbed into the system.
In incipient and chronic cases of Scrofula, consumption, and mesenteric disease, it is possible that Mercurials may act yet in another way. They no doubt stimulate the formation of the Pancreatic secretion, which is similar in nature to the saliva. By doing so they may assist the absorption of the fatty matters of the food in the case of thin and emaciated subjects, in whom it is probable that the function of this gland is frequently impaired. (Vide p. 313.)
Mercury assists the operation of all other medicines which act upon the secretions. In cases of hepatic dropsy it helps the action of Diuretics, and tends to remove the cause of the congestion, by stimulating the function of the liver.
The blood-operation of Mercury, by which it is enabled to counteract morbid processes, is involved in considerable obscurity. The same must be said of all Catalytic medicines (Vide p. 179.) Dr. Billing (Principles of Medicine, p. 73) is of opinion that Mercury acts by contracting the capillary vessels. I have already stated my reasons for dissenting from this notion. (p. 163.) The same author denies the specific action of Mercury in Syphilis. But if we only admit that there is no other medicine that will cure primary Syphilis so well as Mercury, we cannot then surely deny that its action in that disorder is of a special nature. He considers it to be neither stimulant nor sedative,—but tonic. (Op. cit. p. 101.) No one of these terms appears to me to be strictly applicable, but perhaps the last is the best of the three.