All the officinal preparations of mercury, including the metal itself, and excepting only the sulphurets, which appear to be quite inert, are capable of producing the peculiar effects of this medicine on the animal economy. They vary, however, very much in the degree, rapidity, and certainty of their action. M. Mialhe has rendered it probable that they all operate by conversion into bichloride of mercury, and that their difference is owing to the different degrees of facility with which they undergo this conversion. Before the experiments of Mialhe, Prout had stated that calomel, or the protochloride of mercury, in the presence of an alkaline chloride, was converted partially into bichloride. The former chemist has demonstrated that this fact is true of all the officinal preparations, except the sulphuret, and that the action is much more rapid in the presence of the atmospheric air, or a fluid containing it. Another important fact ascertained by M. Mialhe is, that the change into the corrosive sublimate is proportionate, not to the amount of the mercurial present., but to that of the alkaline chloride, and the concentration of its solution. Applying these results physiologically, it would seem that any of the officinal preparations of mercury taken into the stomach, meeting, as they necessarily do, in the alimentary canal, with one of the chlorides, especially that of sodium, and with atmospheric .oxygen, either free or held in solution in the liquids, must undergo a partial conversion into corrosive sublimate, by which, if originally insoluble, they become soluble, and consequently capable of being absorbed. Nor is it only the internal operation of the mercurials which is thus explained. When the medicine is applied to the skin, or inhaled in vapour into the lungs, the instant that it meets with any fluid of.the body containing chloride of sodium, and there are few which do not, it becomes more or less soluble, and is thus rendered capable of absorption, and circulation with the blood. it will be found that this simple view affords a happy explanation of many peculiarities in the action of the mercurial preparations. One of these may be mentioned here; namely, the relative insusceptibility of children to the mercurial influence. This is ascribed, upon the hypothesis of Mialhe, to the fact, that little salt is used in their diet, and that consequently the conditions requisite to the change do not exist in their alimentary canal. Of the particular reactions which take place with the several preparations, an account will be given when they are individually considered.

It might be inferred from the view here given of the action of the mercurials, that, as all operate through the bichloride produced, it would be unnecessary to use any other than this preparation. But a little consideration will show that this does not follow. The change into corrosive sublimate is, with some of the preparations, exceedingly slow; and particle by particle is presented for absorption as it forms, so that no irritation of the stomach is produced by it; and it is impossible to exhibit corrosive sublimate itself, in doses so small and so frequently repeated, as to obtain the same end. Confirmatory of this statement is the fact, that the different preparations operate with a mildness inversely proportionate to the rapidity of their convertibility into corrosive sublimate, when placed under influences favourable to this result out of the body. Besides, in consequence of the slowness with which the bichloride is generated, the milder preparation has the opportunity of reaching the bowels before the change can take place, and does not, like the corrosive sublimate, exert its first effects on the stomach.

Another practical inference is that, when it is desired that a mercurial should operate mildly, common salt should be avoided; when it does not act, that a little of the salt might be taken at the same time. Caution, however, is requisite on this point; as, by too free an administration of the alkaline chlorides, we might cause an over-production of corrosive sublimate, and consequently irritant effects. This is particularly the case with chloride of ammonium, or muriate of ammonia, which has been ascertained to produce the change more rapidly than any other of the chlorides, and has, on this account, produced poisonous effects when taken internally with calomel.

When any one of the preparations, taken internally, occasions undue irritation of the alimentary canal, Mialhe proposes the hydrated protosulphuret of iron as a corrective. This acts by producing immediately, with any soluble preparation of the metal, an insoluble and inert sulphuret of mercury. in cases of excessive salivation, the same compound may be exhibited, in order to prevent further absorption of any portion of the mercurial, which may still remain in the alimentary canal.

The preparations of mercury may be arranged under the following heads; namely, 1. those in which it is in the metallic state, 2. the oxides, 3. the chlorides, 4. the iodides, 5. the sulphurets, and 6. the salts.