Hermstaedt recommends a process of preparing calomel from the sulphat of mercury, to which nearly the original quantity of mercury is to be united by trituration. The muriat of soda is then added, the whole mass sublimed, the trituration and sublimation a second time repeated. This preparation has not been chemically examined, and we do not know its peculiar advantages. On the whole, perhaps, the old method of preparing the calomel forms the most certain and best preparation, if the subordinate agents can be depended on in the trituration; and as their error can be detected by a nice eye, inconveniences will not often occur: indeed, in the shops of the greater number of apothecaries no such are found. Calomel, according to Mr. Chevenix, contains 88.5 of oxide of quicksilver, and 11.5 of muriatic acid: the oxide amounts to 0.107, while the muriate contains 0.15 of oxygen. Fourcroy estimates the oxides differently: he makes three species, the black, the red, and one other still higher, not to be obtained separately; the black and red containing, respectively, 0.04 and 0.08 of oxygen. These different results have not been reconciled, and as the disquisition would be purely chemical we shall not at-tempt it.

Chemists expected to form milder and more convenient preparations of mercury with the acetous acid; and the reputation of Keyser's pills, which were found to be a combination of this kind, seemed to confirm the opinion. In the preparations of the three colleges nitrate of mercury is first formed with a gentle heat, which neither occasions it to take up an excess of acid, nor, as in case of boiling, to absorb and oxidate a larger proportion of the metal. A solution of acetated potash is then added, and the acetite of mercury crystallises, leaving the nitrate of potash, formed, in the preparation on account of its greater solubility in the fluid. It dries slowly, and should be compressed in bibulous paper. We have not found it to possess any advantages above the other mercurial preparations.

Combined with sulphur, mercury is seldom employed internally. Of the medical effects of the AEthiops mineral we have already spoken; but we must now consider shortly the chemical relations of this union, to complete the chemical history of the metal before us.

The black sulphurated quicksilver is not merely a combination of the sulphur and the metal, as authors have supposed. Quicksilver never assumes the form a black powder, without having absorbed some portion of oxygen. Others have suspected that hydrogen is also united, and think that the process is expedited by adding a little water, whose decomposition supplies both. It is dissolved by the aqua kali, but unaffected by nitrous acid. From the solution of kali it is recovered unchanged by acids, and in the fire it suffers no alteration. When hot quicksilver is thrown into melted sulphur, and the whole stirred till cold, the same preparation in appearance results: the union is not however so complete; it is not soluble in the solution of kali, and is changed by the air. Berthollet supports the idea of its containing hydrogen by this remark, that the AEthiops mineral may be prepared by agitating mer -cury with sulphurated, hydrogenated ammonia. This-preparation also admits of change from the air.

The hydrargyrum sulphuratum rubrum is the factitious cinnabar, a medicine formerly used as a tonic, a stimulant, and a deobstruent; in short, for every object of which the prescriber had no distinct idea. It is now only employed as a fumigation in venereal complaints. It is not soluble in any acid; but the nitro-muriutic takes up the metal and leaves the sulphur. Alkalis, in a boiling heat, will not affect it; but, when melted, these and many of the metals decompose it. M. Proust supposes, that the quicksilver which it contains is not oxidated, but that it is in the proportion of 85 to 100, and that the remainder is sulphur.

The variety of other preparations of mercury, employed by physicians and surgeons of different countries, at different periods, would fill a volume. As we cannot enumerate every remedy of this kind, so preparations often celebrated must not be wholly overlooked. We shall not immediately follow the same order; but first divide the preparations according to their pharmaceutical forms, viz. ointments, plasters, pills, syrups, troches, drops.

The mercurial ointments have been varied in every possible way according to the objects for which they were designed. Turpentine was formerly the general intermede to divide the crude mercury, and the additions were adapted to the disease for which it was employed. In the various unguenta ad pediculos we find the seeds of stavisacre, extract of tobacco, the roots of white hellebore, and oleum laurinum added. In Mynsicht's formula, the quicksilver is divided by the saliva of a person fasting. When to cure the itch, sulphur, alum, and white hellebore, are united with the metal; against worms, the gall of an ox, and oil of bitter almonds; in cutaneous diseases, by Stahl, preparations of lead and a portion of camphor; and, in the ungucntum ophthalmicum of Hecker, nitrated mercury is united with camphor.

The mercurial plasters and cerates have not been greatly varied from those directed in the different British Pharmacopoeias. Plenck's cerate is made with mercury, divided by mucilage, as in his other preparations. It has been doubted, whether in this form the mercury is absorbed. Mercurial plasters have often no effect; but after their application we sometimes have found pains in the stomach and bowels, which are relieved by removing the plaster; and, in one or two instances, salivation has followed.

The mercurial pills have been very various. Barbarossa's pills,named from the celebrated Algerine who gave the process to Francis I. consisted of mercury, with a small proportion of rhubarb and scam-mony, formed into a mass with lemon juice. Plenck's, pilulae ex mescurio gummosa, consisted each of a grain of mercury, extinguished by starch and gum arabic, with sometimes a small proportion of rhubarb; and, indeed, some of the forms used in this country contain a mixture of some active cathartics. It were endless to follow all the varieties directed in different Dispensatories, varying only by the mode of extinguishing the quicksilver, and the peculiar additions. The magnetic pills of Ostius are prepared with the mercurius cal-cinatus, with a large proportion of some vegetable extract, which has not been accurately ascertained"; and Keyser's pills, with the acetite of mercury. These last are now disused; and, whatever was once their credit, the testimony of Murray, Girtanner, Quarin, and Co-lumbier, seems to have destroyed it.