A pure oxide of mercury with a larger proportion of oxygen occurs in the hydrargyrum calcinatum of the London and Dublin Dispensatories. The process is slow and tedious; but the preparation, if carefully levigated, highly useful. Its acrimony must, however, be guarded by opium, or it will prove emetic and purgative. In a long practice, it is not uncommon to feel at different periods a predilection for different preparations of mercury; and, perhaps, at this time we may be peculiarly fond of the present preparation. It seems, however, to perform every thing which the most celebrated mercurials have effected, and not to be more inconvenient. Yet, perhaps, in every instance, calomel will do as much.

Mercury, oxidated by nitrous acid, has been much employed; hydrargyrus nitratus ruber of the London Dispensatory; oxidum hydrargyri rubrum per oxidum nitricum of the Edinburgh; and hydrargyrum subnitratum of the Dublin: yet it scarcely differs from the mercurius calcinatus, except in the convenience and the facility of the preparation. The metal is first united with the acid; then the latter decomposed and separated by heat. In general, the acid exceeds in a small proportion the weight of the quicksilver; but the London college renders the proportions equal, adding a little muriatic acid, which it is supposed increases the size and beauty of the red scales. This advantage is, however, equivocal; and the little difference in the proportion of the acid of scarcely any importance except in an economical view. If too small, the continued heat compensates the inconvenience. The ointment contains one part of the oxide to eight of hogs' lard.

The oxides of mercury, combined with acids, are preparations of great importance. In the old unguentum citrinum, unguentum hydrargyri nitrati, we find the only instance of its combination with the nitrous acid. With the vitriolic acid it forms the hydrargyrus vitriolatus of the London, and the subsulphas hydrargyri fla-vus of the Edinburgh. The very singular process by which this medicine is prepared, M. Fourcroy has very industriously and ingeniously analysed. When the acid is added to the metal, no action takes place; but when heat is applied, a part of the acid is decomposed and the metal oxidated, uniting with the remainder of the acid. The salt thus formed is white, but the acid is in excess. When the seperfluous acid is separated, this salt crystallizes in fine prismatic needles; is soluble in about five hundred parts of cold water, and in half that proportion of warm, without decomposition. A little sulphuric acid increases its solubility. In this state Pourcroy found that it contained seventy-five of the metal, twelve of the acid, eight of oxygen, and five of water, in one hundred parts.

In the processes of the colleges, the saline solution is boiled to dryness, so that more of the acid is decomposed, and a larger proportion of sulphureous acid gas evolved. If then, as directed, it is thrown into boiling water, a yellow powder, the hydrargyrus vitriolatus, is formed. What is effected by this part of the process M. Fourcroy discovered by examining the different portions. The water contained a salt with more than its proportion of acid: of course, the powder had a diminished proportion, and the affusion of the hot water dissolving as much of the salt as it could take up with its acid, left the powder deprived of it. It consisted, on analysis, of seventy-six parts of mercury, and eleven of oxygen. The taste is acrid; it is soluble in two thousand parts of cold water, decomposed by the nitrous and muriatic acids, oxidizes quicksilver, and is converted by trituration with it into a black powder. Such is the outline of this author's labour; and chemistry does not furnish an analysis at once so accurate, so elegant, and satisfactory.

The muriatic acid furnishes preparations of the greatest use and importance in medicine. In the calomel of the London Pharmacopoeia, the submurias hydrargyri of the London, and the hydrargyrum muriatum mite sub-limatum of the Dublin, prepared by sublimation, the metal is oxidated more slightly. But before we can notice these we must step forward in the table to the muriats, and the three different titles express only the combination of the metal with the muriatic acid, generally known by the appellation of corrosive sublimate. In all the preparations the mercury is oxidated by sulphuric acid, and then combined with the muriatic, by subliming it with a mixture of common salt. The taste is peculiarly acrid and styptic: it is soluble in twenty parts of cold, and in about two of boiling water; in nearly four parts of alcohol at 70°, and in an equal weight of alcohol in a boiling state. It is unaltered in the air, in sublimation, or by either of the mineral acids. Its solubility is increased by the addition of a small proportion of crude sal ammoniac. It is precipitated by all alkalis and earths; those not carbonated form a yellow precipitate; the others an orange yellow, changing to a brick red. It consists of oxide of mercury 0.82, and of acid 0.18. The oxide itself containing 0.15 of oxygen.

The calomel, and the synonymous formulae which precede, are prepared from this muriat of mercury. The principle of the preparation is obvious, when it is recollected that earths and metals, unlike alkalis, may be combined with different portions of acid, and still be in appearance neutral. In the muriat of mercury there is no apparent excess of acid; yet nearly an equal proportion of the metal may be combined with it, and partake not only of its acid but its oxygen. About nine ounces of the metal are added to a pound of the muriat, and united by trituration. The newly added metal shows immediately appearances of oxygenation, but the combination is effected only by repeated sublimation and trituration. The London college orders this process to be repeated four times; an extraordinary precaution certainly not necessary. Generally twice is sufficient; but we have sometimes found the union not perfectly complete, and a third sublimation is occasionally necessary. From Mr. Chevenix's experiments also there seems to be an unnecessary waste of quicksilver, a less proportion, 0.54, for instance, appearing sufficient. He considers the excess,however,a necessary precaution. The utility of calomel, which may be perhaps considered as an instar omnium with respect to the other mercurial preparations, for it is in every instance equally useful with any other, and in many cases greatly superior, has induced chemists to attempt with great diligence an easier and more certain mode of preparing it. The great object is to avoid the necessity of very long-and careful trituration; for the calomel forming by sublimation a very hard solid mass, consisting of a confused aggegration of tetraedral prisms, terminated by pyramids, the most careful attention is necessary lest any spiculae should remain. Calomel has, consequently, been prepared in the humid way; first, if we recollect rightly, by Scheele, and afterwards more accurately by Gottling. It is now found in the Dublin and Edinburgh Dispensatories with the epithet praecipitatum; and in that of London, under the title of hydrargyrus muriatus mitis. The principle of the process consists in forming a nitrated solution of mercury with an excess of oxide, to which a dilute solution of common salt is added. It was supposed that the decomposition of the nitrated mercury immediately took • place, and that the powder deposited was calomel. Mr. Chevenix, however, has given us some reason to doubt the justness of this conclusion; and as water, the menstruum of the salt, will decompose nitrated quicksilver, the precipitate is probably a subnitrate of quicksilver, together with the sub-muriate. He consequently proposes adding to the water a little muriatic acid, or to employ a nitrated quicksilver without heat. To either proposal no reasonable objection can be made; yet it has been suggested by good authority, that the quicksilver in the nitrated salt is too highly oxidated - for, if the preparation be made by boiling, the proportion of oxygen is greater - and that the solution has been found to contain muriated quicksilver. We have repeated the process with this view-without finding the same result; yet we think it should teach us to render our solution of common salt more concentrated, and the two fluids should be added at once, that the moment a particle of water touches the metallic solution, the latter should be met also by a portion of the common salt. In the usual"preparations there is always a little of the nitrated mercury; for on rubbing the precipitates with lime water, the powder is grey and not black. When again sublimed, this nitrated mercury is decomposed, and a sublimation is conse-qtienuy recommended. We find in our minutes a proa R G 174 Arg posal of again repeating the addition of"a solution of common salt, and edulcorating the powder by washing; but do not recollect that the experiment was tried.