Preparation

As the Prussian blue of commerce is always more or less impure, this preparation should be made specially for medical use. The U. S. Pharmacopoeia directs that to the officinal solution of tersul-phate of iron a solution of ferrocyanide of potassium should be added, and that the precipitate produced should be washed on a filter, and dried. Portions of the iron of the tersulphate, and of the potassium of the ferrocyanide, change places, so as to form sulphate of potassa and ferrocyanide of iron, of which the former either remains in the solution, or is dissolved out in the washing, leaving the latter pure.

Composition

Ferrocyanide of iron consists of three equivalents of ferrocyanogen (which contains one equivalent of iron and three of cyanogen), four equivalents of iron, and six eqs. of water; the last ingredient being essential; as it cannot be separated without decomposing the salt. The compound, however, as in the case of other haloid salts, may be looked on as consisting of a hydracid and an oxide of iron, the hydrogen of the water going to the ferrocyanogen, and the oxygen to the iron. In this view, the salt would be a hydroferrocyanate of the sesquioxide of iron, and would consist of three eqs. of hydroferrocyanic acid (each containing one eq. of ferrocyanogen and two of hydrogen), and two eqs. of sesquioxide of iron. Commercial Prussian blue always contains, besides the above compound, a portion of uncombined alumina and sesquioxide of iron.

Properties

The pure salt is in the form of a rich deep-blue powder, inodorous and tasteless, insoluble in water and alcohol, and unaffected by the dilute acids. Strong sulphuric acid dissolves without decomposing it, concentrated nitric and muriatic acids decompose it, and the alkalies separate sesquioxide of iron.

Effects on the System

From the insipidity of Prussian blue, its insolubility even in dilute acids, and the fact that it is formed whenever ferrocyanide of potassium meets a salt of iron in the body, even in tin-circulating liquor sanguinis, a fair inference is, that it would probably have no other effect on the system than such as might result mechanically from the presence of an insoluble powder in the stomach and bowels; and this view would seem to be supported by the experiments of M. Coullon, who found it without action in several different specie3 of animals. (Merat et De Lens, ii. 531).

Therapeutic Application

Notwithstanding, however, its presumed inertness, it has been much commended by some practitioners. Dr. W. Zollickoffer, of Maryland, first called the attention of the profession to it as a remedy in intermittent and remittent fevers, in which ho was induced by his observations to consider it superior to Peruvian bark. (Am. Med. Recorder, July, 1822, v. 540.) Dr. Samuel Jackson, late of Northumberland, employed it with success in a large number of cases, but found it to fail in many. (Am. Journ. of Med. Sci., ii. 336, a.d. 1827.) Dr. Hasse, a German physician, found it effectual in an epidemic intermittent which prevailed at Gastrow in the spring of 1827; but he gave doses of a grain of the salt with a scruple of pepper, the latter of which was probably much the more efficacious of the two. (Hufeland's Journ., quoted by Merat and de Lens, ii. 531.) Dr. Eberle speaks of it favourably from his own observation and experience. (Mat. Med. and Therap., 4th ed., i. 321.) Many other physicians have borne a somewhat hesitating or equivocal testimony in its favour. I tried it myself in a number of cases, soon after it first came into notice, with some success, but with so many failures that I soon abandoned it. The medicine has been given with supposed success also in chorea and epilepsy, but associated generally with others, so as to leave the real agent of cure doubtful, and sometimes in such doses as to leave no doubt as to the inefficiency of the Prussian blue. Thus, M. Burguet, of Bordeaux, gave from one-eighth of a grain to four grains a day; and Dr. Kirckhoff, of Ghent, supposed that he cured epilepsy with from half a grain to six grains in the day. Dr. Bridges, of Philadelphia, employed it in a case of obstinate facial neuralgia, with considerable relief, after the ordinary remedies had been tried with little or no effect. It will have been observed that all these diseases, in which efficiency has been claimed for it, are of a character to be readily affected through the mind; and in which anything will occasionally effect apparent cures in which the patient may have confidence, or even strong hope. I am not prepared, however, to say, that the mere presence of an insoluble agent in the primae via-. acting only mechanically on the mucous surface, may not prove useful in nervous diseases, and local affections of the membrane itself.

The dose, as given by different practitioners, varies extremely. Generally, from four to six grains have been given several times a day. Instances are mentioned above in which the dose was reduced to the eighth of a grain, and Dr. Jackson, late of Northumberland, gave in some cases two drachms during the interval in intermittents, finding it to lie easily on the stomach. This latter fact tends strongly to favour the idea of its inertness, in reference to any direct influence on the system in general.

Besides the preparations of iron, other substances have been to a certain extent remedially employed, under the impression that they might supply deficiences, in the system, of certain constituents derived from the mineral kingdom.

Manganese, having been found in the blood, has been like iron supposed to constitute a normal ingredient of that fluid; and, under this impression, has been used in certain conditions of anaemia as an adjuvant of the chalybeates, or a substitute for them. At one time the compounds of this metal were considerably prescribed; but, with one exception, the permanganate of potassa, they have fallen into comparative neglect; and this owes its importance to quite other properties than those of a reconstructive tonic. This salt will be treated of with the Disinfectants; and I may then take occasion to allude to some other preparations of this metal; but, for a full account of them, the reader is referred to the U. S. Dispensatory.

Phosphate of Lime has been supposed to be useful in certain cases of diseased bones, by supplying a deficiency of earthy matter, as in rickets, scrofulous affections of the bones, etc.; but it is not, I believe, the deficiency of material that here constitutes the difficulty, so much as the power of assimilating them; and phosphate of lime is probably almost always sufficiently supplied with the food. Nevertheless, it may be used without disadvantage, even if productive of no great benefit. It has been adopted in the U. S. Pharmacopoeia, under the title of Precipitated Phosphate of Lime (Calcis Phosphas Praecipitata, U. S.), and directions are given for preparing it from calcined bones. (See U. S. Dispensatory, 12th ed.) It is a white, inodorous, tasteless powder, insoluble in water, but readily dissolved by diluted nitric, muriatic, and acetic acids. The acid being tribasic, it consists of three eqs. of lime and one of phosphoric acid. Beneke has suggested its use in diseases of defective nutrition, as scrofula, phthisis, etc., from the circumstance that it is an ingredient in the ultimate cells; but the same reason for doubt exists here as in the former case of diseased bones; it is not the material, but the assimilating power that is wanting; for, in many of these cases, the insoluble phosphates are discharged in excess by the kidneys, showing that there is no deficiency of them in the blood. The dose is from ten to thirty grains. It may be given in powder, or dissolved in water, with the aid of a little muriatic or nitric acid, which will often probably greatly add to its efficiency.

Hypophosphites. On the same view of supplying a supposed deficiency of material in the system, the hypophosphites have been employed to furnish phosphorus to the tissues; and with this object have been especially recommended in the treatment of phthisis. Experience, however, has shown their entire want of control over this disease; and there does not appear to me to be sufficient reason for introducing them into the already overburdened catalogue of remedies. All needful information in relation to them will be found in the third part of the U. S. Dispensatory (12th ed.).