This was introduced into practice as a substitute for the old rust of iron. Its claim to the title of subcarbonate cannot be sustained on chemical grounds. Only one compound of carbonic acid and iron is known, and this consists of equivalent proportions of the acid and protoxide. There is, therefore, no known subcarbonate. The preparation under consideration consists mainly of hydrated sesquioxidc of iron, with which is associated a variable proportion, always, however, small, of carbonate of the protoxide. In the U. S. Pharmacopoeia it was denominated subcarbonate, partly because it had previously held the name of precipitated carbonate, which, when its true nature came to be known, was considered quite inappropriate, and partly under the impression that its virtues were, in great measure, ascribed to the small proportion of carbonate contained in it, which could not, therefore, it was thought, be appropriately excluded from a share of the title. The term subcarbonate may, in this sense, be considered as simply signifying that it contains carbonic acid, but not in quantity equivalent to the basic matter.

This preparation is made by precipitating a solution of sulphate of iron with carbonate of soda, and afterwards washing and drying the precipitate. By reaction between the two salts, carbonate of protoxide of iron is formed, which, though of a bluish-white colour when deposited. rapidly changes on exposure to the air, and, before the close of the washing and drying, is converted mainly into sesquioxide of a reddish-brown colour. The protoxide of iron has so strong an affinity for oxygen that, whether separate or combined with an acid, it quickly becomes sesquioxidized on exposure; and, if previously combined with carbonic acid, gives so much of it off as was united with the portion converted into sesquioxide. In the present preparation, however, a small proportion of the carbonate precipitated remains unchanged. The manufacturer sometimes calcines it to improve its colour, thus driving off the water, and probably converting the small residue of carbonate into quioxide, very much to the detriment of the preparation. It is said that the beautiful bright reddish-brown powder, often found in the shops, has always undergone this treatment, and should, therefore, be rejected. Unless the powder effervesce somewhat when dissolved in muriatic acid, its fitness for medicinal use may be doubted.

Properties

The subcarbonate of iron is a dull reddish-brown, or somewhat chocolate-coloured powder, inodorous, of a slightly styptic and ferruginous taste, insoluble in water, and soluble with difficulty in the acids. except the muriatic, which dissolves it with some effervescence, owing to the escape of carbonic acid.

Effects on the System

It has little local action on the stomach, and may be taken to an almost unlimited amount, with no other effect than to occasion feelings of epigastric weight and oppression, and sometimes probably slight nausea and vomiting. It may, too, when taken excessively, accumulate in the bowels, and produce some mechanical inconvenience. By far the larger portion passes out of the bowels with the stools, which it blackens. Of a considerable number of chalybeate preparations examined by Quevenne, this gave to the gastric liquor the least proportion of iron. As it can impart to the circulation only so much of the metal as it yields to that liquor, it must be inferred to be among the feeblest in its effects on the system. Nevertheless, experience has shown that it is capable of producing all the general effects of the chalybeates; and, though it must be given in larger doses than most other preparations, it is yet well borne by the stomach, so that the dis-advantage of its relative feebleness is in some degree counterbalanced. These large doses are rendered necessary by the small proportion it contains of the carbonate, which, probably, is the ingredient through which, mainly, it is capable of affecting the system.

Therapeutic Application

Little good can be expected from the subcarbonate, as a simple tonic, in reference to its direct action on the stomach and bowels; and it is seldom given for such a purpose. But, with regard to its effects on the system, before the discovery of the protective power of sugar over carbonate of iron, and the consequent adoption of that salt, and while yet the extraordinary chalybeate virtues of the powder of iron reduced by hydrogen were unknown, this was among the most popular of the ferruginous preparations, partly from its ascertained efficiency, and partly from its innocence, even in very large doses; and it is still employed to a considerable extent for special purposes. It is unnecessary to repeat an account of the diseases to which the chalybeates in general, and consequently this particular preparation, are applicable. For this the reader is referred to the general remarks upon the subject of iron. It will be sufficient here to call attention to the special purposes just alluded to.

The therapeutic application for which the subcarbonate is most highly esteemed is to the cure of neuralgia. About forty years since, Mr. B. Hutchinson, in a pamphlet on the subject, called the notice of the profession to several cases of tic douloureux, which had been successfully treated by this remedy. The practice was quickly imitated by others, with results so frequently favourable, that the confidence of the profession in its efficiency became established. I have frequently employed it myself in this painful and often very obstinate affection, and do not think that I have obtained so much advantage from any remedy, as from a joint use of the subcarbonate of iron and the narcotic extracts, especially that of belladonna. Many cases certainly will resist the influence of this remedy, and other- may succeed where this has failed; for there are few affections having a greater diversity of origin, or requiring greater diversity of treatment than neuralgia; but it may, I think, rank among the most efficacious. Its usefulness in this disease suggested its employment in other obstinate nervous affections; and it has been given in a considerable number of cases of traumatic tetanus with asserted success. It has also been employed advantageously in chorea, and in the second stage of hooping-cough, when its nervous character has begun to predominate. As a remedy in these nervous affections, particularly neuralgia and tetanus, the doses employed are much larger than those ordinarily administered merely for the improvement of the blood. From half a drachm to three drachms are given three times a day; and, in one of tetanus, it was carried to half an ounce every two or three hours. Care should be taken, when these large doses are given, that the bowels should be duly evacuated; and I have generally combined the chalybeate with a little ginger, to obviate its disagreeable effects on the stomach.

How the medicine operates in the nervous diseases, whether merely as a chalybeate, by improving the blood, and exercising a tonic influence directly on the nervous centres, or by some additional and peculiar influence, it is difficult to determine. If upon the former principle alone, the same effects ought to be produced by other chalybeates, still more efficient than it in obviating anaemia. I cannot help suspecting that the operation of its mere mass on the interior surface of the stomach and bowels may have something to do with the result, probably through the sympathies connecting the alimentary canal with the brain and spinal marrow. In a case of very severe neuralgia of the bowels, which occurs to my recollection, the curative effect was so speedy that it could scarcely be ascribed to the absorption of the iron. It seemed to me highly probable that the powder, retained by its weight in contact with the surface on which it was spread, acted as a protective to the mucous membrane, and thus prevented the neuralgic paroxysms, which might have depended on an irritant action of the intestinal contents on the excessively sensitive surface.

Another purpose for which this preparation may be employed is to act as an antidote to arsenious acid. Though not equal to the freshly precipitated hydrated sesquioxide, it has considerable efficacy, and should be resorted to when the other cannot be obtained. Hut if previously exposed to a red heat, it becomes inapplicable to this purpose, in consequence of the same molecular change which renders it insoluble in dilute acids. It may be given ad libitum.

The dose for the ordinary purposes of the chalybeates is from five to thirty grains, three times a day. It may be administered in the form of an electuary mixed with syrup or molasses, and may often be usefully associated, to meet special indications, with aromatic, tonic, and laxative powders, as ginger, columbo, and rhubarb.

The Iron Plaster (Emplastrum Ferri, U. S.) is made from this preparation, by incorporating it with lead plaster and Burgundy pitch, previously melted together. Under the impression that this plaster serves to strengthen debilitated parts, it has commonly been called strengthening plaster, and employed in weakness of the loins and joints. But there is no reason whatever to suppose that the chalybeate can penetrate the cuticle so as to reach the part affected; and the notion of the strengthening influence of the iron is probably quite illusory. Nevertheless, the plaster may prove useful in some cases of chronic rheumatism, or other inflammatory affection of these parts, through the revulsion effected by the gentle irritation of it sustains upon the surface; and the muscles or joints, being thus relieved of the disease which interferes with their functions, seem to be strengthened.