CARBONATE OF IRON.

553

pheric air being carefully excluded. The carbonate of the protoxide ofiron is precipitated. When we attempt to collect and dry it, decomposi-tion takes place ; oxygen of the air is absorbed, carbonic acid escapes,and sesquioxide of iron remains (ferri sesquioxydum, Pli. L. vide p. 538).Hence when employed in medicine it must be prepared extempo-raneously.

Properties. —Native protocarbonate of iron is yellow: the primaryform of its crystals is the obtuse rhombohedron. Carbonate of iron pre-pared as above directed is a white precipitate, which by exposure to theair becomes at first greenish, then brown (sesquioxide). It is insolublein water, but dissolves in sulphuric or hydrochloric acid with effervescence.It also readily dissolves in carbonic acid water: the acidulo-chalybeatewaters are natural solutions of this kind ( vide p 145.)

Characteristics.— It dissolves in diluted sulphuric acid with efferves-cence. The solution possesses the before-mentioned properties of theferruginous solutions ( vide p. 533).

Composition. —Carbonate of the protoxide of iron is thus com-posed :—

Eq. Eq.Wt. Per Cent. Stromeyer.

(Native.)

Protoxide of Iron . . . 1 . . . 36 . . . 62 . . . 59*6276

Carbonic Acid . . . . ) ... 22 ... 38 .. . 38*0352

Carbonate of Iron ... 1 ... 58 .. . 100 . . . 97*6628

Physiological Effects. —It is one of the most valuable of the ferru-

ginous compounds, on account of the facility with which it dissolves inthe fluids of the stomach and becomes absorbed. Its local effects arevery mild.

Mistura Ferri Composita, Ph . Lond. and Dubl. (Myrrh ,powdered, 5ij.; carbonate of potash, 3j.; rose-water, fSxviij.; sulphateof iron, powdered, 3iiss.; spirit of nutmeg, fSij.; sugar, 5ij. llubtogether the myrrh with the spirit of nutmeg and the carbonate of potash,and to these, while rubbing, add first the rose-water with the sugar, thenthe sulphate of iron. Put the mixture immediately into a proper glassvessel, and stop it).—This is a professed imitation of Dr. Griffith’scelebrated antihectic or tonic mixture (Dr. M. Griffith, Observ. on theCure of Hectic and Slow Fevers, and the Pulm. Consump. 1776): henceit is frequently termed Griffith's Mixture ( mistura Griffithii).

In the preparation of it, double decomposition takes place: by themutual reaction of carbonate of potash and sulphate of iron we obtainsulphate of potash, which remains in solution, and carbonate of pro-toxide of iron, which precipitates. To prevent the latter attracting moreoxygen, it is to be preserved in a well-stoppered bottle. As morecarbonate of potash is used than undergoes decomposition, the excesscombines with the myrrh, and forms a kind of saponaceous compound,which assists in suspending the carbonate of iron in the liquid.

When first made, this mixture has a greenish colour, owing to theferruginous carbonate ; but by exposure to the air it becomes reddish,owing to the absorption of oxygen, by which sesquioxide of iron isformed, and carbonic acid evolves: hence it should only be preparedwhen required for use.

It is one of the most useful and efficacious ferruginous preparations,and which is supposed to be owing to its being readily soluble, and con-