PART III.

Metallic Preparations. — Iron.

519

Sulphas Ferri. Dub. Sulphate of Iron.

" Take of iron wire, two ounces; sulphuric acid, threeounces and a half; water, one -pint. Mix gradually the acidwith the water; then add the wire cut into pieces, and digestthe mixture that the metal be dissolved, after which filter thesolution through paper ; finally, after due evaporation, set itapart, that crystals may form by slow cooling."

In these processes part of the water is decomposed, the ironis oxidized by combining with its oxygen, while its hydrogen isdissipated in the gaseous form; and the oxide thus producedunites with the acid and forms sulphate of iron, or rather sul-phate of oxide of iron; which is dissolved in the undecom-posed portion of the water. Concentrated sulphuric acid,nevertheless, scarcely exerts any action on iron at a low tem-perature, and water alone is very slowly decomposed by it,so that the rapid decomposition of the diluted acid by the ironmust be ascribed to the sum of the affinities of the base of theacid for oxygen, and of the iron for oxygen being superior tothat of the oxygen to the hydrogen of the water, which istherefore decomposed. The solution is of a pale green colour,and when evaporated directly, yields crystals of sulphate ofiron 1 ; but if it be exposed for some time to the atmosphere itattracts oxygen, becomes turbid, a subsulphate is precipitated,and the salt obtained is an oxysulphate.

Qualities. — Sulphate of iron has a strong styptic taste: itcrystallizes in transparent rhomboidal prisms, of a fine greencolour, which redden the vegetable blues; are soluble in twoparts of water at 60° and Jths of their weight of boiling water,and are insoluble in alcohol. When exposed to the air,the crystals become opaque, and are covered with a yellowpowder, owing to the attraction of the oxygen of the atmos-phere by the salt, during its efflorescence. Exposed to heatsulphate of iron undergoes the watery fusion; and in an in-creased heat the acid is driven off, and the base remains in thestate of a red oxide, the colcothar of vitriol of commerce.According to Dr. Thomson*, 100 parts of the green sulphateconsist of 26.7 of sulphuric acid, 28*3 oxide of iron, and 45 - 0of water. 3 The following substances decompose sulphate ofiron: the earths, the alkalies and their carbonates; borate of

1 This salt, which is known in commerce by the name of green vitriol, is preparedon the great scale from native sulphurtts of iron, by exposing them to the air andmoistening them, till a crust of sulphate of iron is formed on their surface, which isafterwards obtained in crystals by solution and evaporation.

2 System of Chemistry, 4th ed. lit. 1225.

3 Of this quantity of water, 8 parts are water of composition, the oxide being in thenate of a hydrate.

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