_IRON AND CHROME TANNAGES._ _Iron tannages_ may be very shortly dismissed, as their practical interest is at present either historical or prospective, but iron salts enter in so many ways into the chemistry of leather manufacture, that their properties must be briefly considered. Iron exists in salts in two states, the ferrous, and the ferric, in the first of which it is divalent, and in the second trivalent. Thus ferrous chloride is FeCl₂; ferrous oxide, FeO; ferrous sulphate, FeSO₄; ferrous hydrate, Fe(OH)₂. The compounds of ferrous iron are mostly green, like ferrous sulphate (“green vitriol,” “copperas”): exposed to air and moisture, they easily absorb oxygen, and pass into the ferric form. Ferric chloride is FeCl₃ (or, as it is sometimes written without much reason, Fe₂Cl₆), ferric hydrate Fe(OH)₃, ferric oxide Fe₂O₃, ferric sulphate Fe₂ (SO₄)₃, and so on. The atomic weight of iron is 56. Ferric salts are mostly yellow or orange, ferric hydrate is yellow-brown, and on ignition is converted into deep red ferric oxide, which is very difficultly soluble in acids. Ferric salts in contact with more easily oxidisable matters, readily give up oxygen, and pass into the ferrous state; and especially does this happen in the presence of organic matter, under the influence of sunlight. Thus iron-salts often act as carriers of oxygen, and oxidisers of organic matter, absorbing oxygen from the air, and giving it up again under the influence of light or heat. There are several other oxides of iron which do not form salts, and there is a ferric acid, apparently corresponding to chromic acid, which is so unstable that it has been very imperfectly investigated. Ferric salts correspond in structure to those of alumina, and like these are powerful tanning agents, and readily form basic salts, while the ferrous salts have no tanning effect until they become oxidised, when they form basic ferric salts. Ferric salts are characterised by giving blue-black or green-black compounds with tannins, and with many other allied bodies, while the corresponding ferrous compounds are mostly colourless, though they rapidly oxidise and darken.