1. Pn toxide of iron( ferrous oxide) (Fe0). This body, though a very powerful base, and forming, with acids, a well-marked class of com pounds called salts of protoxide of iron, is nevertheless not known in the separate state, so readily does it take up oxygen and form peroxide. In the hydrated state (FeO,HO) it is precipitated in white flocks on adding an alkali to a solution of a protosalt of iron. If care is not taken to exclude every trace of air from the liquid used in its pre paration it is contaminated with more or less peroxide, and then falls down as a light green or blue precipitate. In contact with air it gradually becomes entirely converted into red peroxide.
2. Peraride of iron (sealuiJxide or ferric oxide) (Fe,0„). This oxide occurs native as hie matitc, specular iron ore, &e. [litosr, in NAT. HIST. Div.] It is artificially prepared by heating the hydrate, or by igniting the protosulphate. Under the names of colcothor, crocus of Mars, rouge, &c., it is largely used as a pigment, and for polishing glass and jewellery. The hydrated oxide x HO) is best prepared by precipitating a solution of the perchlorida or persulphate with ammonia, well washing and gently heating. When first precipitated this hydrate is very flocculent and bulky, but contracts remarkably on drying. It has a dull red colour, not nearly so bright as the a„nhydrous oxide. It forms salts with acids, but is a weaker base than the protoxide.
Hydrated peroxide of iron is now largely used for the purificAtion Of coal-gas. [GAS-LIGHTING.]
3. Ferric acid is not known in the free state. Combined with potash it is obtained on heating to bright redness peroxide of iron with four times its weight of nitrate of potash. I t is very soluble in water, forming a dark violet solution. Other ferrates are produced by double decomposition ; they are all uncrystallisable and very unstable, readily yielding up their oxygen, especially to organic matter.
Iron and sulphur combine in several proportions. The two most important sulphides are :— 1. Protosulphide of iron . . . Fed 2. Eisulphide of iron . 1. Protosulphide of iron (FeS) is a bye-product in certain metallurgic operations with copper. It is of little value, being chiefly employed in chemical laboratories for the generation of sulphuretted hydrogen. For this purpose it is sometimes prepared by fusing together equivalent proportions of iron (filings) and sulphur, or better, by rubbing rolls of sulphur ou a bar of iron heated to whiteness in a smith's forge, the liquid sulphide being allowed to flow down into cold water.
2. Bisulphide of iron (FeS,I (iron pyrites) occurs native in beautiful brass-yellow crystalline masses. It is valueless as a source of metal, but under the name of atrodie is extensively used as a source of sulphur in the manufacture of sulphuric acid. Some varietiess of it decompose hen exposed to air, being gradually converted into sulphate of iron.