1BON. COBALT, AND NICKEL 821 



Iron is also found in the form of various other compounds for 

 instance, in certain silicates, and also in some phosphates ; but these 

 forms are comparatively rare in nature in a pure state, and have not 

 the industrial importance of those natural compounds of iron pre- 

 viously mentioned. In small quantities iron enters into the composi- 

 tion of every kind of soil and all rocky formations. As ferrous oxide, 

 FeO, is isomorphous with magnesia, and ferric oxide, Fe 2 3 , with 

 alumina, isomorphous substitution is possible here, and hence minerals 

 are not unfrequently found in which the quantity of iron varies con- 

 siderably ; such, for instance, are pyroxene, amphibole, certain varieties 

 of mica, &c. Although much iron oxide is deleterious to the growth of 

 vegetation, still plants do not flourish without iron ; it enters as an 

 indispensable component into the composition of all higher organisms ; 

 in the ash of plants we always tind more or less of its compounds. It 

 also occurs in blood, and forms one of the colouring matters in it ; 

 100 parts of the blood of the highest organisms contain about 0*05 of iron. 



The reduction of the ores of iron into metallic iron is in prin- 

 teiple very simple, because when the oxides of iron are strongly heated 

 with charcoal, hydrogen, carbonic oxide, and other reducing agents,* 

 they easily give metallic iron. But the matter is rendered more 



special sorting apparatus (generally acting with water to wash the ore), and is subjected 

 to roasting and other treatment. In every case the ore contains foreign matter. In the 

 extraction of iron, which is one of the cheapest metals, the dressing of an ore is in most 

 cases unprofitable, and only ores rich in metal are worked namely, those containing at 

 least 20 p.c. It is often profitable to transport very rich and pure ores (with as much as 

 70 p.c. of iron) from long distances. The details concerning the working and extraction 

 of metals will be found in special treatises on metallurgy and mining. 



4 The reduction of iron oxides by hydrogen belongs to the order of reversible re- 

 actions (Chapter II.), and is therefore determined by a limit which is here expressed 

 by the attainment of the same pressure as in the case where hydrogen acts on iron 

 oxides, and as in the case where (at the same temperature) water is decomposed by 

 metaHRc iron. The calculations referring to this matter were made by Henri Sainte-Claire 

 Deville (1870). Spongy iron was placed in a tube having a temperature t, one end of 

 which was connected with a vessel containing water at (vapour tension = 4*6 mm.) 

 and the other end with a mercury pump and pressure gauge which determined the 

 limiting tension attained by the dry hydrogen p (subtracting the tension of the water 

 vapour from the tension observed). A tube was then taken containing an excess of iron 

 oxide. It was filled with hydrogen, and the tension p l observed of the residual hydrogen 

 when the water was condensed at 0. 



t = 200 440 860 1040 



p = 95' 9 25'8 12'8 9'2 mm. 



p= 12' 8 9 '4 mm. 



The equality of the pressure (tension) of the hydrogen in the two cases is evident. The 

 hydrogen here behaves like the vapour of iron or of its oxide. 



By taking ferric oxide, Fe 2 O 3 , Moissan observed that at 850 it passed into 

 magnetic oxide, Fe 5 O 4 , at 500 into ferrous oxide, PeO, and at 600 into metallic iron. 

 Wright and Luff (1878), whilst investigating the reduction of oxides, found that (a) the 

 temperature of reaction depends on the condition of the oxide taken for instance 



