IRON, COBALT. AND NICKEL 345 



and RO 3 . It might have been expected that there would be inter- 

 mecliate stages RO 2 (corresponding to pyrites FeS 2 ) a^nd R 2 O 5 , but for 

 iron these are unknown. 26bis The lower oxide has a, distinctly basic 

 character, the higher is feebly acid. The only one which is stable in 

 the free state is ferric oxide, Fe 2 O 3 ; the suboxide, FeO, absorbs 

 oxygen, and ferric anhydride, FeO 3 , evolves it. It is also the same 

 for other elements ; the character of each is determined by the relative 

 degree of stability of the known oxides. The salts FeX 2 correspond 

 with the suboxide, the salts FeX 3 or Fe 2 X 6 with the sesquipxide, and 

 FeX 6 represents those of ferric acid, as its potassium salt is Fe0 2 (OK) 2 , 

 corresponding with K 2 SO 4 , K 2 Mn0 4 , K 2 Cr0 4 , &c. Iron therefore 

 forms compounds of the types FeX 2 , FeX 3 , and FeX 6 , but this latter, 

 like the type NX 5 , does not appear separately, but only when X re- 

 presents heterogeneous elements or groups ; for instance, for nitrogen 

 in the form of NO 2 (OH), NH 4 C1, &c., for iron in the form of 

 Fe0 2 (OK) 2 . But still the type FeX 6 exists, and therefore FeX 2 and 

 FeX 3 are compounds which, like ammonia, NH 3 , are capable of further 

 combinations up to FeX 6 ; this is also seen in the property of 

 ferrous and ferric salts of forming compounds with water of crystallisa- 

 tion, besides double and basic salts, whose stability is determined by 

 the quality of the elements included in the types FeX 2 and FeX s . 26 tri 

 It is therefore to be expected that there should be complex compounds 



remarkable ; a galvanic current (from 6 Grove . elements) is passed through cast-iron 

 and platinum electrodes into -a strong solution of potassium hydroxide. The cast- 

 iron electrode is connected with- the positive pole, and the platinum electrode is sur- 

 rounded by a porous earthenware cylinder. Oxygen- would bo evolved at the cast- 

 iron electrode, but it is used up in oxidation, and a dark solution of potassium ferrate is 

 , therefore formed about it. It is remarkable that the cast iron cannot be replaced by 

 wrought iron. 



86t>i When Mond and his assistants obtained the remarkable volatile compound 

 Ni(CO) 4 (described later, Chapter XXII.), it was shown subsequently by Mond and 

 Quincke (1891), and also by Berthelot, that iron, under certain conditions, in a stream of 

 carbonic oxide, also volatilises and forms a compound Jike that given by nickel. Koscoe 

 and Scudder then showed that when water gas is passed through and kept under 

 pressure (8 atmospheres) in iron vessels, a portion. of the iron volatilises from the 

 sides of the vessel, and that when the gas is burnt it deposits a certain amount of oxides 

 of iron (the same result is obtained with ordinary coal gas which contains a small amount 

 of CO). To obtain the volatile compound of iron with carbonic oxide, Mond prepared 

 a finely divided iron by heating the oxalate in & stream of hydrogen, and after cooling it 

 lo 80 45 he passed CO over the powder. The iron then formed (although very slowly) 

 ft volatile compound containing Fe(CO) 6 (as though it answered to a very high type, 

 PeXjo), which when cooled condenses into a liquid (slightly coloured, -probably owing to 

 Incipient decomposition), sp. gr. 1-47, which solidifies at -21, boils at about 103, and 

 has a vapour density (about 6'5 with respect to air) corresponding to the above formula ; 

 > decomposes at 180. Water and dilute acids do not act upon it, but it decomposes 

 wider the action of light and forms a hard, non-volatile crystalline yellow compound 

 ITe 3 (CO) 7 which decomposes at 80 and again forms Fe(CO) 5 . 



80 tn When the molecular Fe 8 Cl 6 is produced instead of FeCl 3 this complication o'i. 

 t the type also occurs, 



