ERON, COBALT, AND NICKEL 337 



is not precipitated by alkalis and alkali carbonates, which fact points 

 to the formation of double ammonium salts. 18 The ferrous salts have 

 a dull greenish colour, and .form solutions also of a pale green colour, 

 whilst the ferric salts have a brown or reddish-brown colour. Thu 

 ferrous salts, being capable of Oxidation, form very active reducing 

 agents for instance, under their action gold chloride, AuCl 3 , deposits 

 metallic gold, nitric acid is transformed into lower oxides, and the 

 nighest oxides of manganese also pass into the lower forms of oxidation. 

 All these reactions take place with especial ease in the presence of an 

 excess of acid. This depends on the fact that the ferrous oxide, FeO 

 (or salt), acting as a reducing agent, turns into ferric oxide, Fe 2 C>3 (or 

 salt), and in the ferric state it requires more acid for the formation 

 of a normal salt than in the ferrous condition. Thus in the normal 

 ferrous sulphate, FeSO 4 , there is one equivalent of iron to one 

 equivalent of sulphur (in the sulphuric radicle), but in the neutral 

 ferric salt, Fe 2 (S0 4 ) 3 , there is one equivalent of iron to one and 'a 

 half of sulphur in the form of the elements of sulphuric acid. 19 



The most simple oxidising agent for transforming ferrous into ferric 

 salts is chlorine in the presence of water for instance, 2FeCl 2 4- Cl a 



18 Ferrous sulphate, like magnesium sulphate, easily forms double salts for instance, 

 (NH 4 ) a SO4,FeSO 4 ,6H 2 O. This salt does not oxidise in air so readily as green vitriol, and 

 is therefore used for standardising K^MnO 4 



19 The transformation of ferrous oxide into ferric oxide is not completely effected in 

 air, as then only a part of the suboxide is converted into ferric oxide. Under these 

 circumstances the so-called magnetic oxide of iron is generally produced, which contains 

 atomic quantities of the suboxide and oxide namely, FeO,Fe2O5 = Fe 3 O 4 . This sub- 

 stance, as already mentioned, is found in nature and in iron scale. It is also formed 

 when most ferrous and ferric salts are heated in air; thus, for instance, when ferrous 

 carbonate, FeCO 5 (native or the precipitate given by soda in a solution of FX 2 ), is 

 heated ifc loses the elements of carbonic anhydride, and magnetic oxide remains. This 

 oxide of iron is attracted by the magnet, and is on this .account called magnetic oxide, 

 although it does not always show magnetic properties. If magnetic oxide be dissolved in 

 any acid for instance, hydrochloric which does not act as an oxidising agent, a ferrous 

 salt is first formed and ferric oxide remains, which is also capable of passing into 

 solution. The best way of preparing the hydrate of the magnetic oxide is by decomposing 

 a mixture of ferrous and ferric salts with ammonia ; it is, however, indispensable to pour 

 this mixture into the ammonia, and not vice versd, as in that case the ferrous oxide would 

 at first be precipitated alone, and then the ferric oxide. The compound thus formed has a 

 bright green colour, and when dried forms a black powder. Other combinations of 

 ferrous with ferric oxide are known, as are also compounds of ferric oxide with other 

 bases. Thus, for instance, compounds are known containing 4 molecules of ferrous oxide 

 to 1 of ferric oxide, and also 6 of ferrous to 1 of ferric oxide. These are also magnetic, 

 and are formed by heating iron in air. The magnesium compound MgO,Fe 2 O 3 is 

 prepared by passing gaseous hydrochloric acid over a heated mixture of magnesia and 

 ferric oxide. Crystalline magnesium oxide is then formed, and black, shiny, octahedral 

 crystals of the above-mentioned composition. This compound is analogous- to the 

 aluminates for instance, to spinel. Bernheim (1888) and Rousseau (1891) obtained 

 many similar compounds of ferric oxide, and their composition apparently corresponds 

 to the hydrates (Note 22) known for the oxide. 



