840 PEINCIPLES OF CHEMISTRY" 



longed ebullition of water in which ferric hydroxide prepared by the 

 oxidation of ferrous oxide is suspended, and also sometimes by similar 

 treatment of the ordinary hydroxide after it has been for a long time, 

 in contact with water. The transition of one hydroxide to another is 

 apparent by a change of colour ; the easily-soluble hydroxide is redder, 

 and the sparingly-soluble hydroxide more yellow in colour. 22 



The normal salts of the composition Fe 2 X 6 or FeX 3 correspond 

 with ferric oxide for example, the exceedingly volatile ferric chloride, 

 Fe 2 Cl 6 , which is easily prepared in the anhydrous state by the action 

 of chlorine on heated iron. 23 Such also is the normal ferric nitrate, 



22 The"* two ferric hydroxi'des are not only characterised by the above-mentioned 

 properties, but also by the fact that the first hydroxide forms immediately with potassium 

 ferrocyanide, K 4 FeC b N tJ> a blue colour depending on the formation -of Prussian blue, 

 whilst the second hydroxide does not give any reaction whatever with this salt. The 

 first hydroxide is entirely soluble in nitric, hydrochloric, and all other acids; whilst the 

 second sometimes (not always) forms a brick-coloured liquid, which appears turbid 

 and does not give the reactions peculiai^ to the ferric salts (Pean de Saint-Gilles, 

 Scheurer-Kestner). In addition to this, when the smallest quantity of an alkaline salt 

 is added to this liquid, ferric oxide is precipitated. Thus a colloidal solution is formed 

 (hydrosol), which is exactly similar to silica hydrosol (Chapter XVII.), according to 

 which example the hydrosol of ferric oxide may be obtained. 



If ordinary ferric hydroxide be dissolved in acetic acid, a solution of the colour of red 

 wine is obtained, which has all the reactions characteristic of ferric salts. But if this 

 solution (formed in the cold) be heated to the boiling-point, its colour is very rapidly 

 intensified, a smell of acetic acid becomes apparent, and the solution then contains a 

 new variety of ferric oxide. If the boiling of the solution be continued, acetic acid is 

 evolved, and the modified ferric oxide is precipitated. If the evaporation of the acetic 

 acid be prevented (in a closed or sealed vessel), and the liquid be heated for some time, 

 the whole of the ferric hydroxide then passes into the insoluble form, and if some alkaline 

 salt be added (to the hydrosol formed), the whole of the ferric oxide is then precipitated, 

 in its insoluble form. This method may be applied for separating ferric oxide from 

 solutions of its salts. 



All phenomena observed respecting ferric oxide (colloidal properties, various forms, 

 formation of double basic salts) demonstrate that this substance, like silica, alumina, 

 lead hydroxide, &c., is polymerised, that the composition is represented by (Fe^QjJn. 



JS The ferric compound which is most used in practice (for instance, in medicine, for 

 cauterising, stopping bleeding, &c. Oleum Martis) is ferric chloride, Fe^Cle, easily 

 obtained by dissolving the ordinary hydrated oxide of iron in hydrochloric acid. It" ia 

 obtained in the anhydrous state by the action of chlorine on heated iron. The experi- 

 ment is carried on in a porcelain tube, and a solid volatile substance is then formed in 

 the shape of brilliant violet scales which very readily absorb moisture from the air, and 

 when heated with water decompose into crystalline ferric oxide and hydrochloric acid : 

 Fe. 2 Cl 6 + 3H 2 O = 6HCl + Fe 2 O.v Ferric chloride is so volatile that the density of its 

 vapour may be determined. At 440 it .is equal to 164'0 referred to hydrogen ; the 

 formula Fe 2 Cl 6 corresponds with a density of 162'5. An aqueous solution of this salt 

 has a brown colour. On evaporating and cooling this solution, crystals separate con- 

 taining 6 or 12 molecules of H.>O. Ferric chloride is not only soluble in water, but also 

 in alcohol (similarly to magnesium chloride, &c.) and in ether. If the latter solutions 

 axe exposed to the rays of the sun they become colourless, and deposit ferrous chloride, 

 FeCl 2 , chlorine being disengaged. After a certain lapse of time, the aqueous solutions 

 of ferric chloride decompose with precipitation of a basic salt, thus demonstrating the 

 instability of ferric chloride, like the other salts of ferric oxide (Note 22). This salt is 



