556 IRON. 



evolution of hydrogen, and is precipitated as a white hydrate, 

 when potash is added to these salts, which becomes black on 

 boiling, from loss of water. The colour of the white precipitate 

 changes by exposure to air to grey, then to green, bluish black, 

 and finally to an ochrey red, when it is entirely peroxide. 



The protoxide of iron is thrown down by alkalies as a hy- 

 drate, and by alkaline carbonates as a carbonate, which are 

 white at first, but soon become of a dirty green, and undergo 

 the same subsequent changes from oxidation. Its salts are not 

 precipitated by sulphuretted hydrogen, the sulphuret of iron 

 being dissolved by strong acids, but give a black sulphuret with 

 solutions of alkaline sulphurets. They give a white precipitate 

 with the ferrocyanide of potassium, which gradually becomes of 

 a deep blue when exposed to air ; with the ferricyanide, a pre- 

 cipitate which is at once of an intense blue, being one of the 

 varieties of Prussian blue. The infusion of gall-nuts does not 

 affect a solution of the protoxide of iron when completely free 

 from peroxide. 



Protosulphuret of iron is prepared by heating to redness, in 

 a covered crucible, a mixture of iron filings and crude sulphur, 

 in the proportions of 7 of the former and 4 of the latter. It 

 dissolves in sulphuric and hydrochloric acids with evolution of 

 sulphuretted hydrogen gas (page 402). 



A subsulphuret of iron, Fe 2 S, appears to be formed when the 

 sulphate of iron is reduced by hydrogen ; one-half of the 

 sulphur coming off in the form of sulphurous acid. This sub- 

 sulphuret will correspond with the subsulphurets of copper and 

 lead, which crystallize in octohedrons. 



Proto chloride of iron crystallizes with 4 HO, and is very 

 soluble. Like all the soluble protosalts of iron, it is of a green 

 colour, gives a green solution, and has a great avidity for 

 oxygen. 



Protiodide of iron is formed when iodine is digested with 

 water and iron wire, the latter being in excess, and is obtained 

 as a crystalline mass by evaporating to dryness. It has been 

 introduced into medical use by Dr. A. T. Thomson. A piece of 

 iron wire is placed in the solution of this salt, to preserve it 

 from oxidising. The protiodide of iron dissolves a large 

 quantity of iodine, without becoming periodide, as the excess 

 of iodine may be precipitated by starch, 



Protocyanide of iron is obtained with the same difficulty 



