PROTOCOMPOUNDS OF IRON. 555 



by supposing combination of the hydrogen and platinum, and 

 that the superficial polar molecule of the metal then consists of 

 hydrogen as the external zincous element, and platinum as the 

 chlorous element, resembling the positive amalgam of zinc, in 

 which zinc forms the external zincous atom, and mercury the 

 chlorous atom of the molecule (page 208). This hydrogen 

 must decompose the hydrated sulphuric acid (H -f SO 4 ), evolv- 

 ing hydrogen, and cause a train of decompositions from the hy- 

 druretted to the clean platinum. The analogy between this 

 voltaic action of hydrogen, and the oxidation of hydrogen gas 

 by spongy platinum, adds to its interest. Again, platinum, 

 gold and silver, by being placed for a few seconds in chlorine, 

 become capable of acting negatively, or are chlorous, when they 

 form a circle with clean platinum in dilute sulphuric acid. 

 Here, also, there must be a compound polar molecule, of which 

 the zincous element is platinum, and the external chlorous ele- 

 ment chlorine ; and this chlorine must combine with the hy- 

 drogen, and evolve the salt-radical of the hydrated acid, thus 

 causing a train of decompositions through the latter. A pellicle 

 of peroxide of lead can be precipitated upon the surface of iron 

 and platinum, and then they become strongly chlorous in a vol- 

 taic circle, with nitric acid, like platinum with the film of chlo- 

 rine above, till the pellicle of peroxide is dissolved off by the 

 acid. Here the excess of oxygen in the peroxide must decom- 

 pose water, or more likely the hydrated acid present, evolving 

 oxygen or the salt-radical of the acid, when the circuit is com- 

 pleted. In such circles, we have the affinity of hydrogen, of 

 chlorine, or of oxygen, originating the rotal action, instead of 

 that of a positive metal, as usual.* 



PROTOCOMPOUNDS OF IRON. 



Protoxide of iron, Ferrous oxide ; FeO ; 439.2 or 35.18. Iron 

 appears to admit of only two degrees of oxidation, the protoxide 

 and peroxide, which are both basic, and correspond respectively 

 with manganous and manganic oxides. The protoxide is not 

 easily obtained in a dry state, from the avidity with which it 

 absorbs oxygen. It exists in the sulphate and other salts of 

 iron, formed when the metal dissolves in an acid with the 



* Schcenbein and Faraday in the Phil. Mag. 3rd Series, vols. 9, 10, 11, 12 

 and 14. 



