65 



reference to the colour of rocks. Wlien freshly prej)ared it is a 

 finely granular preeixiitate of a very pale brownish colour. 

 Like Carbonate of Lime, it is soluble in water containing free 

 Carbonic Acid ; this solution, like those of other protosalts, is 

 rapidly peroxidif.ed when exposed to the air. The Percarbonate, 

 however, does not appear to exist, and when the protocarbonate 

 is peroxidised the hydi-ated peroxide is precipitated, since it is 

 insoluble in Carbonic Acid solution, and Carbonic Acid gas 

 escapes. This action may be noticed in every chalybeate spring, 

 which springs contain the protocarbonate dissolvedby the Carbonic 

 Acid in the water ; in the bed of the stream the hydrated per- 

 oxide, the result of the peroxidation of the protocarbonate on 

 exposxu'e to the air, is deposited in an ochreous layer. Again 

 rocks, which contain protoxide of iron or protocarbonate, will 

 weather of a rusty brown colour from a similar cause ; the 

 Carbonic Acid dissolved in the water which soaks through the 

 ground converting the protoxide of ii'on into carbonate, and 

 then dissolving it. "When this solution is exposed to the air on 

 the surface of the rock, it becomes peroxidised, and consequently 

 the rock assiunes a rusty colour. This may be noticed in several 

 varieties of the Inferior Oolite rocks in the neighbourhood, 

 which are of a pale brown or greenish tinge in the interior, but 

 rusty -brown on the weathered surface. 



The protosilicate is of a greenish colour. It is the cause of 

 the green colour of the smaU particles which occur in certain 

 sandstones, notably in the greensand formation. If, however, 

 this protosilicate is absent, the sandstone will not be green at 

 all, but is sometimes colourless, sometimes of a brown colour, 

 from the presence of the hydi'ated peroxide. 



The peroxide is the most important of the higher oxidised 

 compounds of iron with reference to the present question. It 

 occurs combined with water, as the hydrated peroxide, and un- 

 oombined with water as the anhydrous peroxide. In the former 

 state its colour is of a rusty brown ; in the latter the colour 

 varies with the state of aggregation, varying from a rusty-red, 

 brick-red, to reddish-black, or black in the crystalline condition. 



