CHEMICAL WORK. 123 



1. Oxidation in inorganic materials. — The effects that have special geo- 

 logical importance are the slow oxidation of iron, manganese, sulphur, and 

 some other elements, which takes place in the mineral constituents of rocks 

 when water and air together have access. Little oxidation takes place under 

 water. The iron of minerals undergoes easy oxidation when it is present in 

 the protoxide state, PeO, or when combined with sulphur. The protoxide 

 state is the unstable state of iron. In oxidizing it combines with one half 

 more oxygen, and becomes the sesquioxide, FejOg. This iron oxide is the 

 mineral hematite having a red powder, if free from combined water ; but, if 

 containing water, limonite, which has a yellow or yellow brown color when 

 powdered, if not before (page 71). The latter rust-colored oxide is like that 

 which is produced when the metal iron rusts. But the rust may contain 

 some carbonate besides the iron sesquioxide. 



In a similar manner, when a mineral contains manganese protoxide, 

 MnO, the Mn tends to become MuaOg or Mn02, compounds that have a black 

 powder. Black stains, and black crusts on marble and other rocks, after 

 weathering, usually come from the oxidation of some manganese in the rock. 



The oxides FeO and MnO are vmknown except in combination. But 

 magnetite, FcgO^, is common in disseminated grains in many rocks, besides 

 sometimes constituting thick beds ; it often oxidizes slowly to the sesqui- 

 oxide, FcgOs, producing hematite or limonite. 



Again: the iron sulphides, j)yrite and marcasite, each FeSj, oxidize 

 readily, and especially the latter, as shown by Julien ; the iron, Fe, becom- 

 ing FeO, if there is an acid ready to combine with it, but otherwise FcgOg ; 

 the sulphur, S, becoming SOg, and, with added water, sulphuric acid. This 

 acid, with the FeO and water, may make the iron sulphate, copperas ; but 

 it may combine also with FcgOg, and make other sulphates. If there is 

 limestone at hand, the SOg, or sulphuric acid, may combine with the lime 

 and water, and form gypsum, and may thus make beds of gypsum. When 

 pyrite and marcasite are mixed together, the marcasite makes oxidation 

 easy (Julien). 



2. Oxidation in organic materials, and other chemical changes. — When 

 life ceases, all organic materials tend to decay ; and in this decay, oxidation 

 is the chief process, and oxides the larger part, or all, of the final results. 



Wood, when thoroughly dried, consists approximately of carbon (C) 49-66, 

 hydrogen (H) 6-21, oxygen (0) 43-03, with traces of sulphur (S) and 

 phosphorus (P), nitrogen (K) 1-10. Animal fats contain the same elements, 

 and animal tissues the same with much nitrogen. 



In dried wood, the C, H, are atomically in the proportions nearly 

 C6H9O4. In decay, the oxygen used may be that of the wood, or of the 

 atmosphere or other substances. The C may combine with and make 

 carbon protoxide, CO, the gas which burns with a blue flame in a furnace ; 

 but it generally combines with 2 0, making the more stable and incom- 

 bustible compound CO2, or carbonic dioxide (carbonic acid). The H may 

 unite with and form water, HjO. But instead of all the C combining 



