39 6 COAL 



matter in coal is derived largely from the spores of fossil mosses akin to the lycopodia, 

 and spores of this character, from the giant growths of the carboniferous period, to- 

 gether with the more resistant portion of plants akin to the pine, are the substances 

 which have best resisted the actions taking place during the ages that have elapsed in 

 the formation of coal. 



The vegetable matter consisted chiefly of a basis of celluloses and lignose, together 

 with the extractive matters rich in resin, and as these were unable to undergo rapid 

 decay owing to the low oxygen content of the air, fermentation became the chief factor 

 of change, and, the mass being overlaid gradually by sedimentary and other deposits, 

 pressure and heat added their forces to the actions taking place, the celluloses and lignose 

 becoming converted into humus bodies, having the general percentage composition 

 carbon 63, hydrogen, 5 oxygen 32. During these periods the resins which have the 

 general composition: carbon 79, hydrogen n, oxygen 10 underwent but little change, 

 but in the long ages that have passed some of the resin bodies have been converted 

 into such hydrocarbons as retene and hexahydrides, while the humus bodies have 

 undergone general degradation, resulting when completed in residual carbon. All 

 these constituents are found agglomerated in the coal the humus bodies and resid- 

 ual carbon being luted by the resin bodies and hydrocarbons, the properties of the coal 

 being dependent upon the proportions in which they are present. 



The lignites or tertiary coals, being of younger age, were formed at a period when the 

 air contained a higher percentage of oxygen, so that direct oxidation played a more 

 important part, and as the resins are the bodies most affected by this they were reduced 

 in quantity, but the great difference between the lignite and true coal is that the humus 

 bodies have undergone a minimum of degradation, so that the free carbon is lower than 

 in any other class, while the humus bodies still retain their attraction for water, which is 

 always present in considerable quantity in the Tertiary coal. 



During the formation of coal the gases evolved are carbon dioxide, methane and 

 water, and where the roof of the seam has been impermeable to gas, methane is found 

 occluded in the coal together with some carbon dioxide, but the bulk of the latter gas 

 has dissolved under pressure in water and formed bicarbonates in the sedimentary de- 

 posits, and has then helped in such other changes as the dolomiting of limestone. 



Many classifications of coal have been suggested, some based on their chemical, some 

 on their physical, and others on their coking Iproperties. Of the latter, the most 

 C/.-/SSI//- generally adopted is that of Gruner, in which he tabulates bituminous coals 

 cation of into five classes. Although M. Schondorff , F. Muck, and others have shown 

 coals. j.}^ j t j s no j. a ppii ca iji e j o a u kinds of coal, still this criticism applies to all 



classifications that have been proposed. 



Carbon. Hydrogen. Oxygen. 



1. Dry coal Long flame and non-coking . 75 80 43 5-5 13.0 18.5 



2. Fat gas coal Coke porous and brittle . . 80 85 5.0 5.8 10.0 13.2 



3. Semi-fat or furnace 



coal Good coke, but porous . . 84 89 5.0 5.5 5.5 10.0 



4. Coking coal Best coke 89-91 4-5-5-5 4 5~ 5-5 



5. Lean coals and an- 



thracite Non-coking 9093 3.0 4.3 3-O 4.5 



This arrangement shows not only the coking properties, but also the changes in com- 

 position which the coal undergoes, the concentration of carbon, and reduction in highly 

 oxidised bodies. In the first class there are the dry coals, yielding large volumes of gas 

 and liquid products on distillation; and these as might be expected most resemble the 

 lignites, and share with them the property of non-coking or binding together of the residue 

 on carbonisation. This is due to tne fact that the humus-like bodies are still present in much 

 larger quantities than the resinic compounds and hydrocarbons; and as on distillation they 

 leave no binding material in the residue, the resinic bodies cannot supply enough to give 

 more than a friable mass. In the second class of coals, altered conditions of temperature, 

 pressure and time have led to further decompositions of the humus bodies, and, the resinic 

 constituents and hydrocarbons having increased in ratio by concentration, a point is reached 

 at which coking takes place, although not of a really satisfactory character. In the third 

 class, the action has continued with further concentration of the resin bodies, hydrocarbons 



