364 PALEOZOIC TIME. 



But the change could not be as simple as here indicated, since (1) there is some 

 nitrogen present in plants; (2) the plants would have undergone some change before 

 the complete burial; (3) some water and carbo-hydrogen might also be made and 

 escape, though it is not probable that the amount would be large. The facts still 

 illustrate a possible mode of transformation. But since part of the oxygen remains in 

 all coals, only part of the oxygen of the wood has gone to produce carbonic acid; and, 

 moreover, external oxygen has taken some part in making this gas, or the water that 

 is given off. The amount of oxygen present is much the largest in Brown coal, and 

 probably because external oxygen was more concerned in the transformation thau in 

 the making of Carboniferous bituminous coal. 



Bischof has calculated that, if the escaping product is carbonic acid and water, 

 derived from the elements of the wood (which might be the case if external oxygen 

 were completely excluded), the amount of coal left, in the case of bituminous coal, 

 would be about 54 per cent. If the escaping gases were carbonic acid and hydrogen, 

 the latter combining with external oxygen to form water, the amount of bituminous 

 coal left would be about 42 per cent. 



It 13 also to be noted, that, in the derivation of coal from vegetable matters, there 

 may be. as suggested by S. W. Johnson, a process carried forward of molecular con- 

 densation, such as organic chemistry affords many examples of, which may account 

 for the increased density of the product, and for the occurrence of the maximum 

 density in anthracite. 



In the formation of peat, — the first step toward Brown coal, — both carbonic acid 

 and water escape, with also a little carbo-hydrogen gas (marsh-gas) and nitrogen; and 

 the peat, which results, is chiefly, according to late experiments, humic acid. Brown 

 coal also contains probably some humic acid, as is indicated by the brown color it gives 

 to a solution of potash when heated with it. No such color is obtained with bituminous 

 coal or anthracite. 



The gas bubbling up from a marsh afforded Websky: Carbonic acid (CO 2 ) 2-97, 

 marsh-gas (CH 4 ) 43-36, nitrogen 53-67 = 100. The carbonic acid is proportionally small; 

 because it is soluble in water, and also because it may enter into combination with 

 earthy ingredients present in the ash. The amount of escaping nitrogen shows that 

 coal retains but little of that in the vegetable and animal life of the marsh. 



See, further, on the making of Coal and Peat, Bischof 's "Chemical Geology" 

 Websky, in the Jour. f. pr. Chem., xcii. ; Hunt, in Am. Jour. Sci., II. xxxv., and the 

 Canadian Naturalist, vi. 241; S. W. Johnson, on "Peat and its Uses." 



Impurities of the Coal. — The impurities of the coal are in part 

 derived from the wood. 



1. Silica is present in the exterior part of the stems of Equiseta 

 (the representatives of the ancient Calamites), to such an extent that 

 the plants sometimes afford 25 per cent, of ash, with half this silica, 

 that is, 100 lbs. of the dried plants contain 12^- lbs. of silica; and it 

 exists in smaller proportions in the interior of all plants. 



2. Alumina, while absent from most plants, constitutes 22 to 50 

 per cent, of the ash of some modern species of Lycopods. 



3. Lime and Magnesia are present in small proportions in the ash 

 of all plants. In Charce, species that existed in the Carboniferous 

 era, and which afford 30 per cent., or more, of ash, 95 per cent, are 

 carbonate of lime. 



4. Oxyd of iron is present in many plants. The ash of one Lyco- 

 pod afforded 6 per cent, of this oxyd ; and the same is true of a 

 Sphagnum. 



