124 DYNAMICAL GEOLOGY. 



with 0, part, especially when the decomposition goes on under water, or 

 where atmospheric oxygen is excluded, may combine with H and produce 

 the hydrocarbon CH^ — called marsh-gas, because sometimes bubbling up 

 through marsh waters ; it is the gas which burns and makes the flame of a 

 wood fire. Other related hydrocarbons also might form. But the burning 

 of this gas when complete ends in producing CO^ and H2O. This is the final 

 result when plants decompose in the air, except minor results from the 

 nitrogen (K) and sulphur (S) present, among which are making, with 

 the nitrogen, ammonia, NHg; and, with oxygen, nitrous acid (NgOg), and 

 nitric acid (NoOs) ; and making, with the sulphur, hydrogen sulphide (sul- 

 phuretted hydrogen) HoS, and with oxygen, sulphurous acid (SO2) and 

 sulphuric acid (SO3). 



In smothered combustion (as in making charcoal by burning wood under a cover of 

 earth), nearly all the H and disappear as CO, CO2, and HoO, without a consumption 

 of all the carbon ; and this happens when plants decompose under a complete covering of 

 water or earth, because this excludes the air and confines the changes to the elements of 

 the plants ; and the more complete the protection, the greater will be the proportion saved 

 of carbon and hydrogen, the combustible elements for the making of coal. With reference 

 to the making of mineral oil or gas, it is to be noted that if the outside air is wholly 

 excluded through overlying fine sediments, they may be produced by the direct decomposi- 

 tion of woody tissues or of animal oils. Thus, if the carbon of the wood (C6H9O4 nearly) 

 combines with all the oxygen, making thereby 2 CO2 , it will leave C4H9 , and 2 C4H9 = CgHig , 

 which is the composition of some mineral oil. So in animal oils, as oleic acid, C18H34O2 , 

 on separating CO2, there would be left C17H34, one of the ethylene oils ; or from margaric 

 acid, C17H34O2 , the product would be C16H34 , or a combination of marsh-gas oils. Fossil 

 fishes are often numerous in coaly beds that afford much oil. (D., Min., 1868, p. 726.) 



In the change to ordinary bituminous coal the loss in the hydrogen of the wood, 

 proportionally to that of the carbon, is about tioo fifths, and that of the oxygen about 

 four fifths — about 5-5 per cent of such coal (ash excluded) being hydrogen, and 12 to 15 

 per cent oxygen, with 80 to 81 per cent carbon. 



The carbonaceous products from the decomposition of plants and animals 

 give the black color to soils. In wet soil, other acid products sometimes 

 form, called humus acids, from the Latin humus, soil, or earth. 



The returning to the air of the constituents of a plant, by decay, in the form of 

 carbonic acid and water, is restoring what was taken and used in the growth of the plant 

 and balancing the account. The storing of part of the carbon and hydrogen in the 

 rocks in the form of coal and mineral oil and gas was an abstraction of carbonic acid from 

 the air, and commenced a debit account which use in combustion by man is doing only a 

 little in the way of settling. Happily the world is better off for the purification of its 

 atmosphere. 



3. Deoxidation, or the abstraction of oxygen from a compound by any oxi- 

 dizing substance at hand. — Most deoxidation in nature is done by organic 

 substances through the process of decay above described. The affinity in the 

 carbon and hydrogen of the plant for oxygen is so strong that it will take it 

 away from iron oxides or salts, and many other kinds. It may take from 

 FcgOg and reduce it to FeO ; so that if there is then an acid at hand for com- 



