220 SECRETS OF EARTH AND SEA 



tion of the original hydrogen of the woody peat, are the 

 important elements in coal; and we may well ask how 

 they come to be produced as a black or dark brown mass 

 from dead vegetable growths which are often bleached 

 and colourless. It is true that vegetable refuse does not 

 necessarily blacken when left to itself. We know that 

 by roasting or charring wood (or animals' flesh or bone) 

 we can drive off the elements oxygen and hydrogen 

 and nitrogen (if there), and obtain a black mass of 

 carbon (so-called charcoal). That blackness is the 

 actual true tint of carbon. The dead weeds and leaves 

 at the bottom of a stagnant pond break down and form 

 a pitch-black mud. They would not, and do not, go 

 black if exposed to the oxygen of the atmosphere; but 

 at the bottom of a stagnant pond or in a refuse heap they 

 are excluded from the air, and a microbe a bacterium 

 which has been carefully studied, and is of a kind which 

 can only flourish in the absence of free oxygen attacks 

 the dead weeds, producing by change of their substance 

 marsh-gas and black carbon, the black mud emitting 

 bubbles of gas which one may stir up with a pole in such 

 a pond. This chemical attack by anaerobic bacteria goes 

 on in the deeper layers of all marshes and stagnant pools, 

 remote from the oxygen of the air ; and it is fairly 

 certain that the black coal which we find in strata of 

 great geological age was so produced by the action of 

 special kinds of bacteria upon peat-like masses of vege- 

 table refuse. Indeed, by studying microscopic sections 

 of coal, numerous forms of bacteria have been recognized 

 which might be capable of effecting such chemical 

 changes. On the other hand, we must remember that 

 it is not possible to conclude by form alone as to what 

 subtle chemical work a bacterium or bacillus or micro- 

 coccus may be, or may have been, carrying on. The 

 peat-like deposits which became carbonized and so formed 



