Like all woody matter, that from which peat is formed consists of a combination of the elements carbon, hydrogen and oxygen; and these remain in somewhat changed chemical union in the brown coal, bituminous coal, and anthracite. The carbon and a varying and small proportion 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 anaërobic 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 vegetable 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 the "coal" were probably masses of algæ, mosses and soft aquatic plants, which were brought down and accumulated in swampy, forest-covered ground about the mouths of rivers, the deposit being covered in owing to rapid oscillations of level by beds of sand or clay, followed by new growth and deposit.