M.
W.)]

[Sidenote: Cellulose-bacteria.]

Another important advance is in our knowledge of the part played by
bacteria in the circulation of carbon in nature. The enormous masses of
cellulose deposited annually on the earth's surface are, as we know,
principally the result of chlorophyll action on the carbon dioxide of the
atmosphere decomposed by energy derived from the sun; and although we know
little as yet concerning the magnitude of other processes of
carbon-assimilation--_e.g._ by nitrifying bacteria--it is probably
comparatively small. Such cellulose is gradually reconverted into water and
carbon dioxide, but for some time nothing positive was known as to the
agents which thus break up the paper, rags, straw, leaves and wood, &c.,
accumulating in cesspools, forests, marshes and elsewhere in such
abundance. The work of van Tieghem, van Senus, Fribes, Omeliansky and
others has now shown that while certain anaerobic bacteria decompose the
substance of the middle lamella--chiefly pectin compounds--and thus bring
about the isolation of the cellulose fibres when, for instance, flax is
steeped or "retted," they are unable to attack the cellulose itself. There
exist in the mud of marshes, rivers and cloacae, &c., however, other
anaerobic bacteria which decompose cellulose, probably hydrolysing it first
and then splitting the products into carbon dioxide and marsh gas. When
calcium sulphate is present, the nascent methane induces the formation of
calcium carbonate, sulphuretted hydrogen and water. We have thus an
explanation of the occurrence of marsh gas and sulphuretted hydrogen in
bogs, and it is highly probable that the existence of these gases in the
intestines of herbivorous animals is due to similar putrefactive changes in
the undigested cellulose remains.

[Sidenote: Sulphur bacteria.]

Cohn long ago showed that certain glistening particles observed in the
cells of _Beggiatoa_ consist of sulphur, and Winogradsky and Beyerinck have
shown that a whole series of sulphur bacteria of the genera _Thiothrix_,
_Chromatium_, _Spirillum_, _Monas_, &c., exist, and play important parts in
the circulation of this element in nature, _e.g._ in marshes, estuaries,
sulphur springs, &c. When cellulose bacteria set free marsh gas, the
nascent gas reduces sulphates--_e.g._ gypsum--with liberation of SH_2, and
it is found that the sulphur bacteria thrive under such conditions by
oxidizing the SH_2 and storing the sulphur in their