own protoplasm. If the
SH_2 runs short they oxidize the sulphur again to sulphuric acid, which
combines with any calcium carbonate present and forms sulphate again.
Similarly nascent methane may reduce iron salts, and the black mud in which
these bacteria often occur owes its colour to the FeS formed. Beyerinck and
Jegunow have shown that some partially anaerobic sulphur bacteria can only
exist in strata at a certain depth below the level of quiet waters where
SH_2 is being set free below by the bacterial decompositions of vegetable
mud and rises to meet the atmospheric oxygen coming down from above, and
that this zone of physiological activity rises and falls with the
variations of partial pressure of the gases due to the rate of evolution of
the SH_2. In the deeper parts of this zone the bacteria absorb the SH_2,
and, as they rise, oxidize it and store up the sulphur; then ascending into
planes more highly oxygenated, oxidize the sulphur to SO_3. These bacteria
therefore employ SH_2 as their respiratory substance, much as higher plants
employ carbohydrates--instead of liberating energy as heat by the
respiratory combustion of sugars, they do it by oxidizing hydrogen
sulphide. Beyerinck has shown that _Spirillum desulphuricans_, a definite
anaerobic form, attacks and reduces sulphates, thus undoing the work of the
sulphur bacteria as certain de-nitrifying bacteria reverse the operations
of nitro-bacteria. Here again, therefore, we have sulphur, taken [v.03
p.0167] into the higher plants as sulphates, built up into proteids,
decomposed by putrefactive bacteria and yielding SH_2 which the sulphur
bacteria oxidize, the resulting sulphur is then again oxidized to SO_3 and
again combined with calcium to gypsum, the cycle being thus complete.

[Sidenote: Iron bacteria.]

Chalybeate waters, pools in marshes near ironstone, &c, abound in bacteria,
some of which belong to the remarkable genera _Crenothrix_, _Cladothrix_
and _Leptothrix_, and contain ferric oxide, _i.e._ rust, in their
cell-walls. This iron deposit is not merely mechanical but is due to the
physiological activity of the organism which, according to Winogradsky,
liberates energy by oxidizing ferrous and ferric oxide in its protoplasm--a
view not accepted by H. Molisch. The iron must be in certain soluble
conditions, however, and the soluble bicarbonate of the protoxide of
chalybeate springs seems most favourable, the hydrocarbonate absorbed by
the cells is oxidi