zed, probably thus--

2FeCO_3 + 3OH_2 + O = Fe_2(OH)_6 + 2CO_2.

The ferric hydroxide accumulates in the sheath, and gradually passes into
the more insoluble ferric oxide. These actions are of extreme importance in
nature, as their continuation results in the enormous deposits of bog-iron
ore, ochre, and--since Molisch has shown that the iron can be replaced by
manganese in some bacteria--of manganese ores.

[Sidenote: Pigment bacteria.]

Considerable advances in our knowledge of the various chromogenic bacteria
have been made by the studies of Beyerinck, Lankester, Engelmann, Ewart and
others, and have assumed exceptional importance owing to the discovery that
_Bacteriopurpurin_--the red colouring matter contained in certain sulphur
bacteria--absorbs certain rays of solar energy, and enables the organism to
utilize the energy for its own life-purposes. Engelmann showed, for
instance, that these red-purple bacteria collect in the ultra-red, and to a
less extent in the orange and green, in bands which agree with the
absorption spectrum of the extracted colouring matter. Not only so, but the
evident parallelism between this absorption of light and that by the
chlorophyll of green plants, is completed by the demonstration that oxygen
is set free by these bacteria--_i.e._ by means of radiant energy trapped by
their colour-screens the living cells are in both cases enabled to do work,
such as the reduction of highly oxidized compounds.

The most recent observations of Molisch seem to show that bacteria
possessing bacteriopurpurin exhibit a new type of assimilation--the
assimilation of organic material under the influence of light. In the case
of these red-purple bacteria the colouring matter is contained in the
protoplasm of the cell, but in most chromogenic bacteria it occurs as
excreted pigment on and between the cells, or is formed by their action in
the medium. Ewart has confirmed the principal conclusions concerning these
purple, and also the so-called chlorophyll bacteria (_B. viride_, _B.
chlorinum_, &c.), the results going to show that these are, as many
authorities have held, merely minute algae. The pigment itself may be
soluble in water, as is the case with the blue-green fluorescent body
formed by _B. pyocyaneus_, _B. fluorescens_ and a whole group of
fluorescent bacteria. Neelson found that the pigment of _B. cyanogenus_
gives a band in the yellow and strong lines at E and F in the solar
spectrum--an absor