he same conditions influence
their development, the process may be regarded as a whole.

_Difficulty in isolating them._

A great difficulty has been experienced in the attempt to isolate these
micro-organisms for the purpose of studying their nature. This arises
from the fact that they refuse to grow on the ordinary solid cultivating
media used by bacteriologists. Winogradsky, however, has recently
succeeded in cultivating them in _a purely mineral_ medium--viz.,
_silica-jelly_.[108]

_Nitrifying Organisms do not require Organic Matter._

The fact that they can develop in media destitute of organic matter, is
one of very great interest and importance to Vegetable Physiology. It
implies that they can derive their carbon from carbonic acid--a power
which it was believed was possessed by green plants alone among living
structures. For organisms destitute of chlorophyll, the source of their
protoplasmic carbon, it has been hitherto commonly believed, must be
_organic matter_ of some sort. While it would appear that the nitrifying
organisms can, when opportunity affords, feed upon organic matter, yet
it has been proved beyond doubt that they can also freely develop in
media entirely devoid of it, and are capable, under such circumstances,
of deriving their carbon from a purely mineral source.[109] This fact,
which is subversive of what was believed to be a fundamental law of
Vegetable Physiology, is one of the most important of the many important
and interesting facts which these nitrification researches have
elicited.[110]

CONDITIONS FAVOURABLE FOR NITRIFICATION.

We may now proceed to discuss the conditions favourable for
nitrification.

_Presence of Food-constituents._

Among these conditions the first is the presence of certain
food-constituents. To both animal and vegetable life alike a certain
amount of mineral food is absolutely necessary. Among these phosphoric
acid is one of the most important, and in the experiments on
nitrification it has been found that the nitrifying organisms will not
develop in any medium destitute of it. That other mineral
food-constituents are necessary is highly probable, although the
influence of their absence on the development of the process has not
been similarly studied. Probably potash, magnesia, and lime salts are
necessary. In the cultivating solutions used in the experiments on the
subject, the mineral food-constituents added consisted of lime,
magnesia, and po